Position Paper Position Paper
C.J.M. Philippart & M.J. Baptist
Colophon
An explanatory study into effective measures to strengthen diadromous fish populations in the Wadden Sea is produced by the Wadden Academy.
A draft version was externally reviewed by Dutch experts Zwanette Jager (ZiltWater Advies), Peter Paul Schollema (Waterschap Hunze en Aa's), Henk van der Veer (NIOZ) and Erwin Winter (IMARES) and by three independent scientific experts from ICES, i.e. Romuald Lipcius (Virginia Institute of Marine Science, USA), Dennis Ensing (Agri-Food and Biosciences Institute Northern Ireland, IRL), and Joey Zydlewski (University of Maine, USA).
This document can be referred to as: Philippart, C.J.M. & M.J. Baptist, 2016. An explanatory study into effective measures to strengthen diadromous fish populations in the Wadden Sea. Leeuwarden, Waddenacademie, Position Paper 2016-02.
Contactperson Waddenacademie Klaas Deen Secretaris t 058 233 90 31 e [email protected]
Ontwerp BW H ontwerpers Fotografie Saskia Boelsums Druk Hollandridderkerk ISBN 978-94-90289-36-2 Volgnummer 2016-02
© Waddenacademie February 2016 www.waddenacademie.nl
De basisfinanciering van de Waddenacademie is afkomstig van het Waddenfonds. CONTENTS
CONTENTS 3
EXECUTIVE SUMMARY 4
1. INTRODUCTION 10 1.1 Recent changes in Wadden Sea fish 10 1.2 Audit 11 1.3 Target fish species 12 1.4 Long-term changes in the environment 13 1.5 Long-term changes in fish landings 17
2. POSSIBLE DRIVERS OF LOCAL FISH DENSITIES 18 2.1 Introduction 18 2.2 Time series on fish abundance 20 2.3 Statistical analysis survey data 23 2.4 Results & Discussion 24 2.5 Conclusions 49
3. FINDING OPTIMAL LOCATIONS FOR FISH MIGRATION FROM MARINE TO FRESHWATER SYSTEMS 51 3.1 Introduction 51 3.2 Estuarine gradients in the Dutch Wadden Sea 52 3.3 Fish densities in tidal basins of the Dutch Wadden Sea 54 3.4 Fish densities near small freshwater sluices discharging into the Wadden Sea 56 3.5 Conclusions 72
4. NUMERICAL BUDGETS OF LOCAL STOCKS 73 4.1 Introduction 73 4.2 Factors 74 4.3 Stock Budgets 76 4.4 Discussion & Conclusions 81
5. POTENTIAL MEASURES AND COSTS 73 5.1 Introduction 84 5.2 Fisheries 85 5.3 Brackish water zones 86 5.4 Freshwater habitats 89 5.5 Fish passages 90 5.6 Concluding remarks 93
ACKNOWLEDGEMENTS 94
REFERENCES 95
3 EXECUTIVE SUMMARY
Aim of the report Impacts of local barriers and fish passages The strong decline in Wadden Sea fish since the 1980s has called for action to strengthen local fish To assess the consequences of creating (e.g. Afsluitdijk stocks. Proposed measures include the reduction of in 1932) and removing (e.g. fish passages) local barriers fisheries, restoration of natural dynamics in habitats for fish migration for local fish stocks, long-term and and improving connectivity between marine and consistent survey data sets of before and after the time freshwater areas for migratory fish. The Wadden which these interventions took place are required. The Academy was invited by the Waddenfonds to make an data should have the proper spatial scales and include inventory of the most effective measures to strengthen quantitative information on other factors that may (migratory) fish stocks in the Wadden Sea area in have additionally influenced local fish densities (e.g., general, and on the added values of further investments fisheries, predation). Since such data do not exist, in fish passages in particular. Hereto, the following available data and additional information was used to questions were defined: at least qualitatively explore such impacts.
1. To what extent were local stocks of migratory fish Developments in local fish stocks were compared with in the Wadden Sea influenced by the construction large-scale trends (e.g. in the NE Atlantic) to explore of local barriers and fish passages? impacts of large-scale and local impacts, such as the effect of the closure of former estuaries in the Wadden 2. Which local measures would be most promising Sea. Because scientific fish surveys in the Wadden Sea to strengthen the stocks of migratory fish in the did not start before the 1960s, data on registrations of Wadden Sea, and what would be the optimal commercial fisheries that started around 1900 were locations for these measures? used (“Landings”). Within a shorter time frame (< 50 yrs), time series of surveys were analyzed to further 3. What would be the costs and benefits of different explore the nature of large-scale and local drivers, and measures to strengthen the stocks of migratory fish subsequently the present options for managing local in the Wadden Sea? fish stocks (“Surveys”).
Based on the function of the Wadden Sea for fish Landings migration between marine and freshwater systems, In the NE Atlantic, all of the 14 species targeted the audit was focussing on 12 diadromous fish in this study were landed (as catch or by-catch) by species (i.e. River Lamprey, Sea Lamprey, European commercial fishing activities between 1903 and 2013. Sturgeon, European Eel, Allis shad, Twaite Shad, At this scale, migratory fish have strongly declined, European Smelt, Atlantic Salmon, Sea Trout, Houting, often to historic lows. Exploitation is a major factor, Three-spined Stickleback and European Flounder) on top of other human-induced (e.g. pollution, habitat supplemented with 2 species that had large stock sizes loss, blocked migration) and natural factors (e.g. in the former Zuiderzee (i.e. European Anchovy and hydrographical fluctuations). Atlantic Herring). For all of these species, the Wadden In Lake IJssel, the closure in 1932 was followed by Sea is (or was) an essential area to fulfil a particular decrease in landings of Anchovy, Flounder, Herring phase in their life cycle, often during a particular time and Smelt, and a temporal increase in the landings of of the year. European Eel. Eel possibly got stuck in the area
4 and/or no longer needed to migrate further upstream For Twaite Shad, stock dynamics appear to be the river. The decline in Eel since the 1950s remains synchronised in the western and eastern part of the so far unexplained. Landings of Smelt reached Dutch Wadden Sea and the adjacent coastal zone. comparable levels in the 1980s as before the closure. As has been observed for the Scheldt estuary, stock Ongoing fishing pressure contributed, however, to a dynamics of Twaite Shad in estuarine tidal basins depletion of the local Smelt stocks in the 2000s. such as the Marsdiep and the Ems might be driven by In the western Wadden Sea, the closure of the environmental conditions at their freshwater spawning Afsluitdijk was followed by a temporal increase in the grounds. Improvement of these conditions might then landings of Herring and Flounder. This might have enhance stocks in the Wadden Sea. been the result of an increase in fish abundances, but possibly also of enhanced fishing efforts as fishermen For European Smelt and European Flounder, from the former Zuiderzee continued to fish in the synchronisation appears to occur between the North Wadden Sea after 1932. Sea and the westernmost (Marsdiep) and easternmost (Ems estuary) tidal basins of the Dutch Wadden Sea. In Dutch freshwater systems including main rivers, Both basins are characterised by a strong salinity landings of Allis Shad, Atlantic Salmon, Houting and gradient, suggesting freshwater discharges as a driving Sturgeon strongly declined between the 1890s and factor for local stocks. the 1940s. These declines are attributed to habitat destruction (e.g., river engineering, removal of sand Conclusions on local barriers and fish and gravel), deteriorated water quality (i.e. pollution) passages and overfishing. Present data are insufficient to quantitatively analyse the impacts of barriers on local stocks. Long-term Surveys information on fish landings of the Wadden Sea and For all species for which sufficient data was available, Lake IJssel clearly illustrated, however, the impacts local stocks in the Wadden Sea and Lake IJssel appear of the closure of the Zuiderzee in 1932. Whilst some to be more or less influenced by large-scale long- species were negatively impacted (e.g. Anchovy), term drivers such as fisheries, habitat destruction and others (e.g. Eel) increased temporally following the climate. The impact of these large-scale drivers versus closure. Strong fishing pressure finally resulted in the influence of local drivers on fish stocks varies, historical lows (e.g. Smelt). In the Wadden Sea, drivers however, between species and areas. of local stocks appear to be large-scale (e.g. Eel) or local (e.g. Twaite Shad, Smelt, Flounder). If fish stocks For European Eel, for example, synchrony in time series are strongly influenced by local short-term drivers, from North Sea and the Wadden Sea suggest a large- then improvement of local conditions might result into scale driver within the marine waters of the study area. strengthening of local stocks, within the boundaries set Such possible large-scale drivers include atmospherically by the large-scale long-term drivers. driven dispersal of juvenile Eel by ocean currents and changes in hydrology and exploitation. Within Lake IJssel, the effects of these large-scale climate-driven influences may have been obscured by local impacts, including the influence of the Afsluitdijk in 1932 and changes in fishery pressure and restocking hereafter.
5 Measures and optimal locations discharge management (REG), fish-friendly discharge management (FFM) appeared to at least double Fish passages the passing efficiency for European Eel, European The Dutch Wadden Sea area comprises 65 existing and Smelt, Three-Spined Stickleback and Flounder. This 4 planned (or under construction) estuarine gradients. is underlined by an observed increase in Flounder These gradients vary from small tidal creeks at the in Lake IJssel following fish-friendly discharge islands to large freshwater sluices along the mainland management in the early 1990s. An additional fish coast. Existing gradients include freshwater sluices, passage is planned directly west of the discharge sluices ship locks, pumps and culverts of which several are of Kornwerderzand, the so-called Fish Migration structurally operated (8) or equipped with technical River (FMR). For Sea Trout and Flounder, projected facilities (17) to enhance fish migration from the passing via Fish Migration River (FMR) is comparable sea to freshwater systems. Identifying best practices as presently observed during regular discharge and finding optimal locations requires a quantitative management. Compared to fish-friendly discharge assessment of fish passing efficiency of existing fish management (FFM), projected migration via the passages. Despite the generally high costs of building FMR is higher for Herring, Smelt and Three-Spined these facilities, however, no comparable data to Stickleback, and lower for Eel. perform a quantitative evaluation of the construction of fish passages was available. Projections for the FMR were based upon a passing efficiency of 50% of fish present in a semi-enclosed As an approximation of spatial variation in supply of area before the sluices of Kornwerderzand, as was done fish at small freshwater discharges, data on densities for the environmental assessment (MER). Taking of several migratory fish species at 15 discharge points the species-specific variation at present into account, were analysed. Results indicated that local densities an average added value of 50% most probably does are related to local circumstances, including (1) the not correctly addresses future passing efficiencies presence of sufficient flow of freshwater to attract under FMR conditions for all target species. The Fish fish to the entrance location, (2) the closeness of the Migration River could have added value to FFM, entrance to deeper waters that have a salinity gradient, but only when optimally designed. Existing data and and (3) the presence of a deep and wave-sheltered information are presently, however, not sufficient to basin to allow fish to safely wait before passing. Present support design recommendations. However, making and future fish passage facilities should, therefore, the design adequate for testing the factors related pay attention to selecting and/or creating such local to attraction and passing efficiencies could gain this conditions, including optimal habitats before and after necessary knowledge. This would not only be relevant passing. for strengthening migratory fish in the Wadden Sea area, but also in other coastal systems with comparable At the Afsluitdijk, fish can pass to and from Lake man-made barriers world-wide. IJssel via discharge sluices and shiplocks at Den Oever and Kornwerderzand. Based on a very limited small- Fisheries scaled number of tagging experiments and several In the Wadden Sea, Flounder, Twaite Shad, River assumptions, the passing efficiency at Kornwerderzand Lamprey and Smelt are reported as by-catch from under present regular discharge conditions (REG) shrimp fisheries. In Lake IJssel, both European appears to be around 15% for River Lamprey and Sea Eel and Smelt are fished (but fishing on Smelt is Lamprey, 50% for Sea Trout and Atlantic Salmon, and temporally banned). Such fishing activities appear 60% for Houting. Fish migration has been temporally to be an important source of mortality, both in the stimulated by changing the discharge regime and by Wadden Sea (Flounder, River Lamprey, Smelt, Twaite enhancing the migration of fish using the ship locks at Shad) and in Lake IJssel (European Eel, Smelt). In Den Oever and at Kornwerderzand, the so-called fish- the Marsdiep tidal basin, the by-catch from shrimp friendly management (FFM). Compared to regular fishing on diadromous fish is estimated to be almost
6 2.5 million fish per year, comprising of Twaite Shad Lamprey show that both safeguarding of the habitat (213 000 fish per year), Smelt (1 400 000 fish per year), quality and improving the connectivity between Flounder (840 000 fish per year) and River Lamprey various tributaries and the sea is required. (4500 per year). In Lake IJssel, catches were estimated to comprise approximately 6 million Eels and 200 Within the Wadden Sea region, most estuarine million Smelt per year between 2000 and 2014. For gradients are presently characterised by an abrupt these species, this fishing mortality appears to be in the salinity gradient. In general, strongly developed same order of magnitude as the projected migration salinity gradients appear to favour the estuarine via the Fish Migration River. recruitment of marine fish species. In former days, for example, Anchovy, Herring, Smelt and Flounder Reduction of fishing efforts in Lake IJssel and the used the large open brackish water bodies with tidal Wadden Sea might, therefore, be an alternative or influence in this area to spawn, to grow and to mature. additional measure to strengthen local fish stocks In addition to protection and improvement of within the study area as a whole. Reduction of by- remaining estuarine gradients (e.g. small creeks and catch by shrimp fishing in the Wadden Sea as a whole the Ems estuary), restoration of former gradients would result in a decrease of fishing mortality of may aid in strengthening fish stocks in the Wadden approximately 4 million fish per year, and beneficial Sea area. On the mainland bordering the Dutch for their predators (birds, seals). Reduction of the Wadden Sea, several large freshwater bodies exist that shrimp fishing efforts might increase additional natural potentially could be turned into brackish ecosystems, values of the Wadden Sea (e.g. benthic organisms i.e. Lauwersmeer (Marnewaard), Amstelmeer and biogenic structures such as mussel beds), whilst (including the Balgzandkanaal) and the northern part the yield might remain stable. Reduction of fishing of Lake IJssel. Alternatively, the southern part of the activities in Lake IJssel is not only expected to favour Marsdiep tidal basin could be made more permanently local fish stocks, but also as an important source of fish under the influence of freshwater discharges. (in particular Smelt) for local stocks in the Wadden Sea, strengthening fish stocks in the study area as a Construction of brackish zones within freshwater whole. systems should be preceded by feasibility studies, however, including an assessment of the behaviour Habitats of salt water, and possible consequences for other use Once fish has passed from the sea to the freshwater of the water (e.g. agricultural, drinking). To come system, it should be able to reach its final destination up with an optimal design, additional measurements and to close its life cycle. In general, this implies that on flows, tides, salinity gradients and fish passage connectivity between all tributaries, polder ditches and rates under present conditions might be required. If brooks are important to population sizes of migratory a construction of a more permanently brackish zone fish. In addition, fish might require particular at the northern side of the Afsluitdijk is considered, conditions in freshwater habitats, e.g. to spawn. potential impacts of pumps on migrating fish from For Twaite Shad, for example, suited conditions for freshwater to the sea should be kept to a minimum spawning can probably be found in (side channels of) (e.g. by means of using fish-friendly pumps) and an the upstream Ems, a river presently characterized by inventory of possible effects of additional freshwater high fluid mud concentrations and low oxygen levels discharges on local species and habitats in this zone during summer. Water and habitat quality of the river should be made. Ems needs to be improved drastically before becoming an important spawning ground. For Sea Lamprey, local habitat improvement of potential spawning habitats in small streams in (side- channels of) larger rivers such as the Ems and IJssel can be achieved by re-meandering. Studies into River
7 Conclusions on measures and optimal Benefits and costs locations Most promising potential measures to strengthen local With regard to benefits and costs in relation to fish stocks and other natural values of the Wadden strengthen local fish stocks only indications can be Sea region include the reduction of fishing efforts, the given. Mainly because the effectivity of the proposed provisioning of suitable habitats (such as brackish zones measures like the reduction of fisheries, restoration and freshwater spawning grounds) and the facilitation of natural dynamics in habitats and improving of fish migration. connectivity between marine and freshwater areas for migratory fish are only know within large margins of uncertainty and the costs of the various measures also Reduction of fishing efforts in the Wadden Sea and in show a great variety. Subsequently, it is very difficult Lake IJssel is expected to be beneficiary for migratory to estimate the cost effectiveness of the reduction of fish stocks, their predators and bottom life in the fisheries, restoring habitats and for different options for whole Wadden Sea region. Present natural estuarine fish passing the Afsluitdijk (see “Measures and optimal gradients should be safeguarded and, if necessary (e.g. locations”) and the same holds for monetary valuation Ems), be improved for provisioning suitable habitats of the improvement in fish stock. for migratory fish. The Lauwersmeer (Marnewaard), However, several types of costs and benefits can be Amstelmeer (including the Balgzandkanaal), the distinguished and for some a monetary indication can northern part of Lake IJssel and the southern part be given. First of all, an increase in fish stocks has an of the Marsdiep tidal basin are potentially suited intrinsic value in terms of a more natural ecosystem. for turning into large brackish habitats, but actual In addition to that, this may in the medium and long suitability still needs to be checked by means of run also have positive effects for the potential stocks of feasibility studies. fish available for fishing but maybe at a smaller scale. The reduction of fisheries would deprive fishing and Fish migration could be further facilitated by means processing companies of a source of income, with of improving the connectivity with freshwater annual foregone earning being estimated to be around systems, and between freshwater and the sea. Potential 25 million Euro per year for shrimps in the Wadden measures include fish-friendly discharge management Sea and approximately 2 million Euro per year for Eel and fish passages, ranging from relatively simple (e.g. and Smelt in Lake IJssel. fish ladder) to very complex (e.g. Fish Migration River) solutions. At present, the efficiency of such fish As far as could be derived from present information, passages cannot be quantified due to a lack of data. the initial one-time investment costs of the technical The set-up of a Migratory Fish Testing Facility, e.g. solutions to create large brackish zones would be within the area as presently reserved for the FMR, approximately 3 million Euro for Marnewaard would enable exploring the factors related to attraction (Lauwersmeer) and 40 million Euro for the northern and passing efficiencies of various types of technical part of Lake IJssel. One time investment costs solutions in the Wadden Sea region as a whole. (excluding maintenance costs) of technical solutions for fish passages range between approximately 20 thousand Euro for simple technical solutions (e.g. flaps and gates in sluice doors), 50 thousand Euro for more complicated facilities (e.g. fish ladders and spill ways), 500 thousand Euro for fish-friendly by-pass pumps, 4 million Euro for fish passages with salt water return flow systems, and 52 million Euro for the large and complex Fish Migration River. As already indicated, however, the efficiency of the existing fish passages and subsequently the identification of best practices (“value for money”) could not be quantified due to lack of information for the Wadden Sea region.
8 Migratory Fish Testing Facility, under various environmental conditions such as the Monitoring & Modelling creation of large brackish zones, climate change and sea level rise. Given the great interest and considerable Due to the lack of data, at present a quantitative investments in fish passages at a global scale, the assessment of the effectivity of existing fish passages insights derived from the Fish Migration Testing in the Wadden Sea was not possible. With regard to Facilities will lead to more efficient and effective the investment costs of building these structures, it is investments in fish passages in the Wadden Sea and the therefore strongly recommended to not only invest in rest of the world. the strengthening of fish migration by construction of fish passing facilities, but also to invest in appropriate testing and monitoring over a longer time period. Testing and monitoring is required for identification of best practices by means of quantitative evaluation of functioning of present facilities, for potentially allowing modifications in original designs to optimize the investments made, and for more adequate future investments.
Testing and monitoring programs should include measurements of (1) the numbers and characteristics (e.g., species, life phases, condition of fish passing), (2) the efficiency of the attraction flow, (3) the efficiency of the fish passage and, for larger fish passages, (4) the habitat use of the local area. Proper testing of the effectivity of different technical solutions can be done by means of experiments under set environmental conditions (e.g., indoor climate- controlled basins), or by experiments under natural conditions. The latter approach would require more replicates (i.e. the number of distinct and in principle similar experimental units) than the first approach to be able to extract the signal of treatment from the environmental noise.
To estimate the added value of a new fish passage, monitoring should be performed at all other surrounding fish passages before and after a new passage is constructed. This would require an integrated monitoring scheme for migratory fish in the Wadden Sea. With standard techniques, equipment and protocols, and addressing the preferred pathways of migratory fish species at various spatial (e.g., freshwater discharge points, tidal basins) and temporal (tide, season, year-to-year) scales. Incorporating the migratory behavior of fish into hydrodynamic models could produce biophysical models of fish movements through the Wadden Sea. Such models would allow for building scenarios for optimizing investments to strengthen (migratory) fish stocks in the Wadden Sea
9 1. INTRODUCTION
1.1 Recent changes in Wadden Structural monitoring of the fish fauna in the Dutch Sea fish Wadden Sea takes place since 1960-1970 by two major monitoring programs: a fyke program in the western Many fish species rely on shallow coastal habitats for Wadden Sea and an annual beam trawl survey (DFS) at least one of their life stages. A suite of marine fish covering the entire Dutch Wadden Sea. Results from species reaches these areas as postlarvae and spend these surveys show that total fish biomass increased their juvenile phase here (Zijlstra 1972, 1978; Elliott from 1970 to 1980, had a peak in the mid-1980s that et al. 2007; Van der Veer et al. 2000, 2015). Other was followed by a strong decline especially from 1980- species temporarily inhabit the area or pass it on 2000, with a stable trend since then in all tidal basins route to either marine or freshwater spawning sites (Bolle et al. 2009; Jager et al. 2009; Tulp et al. 2015). (diadromous species), during certain times of the year The trends in the Dutch coastal zone differ from those or occasionally (Elliott et al. 2007). In addition to such in the Wadden Sea, especially in the past 10 years, temporary visitors, some species spend (almost) their with an ongoing decline in the Dutch Wadden coast entire life in the shallow waters (Elliott & Hemingway and an increase along the mainland North Sea coast 2002). Naturally such coastal areas support large (Van der Veer et al. 2015; Tulp et al. 2015). The strong numbers of fish that make use of the suitable habitats decline in Wadden Sea fish since the 1980s has called characterised by a high food availability and shelter for action to strengthen local fish stocks. Proposed from predators (Gibson 1994; Elliott & Hemingway measures include the reduction of fisheries, wise sand 2002, Tulp et al. 2015). nourishments, restoration of natural dynamics in habitats and improving connectivity between marine and freshwater areas for migratory fish (Van der Veer & Tulp, 2015).
Legenda
Ondergrens transitie duurzame energiehuishouding NH Waddengebied Provinciegrenzen (2010) Gemeentegrenzen (2006)
g Gemeentegrenzen (2010) nikoo mon Schier Ameland
Eemsmond Delfzijl Terschelling De Marne Dongeradeel
Winsum Ferweradiel
het Bildt Groningen d an iel Vl Franekeradeel Oldambt
en
g
n i l r
a
H
Texel Friesland
Sudwest Fryslân
Den gen Helder in ier Anna W Paulowna Wieringermeer
Hollands Zijpe Kroon
Schagen
Harenkarspel Andijk Noord-Holland Wervershoof Langedijk Wognum Obdam
Hoorn Heerhugowaard Koggenland
Artikel 2 Het oorspronkelijke werkingsgebied zoals gedefinieerd in de Wet op het Waddenfonds blijft na decentralisatie van de middelen uit het Waddenfonds gehandhaafd, te weten: de Waddenzee, de Waddeneilanden, de zeegaten tussen de eilanden, de Noordzeekustzone tot 3 zeemijl uit de kust, alsmede het grondgebied van de 25.000 aan de Waddenzee grenzende vastelandsgemeenten. Voor de transitie naar een duurzame energiehuishouding omvat het werkingsgebied tevens de provincies Fryslân, Groningen en de kop van Noord-Holland voor zover het betreft de gemeenten Hoorn, Wognum, Opmeer, Opdam, Heerhugowaard, Langedijk, Harenkarspel, Zijpe en m het gebied ten noorden daarvan. In geval van gemeentelijke herindeling geldt voor de toepassing van deze regeling het grensbeloop zoals dat ten tijde van het inwerkingtreden Datum: 6 september 2012 van de Wet op het Waddenfonds van kracht was. Papier: A3 Bestuursakkoord Decentralisatie Waddenfonds, Harlingen, 14 september 2011, pagina 6. Bron: provincie- en gemeentegrenzen, Topografische Dienst Nederland BEL/ K & B: C_201111_2238
Fig. 1.1 Boundaries of the area covered by the Waddenfonds (red), i.e. the Wadden Sea, the islands, the tidal inlets, the North Sea coastal zone (up to 3 nautical miles from the coastline) and the municipalities at the mainland bordering the Wadden Sea (www.waddenfonds.nl).
10 1.2 Audit Measures in consideration are restricted to the area covered by the Waddenfonds, being the Wadden Sea, The Wadden Academy was invited by the the islands, the tidal inlets, the North Sea coastal zone Waddenfonds to make an inventory of the most (up to 3 nm from the coastline) and the municipalities effective measures to strengthen (migratory) fish at the mainland bordering the Wadden Sea (www. stocks in the Wadden Sea. Many parties are coming waddenfonds.nl; Fig. 1.1). This implies that potential forward to request funding for building fish passages, measures in other areas (e.g. the North Sea and main including a large fish passage in the Afsluitdijk (52 rivers such as the Rhine and Meuse) will not be MEuro in total, of which a part requested from the addressed in this report. Measures at the boundaries of Waddenfonds). With already several projects on fish the Waddenfonds area (e.g., Lake IJssel) will only be migration funded, the Waddenfonds requested advice discussed if they might have a direct influence on the on the added value of additional investments in fish fish stocks within the study area. passages compared to other measures that could be taken to strengthen migratory fish populations to Local experts and scientists from ICES (International improve the ecological quality of the Wadden Sea. Council for the Exploration of the Sea) have reviewed More particularly, the following questions were the draft report in November 2015. The first results defined: were presented in December 2015 to the Executive Board of Waddenfonds. The final report was presented 1. To what extent were local stocks of migratory fish to the EB of the Waddenfonds in February 2016. in the Wadden Sea influenced by the construction of local barriers and fish passages? 2. Which local measures would be most promising to strengthen the stocks of migratory fish in the Wadden Sea, and what would be the optimal locations for these measures? 3. What would be the costs and benefits of different measures to strengthen the stocks of migratory fish in the Wadden Sea?
COMMON NAME SCIENTIFIC NAME FAMILY GUILD MONTH M. SOURCE River Lamprey Lampetra fluviatilis Petromyzontidae Diadromous 10 – (12) – 3 0.22 Winter et al. 2014 Sea Lamprey Petromyzon marinus Petromyzontidae Diadromous 3 – (4 – 5) 0.14 Winter et al. 2014 European Sturgeon Acipenser sturio Acipenseridae Diadromous 4 – (6 – 7) – 9 0.05 Winter et al. 2014 European Eel Anguilla anguilla Anguillidae Diadromous 2 – (3 – 5) – 6 0.23 Winter et al. 2014 European Anchovy Engraulis encrasicolus Engraulidae Marine seasonal 4 – (5) – 6 1.00 Redeke (1939) Atlantic Herring Clupea harengus Clupeidae Marine juvenile 3 – (4 – 5) – 7 0.60 Redeke (1939) Alis shad Alosa alosa Clupeidae Diadromous 4 – (5) – 6 0.35 Winter et al. 2014 Twalte shad Alosa fallax Clupeidae Diadromous 3 – (4 – 5) – 6 0.31 Winter et al. 2014 European Smelt Osmerus eperlanus Osmeridae Diadromous 2 – (3 – 4) – 5 0.22 Winter et al. 2014 Atlantic Salmon Salmo salar Salmonidae Diadromous 5 – (6 – 11) – 12 0.44 Winter et al. 2014 Sea Trout Salmo trutta Salmonidae Diadromous 5 – (6 – 11) – 12 0.28 Winter et al. 2014 Houting Coregonus oxyrinchus Salmonidae Diadromous 10 – (11) – 12 0.57 Winter et al. 2014 Three-spined Gasterosteus aculeatus Gasterosteidae Diadromous 2 – (3 – 4) – 5 2.96 Winter et al. 2014 Stickleback European Flounder Platichthys flesus Pleuronectidae Diadromous 3 – (4 – 6) – 7 0.41 Winter et al. 2014
Table 1.1 Overview of fish species targeted in this report, their main migration window (see last column for references) and mortality rate (www.fishbase.se).
11 1.3 Target fish species The former Zuiderzee was a major spawning area for Anchovy (Petitgas et al. 2012), and the Anchovy Based on the function of the Wadden Sea for fish larvae stayed in the area until October after hatching migration between marine and freshwater systems, (Redeke 1939). In 1994, eggs of this fish species were the audit is focussing on 12 diadromous fish species found north of the Afsluitdijk (Boddeke & Vingerhoed supplemented with 2 species that had large stock sizes 1996). In May 2010 and May 2011, schools of Anchovy in Lake IJssel, i.e. European Anchovy and Atlantic were found in a pelagic survey in the Marsdiep area. Herring (Table 1.1). For all of these species, the Their mean length (14.5 cm) corresponded to age-1 Wadden Sea is (or was) an essential area to fulfil a fish that are likely to spawn. Hence, for the Wadden particular phase in their life cycle, often at a particular Sea area is important both as nursery as well as for time of the year. For the diadromous fish species, spawning (Couperus et al 2016). Before 1932, the this shallow coastal sea functions as a corridor Zuiderzee might have harboured a subpopulation between freshwater systems and the open North Sea of Zuiderzee Herring, which entered the Zuiderzee when adults and juveniles migrate to and from their from March until July for spawning (Redeke 1939). spawning areas (Winter et al. 2014; Heessen et al. In 2006, Dutch fishermen landed 800 kg of Herring 2015). of which the morphology resembled that of Zuiderzee Herring (Witte, NIOZ), implying that a remnant stock might still be present.
NEW AREA YEAR CHANGE DIKE (KM) NAME HABITAT (KM2)
Zuiderzee (4953 km2 before 1924) 1924 Dike built 3 Lake Amstelmeer 7 1927 Land reclaimed 2 Polder Andijk < 1 1930 Land reclaimed 18 Polder Wieringermeer 200 1932 Dike built 32 Lake IJsselmeer 3440 Marsdiep, Eijerlandse Gat & 1513 Tidal basins Vlie 1942 Land reclaimed 55 Polder Noordoostpolder 480 1957 Land reclaimed 90 Polder Eastern Flevoland 540 1967 Land reclaimed 70 Polder Southern Flevoland 430 1975 Dike built 28 Lake Markermeer 700 Lake IJsselmeer 1100 Lauwerszee (274 km2 before 1969) 1969 Dike built 13 Lake Lauwersmeer 24 Polder Lauwersmeer 67 183 Tidal basins Lauwers & Eilander Balg Ems estuary (460 km2 at present) 1924 Land reclaimed 15 Polder Carel Coenraad 15 1979 Land reclaimed 3 Polder Breebaart < 1
Table 1.2 Overview of major fixed changes in former estuaries Zuiderzee and Lauwerszee and the present Ems estuary during the past 100 years (1916-2015). Sources: Herman et al. (2009), www.britannica.com, https://en.wikipedia.org/wiki/Zuiderzee_Works, https://en.wikipedia. org/wiki/Lauwersmeer, Baptist & Philippart (2015).
12 Fig. 1.2 Maps of the western part of the study area. Left: onwards, with acceleration during the last 150 to 300 Navigational map of the Zuiderzee in 1921, as made years (Wolff 2000; Jackson et al. 2001; Lotze et al. by M.H. Blokpoel of the Royal Dutch Meteorological 2005, Jager et al. 2009). Institute (originally printed by Roeloffzen-Hübner en Van Santen, Amsterdam; sold at the J.L. Willem Seyffardt During the past 100 years, for example, the Dutch bookshop, Amsterdam). Right: Map of the Wadden Sea, part of the Wadden Sea was irreversibly impacted Lake IJssel and Markermeer in 2015 as derived from by damming and land reclamation (Table 1.2). Open Street Map, layout Humanitarian Data Model, Parts of former estuaries, such as the Zuiderzee with present-day coastline, tidal basins, dikes and polders and Lauwerszee, were closed off from the sea by (see Table 1.3 for details). means of dams and turned into freshwater lakes (i.e., Amstelmeer, IJsselmeer, Lauwersmeer). After the closure in 1932, large parts of Lake IJssel (IJsselmeer 1.4 Long-term changes in the in Dutch) were reclaimed, whilst the lake itself was environment further subdivided into two freshwater basins in 1975 (Fig. 1.2; Table 1.2). This reduction and changes in As the result of anthropogenic pressures such as habitat has most probably affected diadromous fish fisheries, the construction of physical barriers between stocks. If considered to be irreversible (e.g. as the the marine and fresh waters, river channelization, result of their role in safety against flooding or the deterioration of water quality and habitat destruction, high economic values of agriculture and freshwater fish stocks in coastal ecosystems including the Wadden supply), this limits the potential for strengthening local Sea have been degrading from the medieval time diadromous fish stocks.
13 River Lamprey Sea Lamprey 35 300 30 25 200 20 15 100 10 50 5 0 0
1890 1910 1930 1950 1970 1990 2010 Atlantic Sturgeon European Eel 15 15 5000 15000 10 10 10000 3000 5 5 5000 1000 0 0 0 0
1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010 European Anchovy Atlantic Herring 4e+06 1500000 120000 6000 3e+06 1000000 80000 4000 2e+06 500000 40000 2000 1e+06 0 0 0 0e+00 1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010 Allis Shad Twaite Shad 10 250 350 350 8 200 250 250 6 150 4 150 150 100 2 50 50 50 0 0 0 0
1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010 European Smelt Atlantic Salmon 3500 25000 10000 2000 2500 15000 6000 1500 1000 500 5000 2000 500 Fig. 1.3 Annual landings 0 0 0 0
1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010 (tonnes live weight) of 14 Sea Trout Houting fish species as caught in the 20 350 2000 Northeast Atlantic (FAO
15 fishing area 27; black dots 1500 250 and left y-axis) and the 10 1000
150 North Sea (FAO area 27
5 subarea IV; white dots and 500
50 right y-axis) from 1903 to 0 0 0 2014. The dashed vertical 1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010 3−Spined Stickleback European Flounder lines indicates the year 1932 150
20000 when the Afsluitdijk was 30000 5000 closed, the dotted lines the 15000 100 closure of the Amstelmeer 20000 3000
10000 and Lauwersmeer in 1924
50 and 1969, respectively. 10000 5000
1000 Source: ICES database on
0 0 0 0 www.ices.dk. 1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010
14 1.5 Long-term changes in fish NE Atlantic and North Sea (1903-2013) landings As can be derived from catch statistics as supplied by International Council for the Exploration of the Sea Comparison of fish stock developments within the (ICES; www.ices.dk), all of the 14 species targeted in study area with large-scale developments (e.g. in the this study are or were landed (as catch or by-catch) NE Atlantic) might help to unravel large-scale and by commercial fishing activities in the North-East local impacts on fish stocks, such as the effect of the Atlantic (Fig. 1.3). Within the North Sea, however, closure of former estuaries in the Wadden Sea. Fish some species were not caught (River Lamprey, Sea surveys in the Wadden Sea did not start before the Lamprey, Sturgeon, Allis Shad and Houting) or hardly 1960s (see paragraph 1.1), however, which is long after caught (Twaite Shad, Three-spined Stickleback) major changes in fish stocks and habitats took place. (Fig. 1.3). Another source of information may be found in fish landings, which are registered since the beginning of On both sides of the Atlantic Ocean, diadromous the previous century. These data sets provide a long- fishes have strongly declined, often to historic lows term (> 100 years) view on the (changes in) intensity (Limburg & Waldman 2009). Exploitation is a major of fishery-induced mortality on fish stocks. Long-term factor (on top of hydrographic fluctuations influencing changes in landings may also potentially reflect long- larval transport, pollution, blocked migration, diseases) term changes in fish stocks or changes in the fishery in European eel decline (Dekker 2004a) and the nearly management. Commercial catch data are usually complete demise of the once-widespread European collected systematically and consistently over long Sturgeon (Williot et al. 2002). In addition, habitat periods, and the data tend to be less noisy than survey loss, pollution, climate change, and replacement with data because of the much greater sampling effort. non-native species (hybridization, escapees from However, proper acknowledgement of potential biases aquaculture) contributed to declines in this group (e.g. misreporting, interaction between fishing fleets, (Limburg & Waldman 2009). technological improvements, commercial value of the fish species etc.) is a prerequisite when performing such interpretations (Engelhard et al. 2011).
15 Zuiderzee (1892-2013) some were more extreme such as landings of Smelt For landings of fish caught in the northern and from Lake IJssel in 1968/1969 of 1588/1329 tonnes by southern parts of former Zuiderzee before and after De Boois et al. (2014) and 103/38 tonnes as supplied the closure in 1932 (Fig. 1.4), data sets from different by Jaap Quak, possibly as the result of including by- periods and sources were combined, i.e. southern catch or not. part in 1892-1906 from Redeke (1907), southern part in 1925-1938 from Redeke (1939), southern part in In landings from the western Wadden Sea, a 1966-2012 for Smelt and Flounder from De Boois et temporally increase of Herring and Flounder was al. (2014), southern part in 1907-1924 and in 1939- observed (Fig. 1.4). This might be the result of an 2013 for Eel from De Graaf & Deerenberg (2015), increase in fish abundances, but also of an increase in and northern part and all other years for the southern fishing efforts as fishermen from the former Zuiderzee part from Fish Auction data as supplied by Jaap Quak continued to fish in the Wadden Sea after 1932 (Wolff (Sportvisserij Nederland). 2005). In local landings from Lake IJssel, the closure of the Afsluitdijk in 1932 was followed by more or less It must be noted that where data sets of different abrupt decreases in landings of Anchovy, Flounder, origin overlapped, values on landings were not always Herring and Smelt, and a temporally increase in the similar. Whilst most of these differences were minor, landings of European Eel (Fig. 1.4).
European Eel European Anchovy 250 5000 4000 1000 200 3000 3000 150 600 2000 100 1000 1000 200 0 0 0 50 1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010 Atlantic Herring European Smelt 3500 15000 10000 2500 2500 10000 6000 1500 1500 5000 500 2000 500 0 0 0 0
1890 1910 1930 1950 1970 1990 2010 1890 1910 1930 1950 1970 1990 2010 European Flounder 400 3000
300 Fig. 1.4 Annual landings (tonnes live weight) of five fish 2000 species as caught in the northern part of the former Zuiderzee 200 (western Wadden Sea since 1932; black dots) and southern
1000 part of the former Zuiderzee (Lake IJssel since 1932; white 100 dots). The dashed vertical lines indicates the year 1932 0 when the Afsluitdijk was closed, the dotted lines the closure 1890 1910 1930 1950 1970 1990 2010 of the Amstelmeer and Lauwersmeer in 1924 and 1969, respectively. Sources: Redeke (1907), De Boois et al. (2014), De Graaf & Deerenberg (2015) and Jaap Quak.
16 Anchovy immediately disappeared after the closure (Willem Dekker, pers. comm.). Alternatively, owing (Fig. 1.4). The loss of Herring spawning grounds to the freshening of Lake IJssel, Eel no longer needed in 1932 led to the nearly complete disappearance of to move away from the brackish Zuiderzee into rivers the Herring stock within a few years (Wolff 2005). and inland waters (Redeke 1939). The cause of the Flounder did not fully disappear after the closure subsequent decline of the local stock of European Eel (Fig. 1.4), possibly because of its tolerance to low in Lake IJssel is still unexplained (Dekker 2004b). salinities (Winter 2009). Landings of Smelt reached Worldwide there are multiple causes identified that comparable levels as before the closure (Fig. 1.4). Smelt have adverse effects on eels but the relative importance has developed a land-locked population (Phung et of each cause is not known yet (Kettle et al. 2011). al. 2015). Ongoing fishing pressure on Smelt in Lake IJssel contributed, however, to a depletion of the local Several freshwater fish and fisheries benefited from the stocks (Masterplan Toekomst IJsselmeer 2014). new conditions after the closure. Landings of Pike- perch (Sander lucioperca), for example, rose from 111 The initial increase in Eel catches in Lake IJssel after kg in 1933 to more than 125.000 kg in 1938 (Redeke 1932 might have resulted from an inability for Eel to 1939). take advantage of selective tidal transport for migrating upstream and, therefore, Eel got stuck in Lake IJssel
Atlantic Sturgeon Allis Shad 800 600 100000 400 40000 200 Rhine & Meuse (n/yr) The Netherlands (n/yr) 0 0
1900 1920 1940 1960 1980 2000 2020 1900 1920 1940 1960 1980 2000 2020 Atlantic Salmon Houting 30000 3000 Rhine (kg/yr) 1000 10000 Rhine & Meuse (n/yr) 0 0
1900 1920 1940 1960 1980 2000 2020 1900 1920 1940 1960 1980 2000 2020
Fig. 1.5 Annual landings (number or kg) of four fish species caught in large rivers of The Netherlands. The dashed vertical lines indicates the year 1932 when the Afsluitdijk was closed, the dotted lines the closure of the Amstelmeer and Lauwersmeer in 1924 and 1969, respectively. Source: www.compendiumvoordeleefomgeving.nl.
17 Rivers (1892-1954) 1.5 Outline of the report Data on landings of fish caught in Dutch tributaries such as the rivers Rhine and Meuse (Fig. 1.5) were To quantify impacts of barriers and passages on taken from a Dutch governmental website supplying fish populations in the Wadden Sea, long-term and basic information on the natural environment of The consistent survey data sets of the period during which Netherlands (www.compendiumvoordeleefomgeving. these interventions took place (e.g., the construction nl). Landings of Allis Shad, Atlantic Salmon, Houting of the Afsluitdijk in 1932) are required, at the proper and Sturgeon as caught in Dutch freshwater systems spatial scales and including quantitative information including main rivers strongly declined between the on other factors that may have influenced local fish 1890s and the 1940s (Fig. 1.5). These declines are densities (e.g., fisheries, predation). Since such data attributed to habitat destruction (e.g., as the result do not exist, we have used available data sets and of river engineering, removal of sand and gravel), additional information to at least qualitatively explore deteriorated quality of the water (i.e. pollution) and such impacts. Limits of using these data sets for this overfishing (De Groot 2002). purpose are addressed throughout the report.
In Chapter 2, annual fish surveys at different spatial scales were compared to see if year-to-year densities of fish in the tidal basins of the Wadden Sea were dominated by local variation, possibly including local effects such as barriers and passages, or by underlying variation at larger spatial scales (possibly indicating larger-scale drivers of variation).
In Chapter 3, the abundance of migratory fish species in the Wadden Sea at a tidal basin scale and at local scale (small freshwater sluices and fish passages) was examined to explore the spatial variation in potential for strengthening populations of migratory fish by means of additional fish migration projects.
In Chapter 4, information on local densities, sources of mortality (fisheries, predation by fish, birds and seals) and transport via sluices, ship locks and the proposed Fish Migration River are compared to identify the relative contribution of these factors to changes in fish populations in the Wadden Sea and Lake IJssel.
In Chapter 5, an overview is given of possible measures to strengthen populations of migratory fish in the Wadden Sea (o.a. by reducing present threats), including an indication of costs and possible effectiveness.
18 2. POSSIBLE DRIVERS OF LOCAL FISH DENSITIES
2.1 Introduction locally regulated populations can be synchronized by large-scale environmental shocks (Moran 1953). Long-term changes in fish stocks are influenced by Synchronous dynamics on smaller spatial scales can a suite of drivers, including fisheries pressure and result from spatial trends in regional variables. For environmental factors such as sea surface temperatures, instance, regional trends in food availability, predation and interactions between these drivers (Pauly et al. and coastal fisheries could lead to distinct regional 2002). Correlations between simultaneously measured synchronization patterns. Synchronous dynamics time series can help to provide insight into possible among nearby stocks can also result from distance- driver-response relationships. If the data were perfect limited dispersal of fish between stocks (Bjørnstad et (noise-free, bias-free, infinitely long), we should be al. 1999; Ranta et al. 2006). able to detect, at least in some cases, which of the coupled systems is the driver (or at which scale the Analyses of synchrony in time series at different spatial driver operates) and which is the response (Fig. 2.1). scales might help to unravel the nature of drivers, and eventually the possibilities and limitations for local Synchronization in population sizes that takes place management of local fish stocks, e.g. by means of over large spatial scales may point to common facilitating fish migrations. large-scale drivers such as climatic conditions. Even
Fig. 2.1 Possible spatial relationships between drivers and synchrony in time series.
19 SOURCE IMARES (I Tulp) NIOZ (H Veer) vd IMARES (I Tulp) IMARES (I Tulp) IMARES (I Tulp) IMARES (I Tulp) IMARES (I Tulp) IMARES (I Tulp) IMARES (I Tulp) / ZiltWater (Z Jager)RWS / ZiltWater (Z Jager)RWS / ZiltWater (Z Jager)RWS IMARES C060/13 C164a/14 IMARES C164a/14 IMARES IMARES C003/15 & C006/04 IMARES C003/15 & C006/04 IMARES C003/15 & C006/04 IMARES C003/15 & C006/04 IMARES C003/15 & C006/04 IMARES C003/15 & C006/04 IMARES (H Heessen) LIFE PHASE Juv. + AdultJuv. + AdultJuv. Juv. Juv. Juv. Juv. Juv. Juv. Juv. + AdultJuv. + AdultJuv. + AdultJuv. + AdultJuv. + AdultJuv. + AdultJuv. Glass eel Glass eel Glass eel Glass eel Glass eel Glass eel Juv. + AdultJuv. MONTHS 9 – 10 3-6 + 9-10 9 – 10 9 – 10 9 – 10 9 – 10 9 – 10 9 – 10 9 – 10 5 + 9 5 + 9 5 + 9 9 – 10 – 10 12 – 10 12 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 4 – 5 8 – 9 -1 -1 -1 ) ) ) 2 2 2 -1 -1 -1 -1 -1 -1 (80m (80m (80m -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 UNIT N net N net N net N net N net N net Index N d N ha N ha N ha N ha N ha N ha N ha N h N h N h N ha N d N d N ha lift net lift net lift net lift net lift net lift net 2 2 2 2 2 2 GEAR Bottom trawls Bottom Kom-fyke trawl Beam trawl Beam trawl Beam trawl Beam trawl Beam trawl Beam trawl Beam Stow net Stow net Stow net trawls Bottom Eel fyke Eel fyke 1 m 1 m 1 m 1 m 1 m 1 m Beam trawls Beam YEARS 1977-2013 1960-2014 1970-2014 1970-2014 1970-2014 1970-2014 1970-2014 1970-2014 1970-2014 2007-2014 2006-2014 2006-2014 1966-2012 1994-2013 1994-2013 1938-2014 1995-2014 1992-2014 1979-2014 1991-2014 1996-2014 1970-2014 AREA North Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Lake IJssel Lake IJssel Lake IJssel Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea Wadden Sea North Sea SURVEY NSER NIOZ Fyke DFS DFS DFS DFS DFS DFS DFS WFD WFD WFD WFD WFD WFD Glass Glass eel Glass eel Glass eel Glass eel Glass eel DFS NAME North Sea Ecoregion Marsdiep tidal inlet Marsdiep Eijerlandse Gat Vlie Borndiep Zoutkamperlaag Lauwers/Schild estuaryEms Spijk Oterdum Terborg Lake IJssel trawl Lake IJssel 01 Lake IJssel 02 Den Oever 1 Den Oever 2 Harlingen Lauwersmeer Termunterzijl StatenzijlNieuwe Dutch coastal zone CODE NS_TER WS_FMD WS_T610 WS_T612 WS_T616 WS_T617 WS_T618 WS_T619 WS_T620 WS_SSP WS_SOT WS_STB LIJ_TLIJ LIJ_F01 LIJ_F02 WS_LD1 WS_LD2 WS_LHA WS_LLA WS_LTZ WS_LNZ NS_T404
Table. 2.1 Overview of surveys and their characteristics as used in this chapter.
20 2.2 Time series on fish abundance and surveys with a fyke, lift-nets and stow-nets) and Lake IJssel (i.e. bottom-trawls and fykes) (Table 2.1, Time series used for the synchronization analyses are Figure 2.2). Note that these surveys differ not only in based on surveys only; landings are not considered sampling area (North Sea, Wadden Sea, Lake IJssel) here. It is assumed that survey data are consistently and sampling gear (bottom-trawls, fykes, lift-nets, sampled in time and space and are not as biased by stow-nets), but also in sampling period. This implies changes in effort and gear as data on landings often that, although targeting similar species, time series on are (Heessen et al. 2015). The time series comprise densities are not fully comparable due to differences surveys of the North Sea (i.e. bottom-trawl surveys in catchability (e.g. low catches of pelagic fish when of the North Sea ecoregion and of the coastal zone using bottom-trawls) and in life-phases (e.g. surveys of north of the islands), the Wadden Sea (i.e. bottom- 1x1m lift nets select for small species and juveniles of trawl surveys of tidal basins including the Ems estuary, larger species).
Fig. 2.2 Locations of the fish surveys. Blue circle: Marsdiep Fyke, Pink circles: Glass Eel survey, Orange circles: Ems estuary stow net survey, Green circles: Lake IJssel fyke survey, Red lines: locations of trawls in Lake IJssel trawl survey, Black lines: locations of trawls in Demersal Fish Survey (the direction of the line does not indicate trawl direction), Brown: Boundaries of Demersal Fish Survey (DFS) areas and codes. North Sea Ecoregion surveys are not shown on this map.
21 North Sea Ecoregion surveys (NS_TER) mesh size (20 mm) results in selection of the smaller The survey index for the North Sea Ecoregion is fish species and younger year classes of (particularly based upon a combination of data from a variety of demersal) fish and other epibenthos. Furthermore, national and international surveys in the North Sea, the timing of occurrence in the Wadden Sea needs to which use otter trawls and beam trawls (Heessen et coincide with the survey period (September-October) al. 2015). All data were scrutinized (e.g. for reporting (Tulp et al. 2015). of less common species, misidentifications), corrected (e.g. for differences in coding and fish lengths), and Kom-fyke surveys Marsdiep (WS_FMD) standardized (Heessen et al. 2015). Since 1960, a passive kom-fyke trap has been operating at the entrance of the Marsdiep basin in the western It should be noted that the sampling was targeted Dutch Wadden Sea (Van der Veer et al. 1992). Fishing at commercial species (such as Herring, Cod and normally started in March - April and lasted until Plaice) and not geared to get a representative index for October. In winter the trap was removed because Lampreys, Salmon and Trout (Henk Heessen, of possible damage by ice scouring and from 1971 IMARES, pers. comm.). This might imply that trends onwards no fishing took place during part of the for the latter group of fish species contain relatively summer (July-August) because of fouling and potential low signal to noise ratio. clogging of the net by macroalgae and jellyfish. All catches were sorted out immediately and identified Beam-trawl surveys North Sea & Wadden to species level (Van der Veer et al. 2015). Fixed Sea (NS_T404, WS_T612, WS_T616, WS_T617, gears require mobility (active movement or passive WS_T618, WS_T619 & WS_T620) transport) of the organism. The location of the fyke The Demersal Fish Survey (DFS) is part of an near a tidal inlet implies that in spring catches will international coastal and inshore survey, which contain fish that were migrating from the North Sea started in 1970, is performed in autumn (Sep-Oct) into the Wadden Sea and in autumn fish on their way and aiming at monitoring young plaice and sole in to migrate to the North Sea including the locally their nursery grounds in the North Sea coastal zone, produced young-of-the-year (Fonds 1983). Wadden Sea and South Western Delta. The Dutch survey areas included in this analysis comprise the Stow-net surveys Ems estuary (WS_SSP, WS_ coastal zone of the North Sea north of the Wadden Sea SOT & WS_STB) islands (DFS404), the Wadden Sea (from west to east: Within the Ems estuary, WFD (Water Framework DFS610, DFS612, DFS616-619), and the Ems-Dollard Directive) fish monitoring is carried out at three estuary (DFS620) (Figure 2.2). stations, i.e. Spijk (coded as WS_SSP in this report, see Table 2.1), Oterdum (WS_SOT) and Terborg Within the Wadden Sea and Ems estuary, fishing with (WS_STB), in the Ems at river 70.5, 52.5 and 24.5 a 3-m beam trawl was restricted to the tidal channels km, and thus relates to the polyhaline, mesohaline and and gullies deeper than 2 m because of the draught oligohaline zone, respectively (BioConsult 2009; Jager of the research vessel, whereas the North Sea coastal et al. 2011). The fishing gear is a stow net with a width zone was fished with a 6-m beam trawl. The mean of 10 m and a variable vertical opening of max. 10 m. abundance per area was calculated for all subareas in For fishing a commercial fishing vessel is chartered in the period 1970-2013 weighed by surface area for spring (May) and autumn (September). The vessel puts five depth strata (intervals of 5 m) within the subareas out its anchor and deploys the nets; the tidal current (Tulp et al. 2015). flows through the net during 3.0 to 4.5 hours. One catch per station was carried out over most of the ebb As result of the sampling gear, the DFS results are tide phase and one catch over most of the flood tide considered to reflect the densities of demersal fish, phase. All fish catches were standardized to annual such as Flounder, best and less those of the other target averages of numbers per hour per 80 m2 surface of the species (Ingrid Tulp, IMARES, pers. comm.). The net opening per station (Jager et al. 2011). combination of low fishing speed (2-3 knots) and fine
22 Lift-net surveys freshwater sluices (WS_LD1, the open waters of Lake IJssel. In 2002, the sampling WS_LD2, WS_LHA, WS_LLA & WS_LTZ) frequency was reduced from three times a year (May, Glass Eel is caught per lift net haul at the freshwater Aug, and Oct-Nov) to once a year (Oct-Nov). For sluices in Den Oever (coded as WS_LD1 in this long-term trends, data are taken for the Oct-Nov report, see Table 2.1) in the period April-May, using period only (Van der Sluis et al. 2014). a 1 m2 lift net with mesh size 1x1 mm, annually since For most fish species, with exception of European Eel, 1938 (Dekker 2002). Monitoring is carried out daily Smelt and Three-spined Stickleback, these gears are in 5 hauls every 2 hours between 22:00-5:00 h. selective for juveniles (Van der Sluis et al. 2014). In addition, recruitment of Dutch waters is monitored at twelve sites along the Dutch coast. Five of these Fyke surveys Lake IJssel (LIJ_F01 & LIJ_F02) are located in the Wadden Sea, i.e. near Den Oever This fishery survey was executed from 1994 to 2013 (WS_LD2), Harlingen (WS_LHA), Lauwersoog in order to sample freshwater fish abundance in large (WS_LLA), Termunterzijl (WS_LTZ) and Nieuwe water bodies of The Netherlands (Van der Sluis et Statenzijl (WS_LNZ). At these stations, glass eel was al. 2014). During this survey, professional fishermen caught by 1 m2 lift nets in 2 hauls between 18:00 under scientific guidance register by-catch of fisheries. and 8:00 h in the period April-May with a weekly The catch effort (number of nets per day and the frequency (De Graaf & Deerenberg 2015). Sampling fishing duration per day) was registered since 1994 efforts were more diverse than the long-term survey at (Van der Sluis et al. 2014). Since 2014 a different Den Oever 1 (WS_LD1), with shorter time series and survey set-up has been followed and these surveys have missing years (De Graaf & Deerenberg 2015). been abandoned.
Bottom-trawl surveys Lake IJssel (LIJ_TLIJ) Fish were caught using Eel fyke nets with a stretched The longest time-series in Lake IJssel stems from the mesh size of 18-20 mm. This survey started with open-water monitoring program with active fishing 33 locations in lakes and rivers but the number of from a vessel. From the start of the survey in 1966 up locations declined. Lake IJssel had two locations, one to 2012, sampling was performed by means of a large at the Afsluitdijk near Kornwerderzand (IJsselmeer01, (7.4 m wide, 26.9 m long, 1 m high) bottom trawl net in this report coded as LIJ_F01, see Table 2.1) and one (in Dutch: “kuil”), which was replaced by a smaller at the Houtribdijk (IJsselmeer02, LIJ_F02), although beam trawl (4 m wide, 19.95 m long, 1 m high) in in fact at this location two nearby fykes were applied 2013 (Van der Sluis et al. 2014). Catches of the new (labelled IJm02a and IJm02b in Figure 2.2). Eel gear were corrected for differences with the old gear fishing, and thus registration of catches, is allowed in catchability for biomass of Smelt and numbers of from December to October (Van der Sluis et al. 2014). Flounder (Van Overzee et al., 2013). Since 1989, sampling is consistently performed at 29 locations in
23 2.3 Statistical analysis survey data In addition, the results for each fish species are presented within schematized maps of the study area Synchronization is defined as the correlation between (Fig. 2.3). Within the maps, the connectivity between the time series of normalized densities of fish species sampling areas is depicted by lines with rounded for the different surveys. For each pair of time endpoints. The colour of these lines is an indication of series, synchronization is calculated using Pearson’s the sign and value of the correlation between the times correlation coefficient (r). Correlation coefficients series of the normalized abundances of fish from both were calculated between areas and stations with non- subareas. If no or not enough data were available to zero densities. This resulted in symmetric correlation calculate the correlation, then this line was not drawn. matrices, of which the size differed per fish species. R 2.15.2 (R Development Core Team 2009) was For each species, the correlation matrix was visualized used for data handling, estimation, and plotting. For by plotting ellipses shaped as contours of a bivariate spatial data handling and statistics the R packages normal distribution with unit variance and correlation sp (Pebesma & Bivand 2005; Bivand et al. 2008), r. More elliptical and more intense colour means rgeos (Bivand & Rundel 2011), and rgdal (Keitt et higher correlation; colour (red or blue) is indicating al. 2011) were used and for mixed modelling the R the respective negative or positive nature of the package lme4 (Bates and others 2012). For plotting, correlation. The points of the contour are given by ellipse (Murdoch & Chow 1996, 2012) and ggplot2 (x,y) = (cos(θ+d/2), cos(θ−d/2)) and d = arccos(r) (Wickham 2009) were used. Data analysis was where θ ϵ [0, 2π] (Murdoch & Chow 1996). performed by Eelke Folmer (EcoSpace).
Fig. 2.3 Compartmentalization of the study area based upon fish abundance as determined by means of surveys in the North Sea (NSER), the North Sea coastal zone (DFS), and the Wadden Sea including the Ems estuary (DFS), and Lake IJssel (LIJ). Within the Wadden Sea, the blue boxes refer to DFS areas (MD = Marsdiep, EG = Eijerlandse Gat, VL = Vlie, BD =Borndiep, ZL = Zoutkamperlaag / Pinkegat, LS = Lauwers / Schild, ED = Ems-Dollard) and the white boxes to islands (TX= Texel, VL = Vlieland, TR = Terschelling, AM = Ameland, SO = Schiermonnikoog, RP = Rottumerplaat). Blue lines with rounded endpoints indicate (potential) corridors for migratory fish between different compartments. Note that potential migration points to inland waters are not indicated (e.g. all sluices along the Dutch Wadden Sea coast).
24 2.4 Results & Discussion of densities of River Lamprey in the North Sea and the Wadden Sea suggests an increase since the 1990s (Fig. 2.4.1 Lampreys (Petromyzontidae) 2.4.1.1), it cannot be excluded that misidentification during the surveys might have caused this (Heessen River lamprey (Lampetra fluviatilis) – River Lamprey et al. 2015). On average, densities in the North Sea was caught during all surveys and in almost all coastal zone and the Wadden Sea are around 0.1 fish surveyed areas, with exception of the tidal basin of the ha-1, and less than 0.01 fish ha-1 in Lake IJssel (Fig. Eijerlandse Gat (Fig. 2.4.1.1). Although the time series 2.4.1.1).
North Sea NS coastal zone Marsdiep inlet Marsdiep 0.12 0.030 4 0.06 0.10 3 0.08 0.020 0.04 0.06 (n/d) (n/ha) (n/ha) 2 survey index (−) index survey 0.04 0.010 0.02 1 0.02 0 0.00 0.00 0.000 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 0.8 1.0 0.25 0.8 0.20 0.15 0.6 0.6 0.15 0.10 0.4 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 0.10 0.2 0.05 0.2 0.05 0.0 0.0 0.00 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 30 0.010 2.5 25 0.3 0.008 2.0 20 0.006 0.2 15 1.5 (n/ha) (n/ha) (n/h/80m2) (n/h/80m2) 0.004 10 1.0 0.1 5 0.002 0.5 0.0 0 0.000 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 0.7 3.5 0.05 0.20 0.6 3.0 0.04 0.5 0.15 2.5 0.03 0.4 2.0 (n/d) (n/d) (n/ha) 0.10 0.3 (n/h/80m2) 0.02 1.5 0.2 0.05 1.0 0.01 0.1 0.5 0.0 0.00 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.1.1 Time series of normalized densities of River Lamprey as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
25 Comparison of trends from the different surveys in time show that year-to-year variations of fish caught by the three stow nets in the Ems estuary are more or less comparable with each other and with the variation of fish caught by beam trawls in the Vlie tidal basin (Fig. 2.4.1.2). In Lake IJssel, the variation in the by- catch of the fyke near Kornwerderzand is negatively correlated with that in the beam trawl surveys in Lake IJssel (Fig. 2.4.1.2).
When comparing the trends in the beam trawl surveys in the study area, it appears that there is some correlation between the North Sea and the Marsdiep tidal basin, between the NS coastal zone and the Vlie tidal basin and between the tidal basins of the Lauwers and the Ems estuary (Fig. 2.4.1.3).
Within the Netherlands, “Gasterense Diepje” (a branch of the small river “Drentsche Aa”) is one of the few locations where these fish are presently known to spawn (Winter & Griffioen 2007). River Lampreys can reach this area via the sluices of Delfzijl Fig. 2.4.1.2 Ellipse matrix of the correlations in the Ems estuary and adjacent canals in the Province between times series of normalized densities of of Groningen (Winter et al. 2013). Other but still River Lamprey as determined by means of surveys unknown spawning areas might be reached when in the North Sea, Wadden Sea and Lake IJssel. River Lamprey migrates upstream in the rivers IJssel Colour indicates positive (blue) or negative (red) and Vecht after accessing Lake IJssel via the Afsluitdijk correlation, colour intensity the (absolute) size of (Erwin Winter, IMARES, pers. comm.). the correlation coefficient.
Fig. 2.4.1.3 Correlations between times series of normalized abundance of River Lamprey between adjacent basins as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
26 Sea Lamprey (Petromyzon marinus) - Sea Lamprey mammals, the catchability of Sea Lamprey with the was caught in all types of surveys (except for the beam nets used in the different surveys is very small (Erwin trawl survey in Lake IJssel), but only in 10 of the 16 Winter, IMARES, pers. comm.). Due to possible surveyed areas and locations (Fig. 2.4.2.1). For half misidentifications, the time series of densities in Sea of these locations, Sea Lamprey was caught only in 1 Lamprey for the North Sea cannot be trusted (Heessen year or in 2 years of the study period (Fig. 2.4.2.1). et al. 2015). In general, catches were too low to Given that adult Sea Lamprey are parasitic feeders perform a correlation analyses. that attach to large preys, i.e. big fish and marine
North Sea NS coastal zone Marsdiep inlet Marsdiep 0.08 6 0.10 0.06 5 0.06 0.08 4 0.04 0.06 3 0.04 (n/d) (n/ha) (n/ha) 0.04 survey index (−) index survey 2 0.02 0.02 1 0.02 0 0.00 0.00 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1.0 1.0 1.0 0.020 0.8 0.8 0.8 0.015 0.6 0.6 0.6 0.010 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 0.4 0.4 0.005 0.2 0.2 0.2 0.0 0.0 0.0 0.000 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 1.0 1.0 0.20 0.20 0.8 0.8 0.15 0.15 0.6 0.6 (n/ha) (n/ha) 0.10 (n/h/80m2) (n/h/80m2) 0.10 0.4 0.4 0.05 0.2 0.2 0.05 0.0 0.0 0.00 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 0.30 1.0 0.4 0.5 0.25 0.8 0.3 0.4 0.20 0.6 0.15 0.3 0.2 (n/d) (n/d) (n/ha) (n/h/80m2) 0.4 0.10 0.2 0.1 0.2 0.05 0.1 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.2.1 Time series of normalized densities of Sea Lamprey as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
27 2.4.2 European Eel (Anguilla anguilla) discharge points in the Wadden Sea). The time series of the adults and juveniles suggest that densities were European Eels were caught during all surveys and in generally higher in the 1980s than in the 1990s and all areas (Fig. 2.4.3.1, Fig. 2.4.3.2). It must be noted 2000s except for the Ems estuary (Fig. 2.4.3.1). At that the surveys can be grouped according to the life sampling stations near the Afsluitdijk, Eels appear phases of the fish caught, i.e. (i) adults & juveniles (all to have declined at both sides during the past surveys in the North Sea and the North Sea coastal decades, i.e. adults and juveniles in Lake IJssel near zone, fyke catches in the Marsdiep tidal inlet and Ems Kornwerderzand (Fig. 2.4.3.1) and Glass Eel at the estuary, and all surveys in Lake IJssel), (ii) (mainly) sampling stations near Den Oever in the Wadden Sea juveniles (all surveys for the tidal basins of the Wadden (Fig. 2.4.3.4). Sea), and (iii) Glass Eel (lift net surveys at freshwater
North Sea NS coastal zone Marsdiep inlet Marsdiep 6 14 60 5 5 12 50 4 10 4 40 8 3 3 30 (n/d) (n/ha) (n/ha) 6 2 2 survey index (−) index survey 20 4 1 1 10 2 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1.0 8 12 5 0.8 10 6 4 8 0.6 3 4 6 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 2 4 2 0.2 1 2 0 0 0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 0.20 6 2.5 0.8 5 0.15 2.0 0.6 4 1.5 0.10 3 (n/ha) (n/ha) 0.4 (n/h/80m2) (n/h/80m2) 1.0 2 0.05 0.2 0.5 1 0 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 60 15 15 2.0 50 40 10 10 1.5 (n/d) (n/d) 30 (n/ha) (n/h/80m2) 5 20 1.0 5 10 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.3.1 Time series of normalized densities of European Eel (juveniles and adults) as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
28 The ellipse matrix of correlations suggests synchronous When comparing the trends in the beam trawl surveys dynamics in European Eel (including Glass Eel) in the study area, it appears that trends in many between the marine waters of the North Sea subareas are positively correlated, in particular the (including the coastal zone) and the Wadden Sea Marsdiep with its adjacent basins (Vlie, Eijerlandse (including the Ems estuary) (Fig.2.4.3.2, Fig.2.4.3.3). Gat) and the North Sea, and the tidal basins of Correlations were particularly high between Glass Borndiep and Zoutkamperlaag (Fig. 2.4.3.2). Eel catches at Den Oever, Harlingen and Lauwersoog Correlations were low or absent between the Marsdiep (Fig.2.4.3.2, Fig.2.4.3.3). At the stations Terborg tidal basin and Lake IJssel, and between the North Sea (TB) and Spijk (SP) in the Ems estuary, (juvenile and coastal zone and the tidal basins of Zoutkamperlaag adult) Eel showed opposite trends compared to most and Lauwers (Fig. 2.4.3.3). other stations and larger marine areas (Fig.2.4.3.2, Fig.2.4.3.3).
Fig. 2.4.3.2 Ellipse matrix of the correlations between times series of normalized densities of European Eel (juveniles and adults) as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
Fig. 2.4.3.3 Correlations between times series of normalized abundance of European Eel (juveniles and adults) between adjacent basins as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
29 Den Oever 1 Den Oever 2 35 120 30 100 25 80 20 60 (n/net) (n/net) 15 40 10 20 5 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Harlingen Lauwersmeer 35 100 30 80 25 60 20 (n/net) (n/net) 15 40 10 20 5 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Termunterzijl Nieuw Statenzijl 70 15 60 50 10 40 (n/net) (n/net) 30 5 20 Fig. 2.4.3.4 Time series of normalized densities of 10 European Eel (Glass Eel) as determined by means of 0 0 surveys in the Wadden Sea. 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
This large-scale synchronicity suggests a common deterioration), have been suggested as causes for the driver of Eel dynamics, e.g. an atmospherically driven decline (a.o., Castonguay 1994; Feunteun 2002; dispersal by ocean currents connecting the putative Friedland et al. 2007; Palstra et al. 2007; Robinet & spawning grounds of the European eel and the Gulf Feunteun 2002). Stream, determining the arrival of juveniles at the European coast (Baltazar-Soares et al. 2014). An Since its closure in 1932, Lake IJssel has been subject alternative explanation is given by Kettle et al. (2011), to a suite of management regulations with regard who point to a change in hydrology and exploitation to Eel, including changes in fishery pressure (e.g. in the southwest corner of the geographical range fishing efforts, fishing season, engine power, and of European Eel, particularly the Atlantic coast increases in mesh size; De Leeuw et al. 2008) and of Morocco. Possibly, Glass Eel trends are more restocking (Dekker & Beaulaton 2016b). Furthermore, dominated by large-scale drivers (hydrographic), the increase in Eel in Lake IJssel directly after the whereas those of the (Yellow and Silver) Eel are closure might have resulted from the fact that Eel more determined by local drivers (e.g., entrance was no longer able to take advantage of selective tidal to fresh/inland water, fisheries) (Zwanette Jager, transport for swimming up rivers and got stuck in this ZiltwaterAdvies, pers. comm.). area (Dekker & Beaulaton 2016a). Alternatively, a new suitable and vast habitat came available for a large part Dekker & Casselman (2014) showed that in Europe of the Glass Eel entering Lake IJssel and therefore there recruitment of (Glass) Eel has declined by 90%–99% was no need for more dispersal further upstream. This since 1980. Global oceanic changes, as well as fill-up principle is also observed in French rivers at the direct and indirect impacts (barriers to migration, Bay of Biscay after barriers were taken away (Erwin contaminants, fisheries exploitation, Winter, IMARES, pers. comm.). habitat loss, parasite introductions, and water quality
30 North Sea NS coastal zone Marsdiep inlet Marsdiep 14 5 12 3 0.6 10 4 8 2 3 0.4 (n/d) (n/ha) (n/ha) 6 2 survey index (−) index survey 4 1 0.2 1 2 0 0 0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 0.4 1.0 0.25 0.08 0.8 0.20 0.3 0.06 0.6 0.15 0.2 (n/ha) (n/ha) (n/ha) (n/ha) 0.04 0.4 0.10 0.1 0.02 0.2 0.05 0.0 0.0 0.00 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 250 1.0 0.08 100 200 0.8 80 0.06 150 0.6 60 0.04 (n/ha) (n/ha) (n/h/80m2) (n/h/80m2) 0.4 100 40 0.02 50 20 0.2 0 0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 1.0 1.0 1.0 0.05 0.8 0.8 0.8 0.04 0.6 0.6 0.6 0.03 (n/d) (n/d) (n/ha) (n/h/80m2) 0.4 0.4 0.4 0.02 0.2 0.2 0.2 0.01 0.0 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.4.1 Time series of normalized densities of European Anchovy as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
2.4.3 European Anchovy (Engraulis 2.4.4.1; Fig. 2.4.4.2). Another peak in 1998 coincided encrasicolus) simultaneously in the tidal basins of Vlie, Borndiep and Zoutkamperlaag (Fig. 2.4.4.1; Fig. 2.4.4.2). Anchovy was caught in 11 of the 20 surveyed areas or locations, and caught only in 1 year or in 2 years of the Correlations in trends between tidal basins and study period within 6 of these 11 areas (Fig. 2.4.4.1). adjacent marine waters (North Sea, North Sea coastal The peaks in the stow-net catches in 2007 in the Ems zone) are poor, with exception of the high correlation estuary (Spijk, Oterdum, Terborg) are in concordance between Vlie, Borndiep and Zoutkamperlaag resulting with the only catch in the beam trawl survey for this from the simultaneous peak in 1998 (Fig. 2.4.4.3). In area, and was also observed in the North Sea (Fig. general, the time series of the Wadden Sea tidal basins
31 Fig. 2.4.4.2 Ellipse matrix of the correlations between times series of normalized densities of European Anchovy as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
Fig. 2.4.4.3 Correlations between times series of normalized abundance of European Anchovy between adjacent basins as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
32 were characterized by one (1998 or 2007) or two declined after the closure in 1932 (see Chapter 1, years (1975 and 1998) with occurrences of Anchovy, Fig. 1.4) and this species is no longer caught in Lake and zero observations for other years within the study IJssel (Fig. 2.4.4.1). Within the western Wadden period (1970-2014). This can be explained by the fact Sea, spawning individuals were reported up to 1962 that Anchovy is a summer (May-July) visitor and is not (sequence of cold winters) and eggs were observed found in autumn in these waters when these surveys again in 1994 north of the freshwater discharge sluices are performed. of Kornwerderzand, close to former spawning areas in the Vlieter tidal gully (Boddeke & Vingerhoed 1996). In the North Sea, anchovy abundance has fluctuated, In May 2010 and May 2011, schools of Anchovy were with periods of high abundance being followed by found in a pelagic survey in the Marsdiep area. Their periods of near absence (Aurich 1953). Data from trawl mean length (14.5 cm) corresponded to age-1 fish that surveys and commercial information landings indicate are likely to spawn. Hence, for anchovy the Wadden a strong increase in Anchovy abundance since 1995 Sea area is important both as nursery as well as for after a period of absence (Beare et al. 2004). This spawning (Couperus et al. 2016). increase is considered to be the result of an improved productivity of existing populations, associated with In historical times, Anchovy formed the basis of a an expansion in thermal habitats and/or wind-induced substantial fishery in the Ems estuary, which came variations in the inflow of Atlantic water in winter, to an end before 1855 (Stratingh & Venema, 1855), and probably not due to a northward shift in the which was before the blooming of the fishery in distribution of a southern (Bay of Biscay) population Zuiderzee between 1880 and 1903 (Gmelich Meijling- (Corten & Van de Kamp 1996; Petitgas et al. 2012). van Hemert 2008). Possibly, the recent catches of The former Zuiderzee used to support the largest Anchovy in the (spring) stow-net survey in the middle spawning population of Anchovy in northwestern and outer part of the Ems estuary indicate a return of Europe (Petitgas et al. 2012), but catches rapidly this fish species in this formerly rich area.
33 2.4.4 Herrings (Clupeidae) trawl catches in the North Sea and the Wadden Sea were low in the 1970s, and increased hereafter (Fig. Atlantic Herring (Clupea harengus) – Herring was 2.4.5.1). The variation in stow-net catches at Terborg caught in all marine survey areas and locations, and appears to be positively correlated with the beam trawl during two years (1991, 1992) in the trawl catches in catches in tidal basins of Eijerlandse Gat, Borndiep and Lake IJssel (Fig. 2.4.5.1). On average, marine bottom- Zoutkamperlaag (Fig. 2.4.5.2).
North Sea NS coastal zone Marsdiep inlet Marsdiep 2000 400 2.0 5000 4000 1500 300 1.5 3000 (n/d) 1000 200 (n/ha) (n/ha) 1.0 2000 survey index (−) index survey 500 100 0.5 1000 0 0 0 0.0 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1000 800 120 600 800 100 600 80 600 400 (n/ha) (n/ha) (n/ha) (n/ha) 60 400 400 40 200 200 200 20 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 800 3000 1500 80 600 60 2000 1000 400 (n/ha) (n/ha) 40 (n/h/80m2) (n/h/80m2) 1000 500 200 20 500 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 1.0 1.0 0.08 600 0.8 0.8 500 0.06 400 0.6 0.6 0.04 (n/d) (n/d) 300 (n/ha) (n/h/80m2) 0.4 0.4 200 0.02 0.2 0.2 100 0 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.5.1 Time series of normalized densities of Atlantic Herring as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
34 Fig. 2.4.5.2 Ellipse matrix of the correlations between times series of normalized densities of Atlantic Herring as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
Fig. 2.4.5.3 Correlations between times series of normalized abundance of Atlantic Herring between adjacent basins as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
35 When comparing the trends in the beam trawl surveys There is evidence that this winter transport of larvae in the study area, it appears that trends in the tidal across the North Sea was interrupted for a number of basins are positively correlated, and that variation years prior to 1980 as the result of a reduced inflow in Herring densities in the eastern tidal basins is from the Atlantic Ocean (Corten 2013), which might correlated with that in the North Sea coastal zone explain the relatively low densities during that time in (Fig. 2.4.5.3). the Marsdiep tidal inlet and the western tidal basins of the Wadden Sea (Fig. 2.4.5.1). In the North Sea, numbers increased steadily in the 1980s reflecting a period of recovery after the collapse There appears to be a synchronisation in Herring of the Herring stock as the result of overfishing in abundances in the coastal zone and the eastern part of the 1970s and remained more or less stable hereafter the Wadden Sea, probably due to local circumstances (Dickey-Collas et al. 2010). Relatively low stocks in reducing fish numbers such as competition for food the 2000s are ascribed to low recruitment (2002–2010) with the invasive cnetophore since 2005 (Kellnreitner caused by an increase in temperature, a regime shift in et al. 2013), temperature-related infections by parasites the ecosystem and/or predation by the large stock of (Schade et al. 2015) or enhanced predation by fish adult Herring on its own larvae (Corten 2013). (Cardoso et al. 2015), birds and seals (Van der Veer et These large-scale changes are, however, not al. 2015). synchronised with changes in the Dutch coastal zone nor in the tidal basins of the Wadden Sea (Fig. The occurrence of Herring in Lake IJssel in 1991 and 2.4.5.2). A critical factor for the survival of the larvae 1992 might have been the result of a change in sluice of Herring born in the central and northern North management in 1991-1993, where discharge sluices Sea appears to be their transport in winter across the were kept open until water levels at both sides of North Sea towards the shallow coastal nursery areas the sluices were equal, as was observed for Flounder in the east, including the Wadden Sea (Corten 2013). (Winter 2009).
36 Allis Shad (Alosa alosa) – Allis Shad was only caught in the North Sea bottom trawl survey, and therefore not further considered for analyses (Fig. 2.4.6.1). In 2013, three young-of-the-year Alosa alosa, probably originating from natural reproduction, were documented for the first time in a period of nearly 100 years in the River Rhine. In 2014, a further increase was observed when 57 juveniles and eight adults were caught; seven of these eight adults were derived from stocking (Hundt et al. 2015).
North Sea NS coastal zone Marsdiep inlet Marsdiep 1.0 1.0 1.0 20 0.8 0.8 0.8 15 0.6 0.6 0.6 (n/d) (n/ha) (n/ha) 10 0.4 0.4 0.4 survey index (−) index survey 5 0.2 0.2 0.2 0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/h/80m2) (n/h/80m2) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/d) (n/d) (n/ha) (n/h/80m2) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.6.1 Time series of normalized densities of Allis Shad as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
37 Twaite (Alosa fallax) – Twaite Shad was caught by Variation at Oterdum was, however, negatively most surveys in the North Sea, the coastal zone of correlated with that of the third (oligohaline) stow the North Sea, in all tidal basins of the Wadden Sea, net at Terborg and with that of the (freshwater) fyke only not by the bottom trawl surveys in Lake IJssel catches at Kornwerderzand (Fig. 2.4.7.2). (Fig. 2.4.7.1). The year 1982 was an outlier with large numbers of fish caught in the coastal zone (Fig. 2.4.7.1; When comparing the trends in the beam trawl Heessen et al. 2015). This peak in 1982 occurred in surveys in the study area, the variation in Twaite several tidal basins of the Wadden Sea as well (Fig. Shad appeared to be synchronised in the North Sea, 2.4.7.1). For two of three stow net surveys (Oterdum, the Dutch coastal zone and the westernmost and Spijk), variation in Twaite Shad was positively easternmost tidal basins of the Wadden Sea correlated with that in the North Sea (Fig. 2.4.7.2). (Fig. 2.4.7.3).
North Sea NS coastal zone Marsdiep inlet Marsdiep 100 15 20 30 80 15 10 60 20 (n/d) (n/ha) (n/ha) 10 40 survey index (−) index survey 5 10 5 20 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 8 4 5 20 4 6 3 15 3 4 2 (n/ha) (n/ha) (n/ha) (n/ha) 10 2 2 1 5 1 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 20 60 100 200 50 80 15 40 150 60 10 30 (n/ha) (n/ha) 100 40 (n/h/80m2) (n/h/80m2) 20 5 50 20 10 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 1.4 1.0 0.08 0.30 1.2 0.8 0.25 0.06 1.0 0.20 0.6 0.8 (n/d) (n/d) (n/ha) 0.04 0.15 (n/h/80m2) 0.4 0.6 0.10 0.02 0.2 0.4 0.05 0.0 0.00 0.00 0.2 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.7.1 Time series of normalized densities of Twaite Shad as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
38 Twaite Shad need tidal freshwater systems to spawn conditions, resulted in a comeback of Twaite Shad successfully and estuaries for nursing, thus the in the Western Scheldt since 1996 with spawning occasional fish in Lake IJssel at present is considered individuals since 2010 (Peter Herman, NIOZ, pers. to be have entered these waters without successful comm.). In the Ems, at least in some years, successful spawning (De Groot 2002). Improvement of the spawning occurs (Erwin Winter, IMARES, water quality, in particular with regard to the oxygen pers. comm.).
Fig. 2.4.7.2 Ellipse matrix of the correlations between times series of normalized densities of Twaite Shad as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
Fig. 2.4.7.3 Correlations between times series of normalized abundance of Twaite Shad between adjacent basins as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
39 2.4.5 European Smelt (Osmerus eperlanus) those in the North Sea and in Lake IJssel (Fig. 2.4.8.3). In addition, some correlation is found between the Smelt was caught in all surveys in the North Sea, the North Sea coastal zone and Zoutkamperlaag tidal Wadden Sea and Lake IJssel (Fig. 2.4.8.1). Stow-net basin, and between the tidal basins of Lauwers and catches at Spijk and Oterdum are positively correlated, Ems estuary (Fig. 2.4.8.3). This suggests that drivers whilst the stow-net catches at Spijk are negatively of the abundance of Smelt at the edges of the Dutch correlated with those of the bottom-trawl surveys and Wadden Sea occur in the North Sea, but not in the the fyke catches near Kornwerderzand in Lake IJssel coastal zone (Fig. 2.4.8.3). It seems, however, unlikely (Fig. 2.4.8.2). that drivers in the North Sea steer Smelt abundance. The main drivers will likely be the two large When comparing the trends in the beam trawl surveys freshwater discharges from Lake IJssel and Ems (Erwin in the study area, annual variations in Smelt in the Winter, IMARES, pers. comm.). Marsdiep tidal basin appears to be correlated with
North Sea NS coastal zone Marsdiep inlet Marsdiep 5 8 4 1.5 1200 6 3 800 1.0 (n/d) (n/ha) (n/ha) 4 2 600 survey index (−) index survey 0.5 400 2 1 200 0 0 0.0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1000 12 800 400 10 800 600 300 8 600 6 (n/ha) (n/ha) (n/ha) (n/ha) 400 200 400 4 200 100 200 2 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 400 600 80 350 60 500 60 300 400 40 250 (n/ha) (n/ha) 40 300 (n/h/80m2) (n/h/80m2) 200 20 200 20 150 100 100 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 40 10000 1000 30 30 8000 800 6000 20 600 20 (n/d) (n/d) (n/ha) (n/h/80m2) 4000 400 10 10 2000 200 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.8.1 Time series of normalized densities of European Smelt as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
40 Fig. 2.4.8.2 Ellipse matrix of the correlations between times series of normalized densities of European Smelt as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
Fig. 2.4.8.3 Correlations between times series of normalized abundance of European Smelt between adjacent basins as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
Smelt was formerly abundant in the Zuiderzee. After (Tulp et al. 2013). Within Lake IJssel, the Smelt closure of the dike in 1932, a landlocked population populations have declined strongly since 1990 (Fig. developed, that matures after only 1 year instead of 2.4.8.1), and Smelt fishery was closed during five years after 2-3 years as for the diadromous smelt, while the in the last decade because of low biomass (De Leeuw diadromous population still inhabits the coastal waters et al. 2008). of the Wadden Sea. The diadromous population may either be seeking spawning grounds in open estuaries Barium and Strontium concentrations in the otoliths (e.g. Ems estuary) or may be entering Lake IJssel of Wadden Sea Smelt suggest a mixture of two through the sluices, although the contribution of populations, i.e. those who were born in the Wadden diadromous smelt is neglible in the current situation Sea (55%) and in Lake IJssel (5%) or Markermeer
41 (45%) and migrated to the Wadden Sea (Phung et al. 2.4.6 Salmonids (Salmonidae) 2015). This implies that variation in the Wadden Sea stocks of Smelt is at least partly driven by import from Atlantic Salmon (Salmo salar) - The time series Lake IJssel. of the North Sea just shows the incidental catches (Heessen et al. 2015; Fig. 2.4.9.1). Within all other Water Board Hunze en Aa’s observes migration of surveys, Salmon was only caught in the two fykes in adult Smelt through the sluice of Nieuwe Statenzijl Lake IJssel (Fig. 2.4.9.1). This species was therefore in the Ems estuary. Monitoring with fykes in not considered for further analyses. Low to zero inland waters shows that they migrate up to 20 km catches are most probably due to the low catchability upstream. Possibly they spawn in the Westerwoldse of Atlantic Salmon, resulting from their strong Aa considering the fact that juvenile 0+ individuals swimming capacity (escaping nets) and their periodic are caught in local fykes (Peter Paul Schollema, occurrence (smolts in late spring, and adults mainly Waterschap Hunze en Aa’s, pers. comm.). during summer and autumn) when using the Wadden
North Sea NS coastal zone Marsdiep inlet Marsdiep 1.0 1.0 1.0 14 12 0.8 0.8 0.8 10 0.6 0.6 0.6 8 (n/d) (n/ha) (n/ha) 6 0.4 0.4 0.4 survey index (−) index survey 4 0.2 0.2 0.2 2 0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/h/80m2) (n/h/80m2) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 1.0 1.0 0.06 0.12 0.05 0.8 0.8 0.10 0.04 0.08 0.6 0.6 0.03 (n/d) (n/d) (n/ha) 0.06 (n/h/80m2) 0.4 0.4 0.02 0.04 0.2 0.2 0.01 0.02 0.0 0.0 0.00 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.9.1 Time series of normalized densities of Atlantic Salmon as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
42 Sea as corridor to the open ocean (Erwin Winter, reports have been more frequent in the 1990s than IMARES, pers. comm.). in the last decade (Heessen et al. 2015). In contrast to Atlantic Salmon (using the Wadden Sea only as a Sea Trout (Salmo trutta) – Within the study area, Sea corridor to the open ocean), Sea Trout is roaming the Trout was caught by the bottom trawls in the North coastal areas and estuaries directly downstream their Sea (irregularly) and Lake IJssel (in 1995 and 1996 spawning rivers to feed. The Wadden Sea will also be only), fykes in the Marsdiep (regularly) and Lake IJssel, partly used as a feeding habitat, but this fast swimmer and some of the stow nets in the Ems estuary (Fig. is difficult to catch with most of the survey gears 2.4.10.1). This species was therefore not considered (with exception of fykes, and possibly stow nets), and for further analyses. Although the small catches in the these surveys will therefore grossly underestimate its North Sea were quite irregular since the late 1970s, abundance (Erwin Winter, IMARES, pers. comm.).
North Sea NS coastal zone Marsdiep inlet Marsdiep 1.0 1.0 4 4 0.8 0.8 3 3 0.6 0.6 (n/d) (n/ha) (n/ha) 2 2 0.4 0.4 survey index (−) index survey 1 1 0.2 0.2 0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 1.0 1.0 1.0 0.05 0.8 0.8 0.8 0.04 0.6 0.6 0.6 0.03 (n/ha) (n/ha) (n/h/80m2) (n/h/80m2) 0.4 0.4 0.4 0.02 0.2 0.2 0.2 0.01 0.0 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 0.4 0.06 0.4 0.3 0.3 0.3 0.04 0.2 (n/d) (n/d) (n/ha) 0.2 (n/h/80m2) 0.2 0.02 0.1 0.1 0.1 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.10.1 Time series of normalized densities of Sea Trout as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
43 Houting (Coregonus oxyrhinchus) - Within the study A successful re-introduction programme, however, re- area, Houting was caught once (2010) in the Marsdiep established a self-reproducing population (Borcherding tidal basin and furthermore only caught in Lake et al. 2014). The River IJssel, a lower branch of the IJssel, in the bottom trawls as well as in the fykes (Fig. River Rhine, and/or its tributaries serve as spawning 2.4.11.1). This species was therefore not considered for ground. Freyhof & Schöter (2005), however, conclude further analyses. Houting was historically distributed that C. oxyrinchus is a globally extinct species and in the Wadden Sea extending from Jutland (Denmark) recently re-introduced coregonids in the Rhine to the Schelde delta (The Netherlands). The species catchment from Danish rivers are in fact C. maraena, a has been considered extinct in the River Rhine since species not native to the Rhine. the 1940s (Fig. 1.5).
North Sea NS coastal zone Marsdiep inlet Marsdiep 1.0 1.0 1.0 0.15 0.8 0.8 0.8 0.6 0.6 0.6 0.10 (n/d) (n/ha) (n/ha) 0.4 0.4 0.4 survey index (−) index survey 0.05 0.2 0.2 0.2 0.0 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/h/80m2) (n/h/80m2) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 0.7 1.0 0.12 0.8 0.6 0.10 0.8 0.5 0.6 0.08 0.6 0.4 0.4 0.06 (n/d) (n/d) (n/ha) 0.3 (n/h/80m2) 0.4 0.04 0.2 0.2 0.2 0.02 0.1 0.0 0.0 0.0 0.00 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.11.1 Time series of normalized densities of Houting as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
44 In 2010, justly hatched larvae were caught directly upstream from Kampen, where the IJssel discharges into Lake IJssel, most probably hatched in upstream areas of the River IJssel (Borcherding et al. 2014). The fish caught in the Lake IJssel survey since 2006 and in the Marsdiep in 2010 are, therefore, most likely originating from the newly established spawning population in the River IJssel. Fyke monitoring programmes and seine-net efforts to catch Houting for telemetry experiments showed that many Houting use Lake IJssel for feeding and that only part of the population migrated to marine environments (Borcherding et al. 2008, 2014).
North Sea NS coastal zone Marsdiep inlet Marsdiep 6 1.0 1.0 10 5 0.8 0.8 8 4 6 0.6 0.6 3 (n/d) (n/ha) (n/ha) 4 0.4 0.4 2 survey index (−) index survey 2 0.2 0.2 1 0 0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 (n/ha) (n/ha) (n/ha) (n/ha) 0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.2 0.0 0.0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 5 7 1.0 1.0 6 4 0.8 0.8 5 3 0.6 0.6 4 (n/ha) (n/ha) 3 2 (n/h/80m2) (n/h/80m2) 0.4 0.4 2 1 0.2 0.2 1 0 0 0.0 0.0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 0.7 7 600 8 0.6 6 500 5 0.5 6 400 4 0.4 (n/d) (n/d) 300 (n/ha) 3 4 0.3 (n/h/80m2) 200 2 0.2 2 1 100 0.1 0 0 0.0 0 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.12.1 Time series of normalized densities of Three-Spined Stickleback as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
45 2.4.7 Three-spined Stickleback (Gasterosteus aculeatus)
Within the study area, Three-Spined Sticklebacks were caught in the bottom-trawl surveys in the North Sea and Lake IJssel, in fykes in the tidal basins of the Marsdiep and Lake IJssel, and in the stow-net surveys in the Ems estuary (Fig. 2.4.12.1). Variations in stow- net catches appear to be correlated with each other, as the stow-net catches near Terborg and the fyke catches near Kornwerderzand, and the bottom-trawls in Lake IJssel with the fyke catches near Flevoland (Fig. 2.4.12.2). In the North Sea, catches have been quite variable from year to year without any clear trend. However, bottom-trawl catches are not representative of annual fluctuations in Three-Spined Sticklebacks (Heessen et al. 2015).
Fig. 2.4.12.2 Ellipse matrix of the correlations between times series of normalized densities of Tree-Spined Stickleback as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
46 2.4.8 European Flounder explained by a stock-recruitment relationship, i.e. high (Platichthys flesus) number of spawning adults in the North Sea result in high numbers of juvenile Flounders in the western Flounder was caught in all surveys in the North Wadden Sea (Zwanette Jager, ZiltwaterAdvies, pers. Sea, the Wadden Sea and Lake IJssel (Fig. 2.4.13.1). comm.). When comparing trends in the bottom-trawl catches in the study area, annual variations in Flounder in Within Lake IJssel, the abundance of Flounder the Marsdiep tidal basin appears to be correlated increased rather suddenly from less than 5 individuals with those in the North Sea, the Vlie tidal basin and per ha up to more 30 individuals per ha in the early Lake IJssel (Fig. 2.4.13.3). This positive relationship 1990s (Fig. 2.4.13.1). between the North Sea and the Marsdiep might be
North Sea NS coastal zone Marsdiep inlet Marsdiep 8 150 3.0 250 6 2.5 200 100 2.0 4 150 (n/d) (n/ha) (n/ha) 1.5 survey index (−) index survey 100 50 2 1.0 50 0.5 0 0 0 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Eijerlandse Gat Vlie Borndiep Zoutkamperlaag 70 200 200 150 60 150 50 150 100 40 100 (n/ha) (n/ha) (n/ha) (n/ha) 100 30 50 20 50 50 10 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Lauwers / Schild Ems estuary Spijk Oterdum 16 120 150 15 14 100 12 80 100 10 10 60 (n/ha) (n/ha) (n/h/80m2) (n/h/80m2) 8 40 50 6 5 20 4 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Terborg Lake IJssel Kornwerderzand Flevoland 35 40 25 6 30 5 20 30 25 4 20 15 20 3 (n/d) (n/d) (n/ha) 15 (n/h/80m2) 10 2 10 10 5 5 1 0 0 0 0
1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010
Fig. 2.4.13.1 Time series of normalized densities of European Flounder as determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
47 This increase has been attributed to a change in sluice management between 1991 and 1993 (Vethaak et al. management in 1991-1993, where discharge sluices 2004). If the diseases led to increased mortality or were kept open until water levels at both sides of the weaker growth and smaller sized fish, this may have sluices were equal allowing some salt water near the affected the reproductive potential of part of the bottom to enter Lake IJssel (originally sluices were population through size-specific fecundity (Rijnsdorp closed when water levels at the seaside were still 10 cm 1994; Van der Veer et al., 2000). lower than in Lake IJssel) (Winter 2009). In addition The cause of the outbreak of disease in Flounder to sluice management, a strong correlation with in front of the Afsluitdijk drainage sluices has been Flounder densities and discharge rates in late summer attributed to cumulating stress factors affecting (Aug/Sept) was observed, with an approximate slope the fish’ immune system, making the fish more of +0.3 individuals per ha per m3 discharge for the susceptible to a variety of diseases. The most important period between 1991 and 2000 (Winter 2009). stress factors include long periods of time in the unfavourable water conditions outside the drainage After an initial decrease of the proportion of Flounder sluices, including abrupt and extreme salinity with skin ulcers in front of the drainage sluices fluctuations due to the discharge of fresh water from of the Afsluitdijk from some 30% to 10% since Lake IJssel (Vethaak et al. 2004). Reduction of these 1988, the situation improved further at Den Oever stress factors may result in increased fecundity and, and Kornwerderzand following the adapted sluice subsequently, in increased stock size.
Fig. 2.4.13.2 Ellipse matrix of the correlations between times series of normalized densities of European Flounder as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
48 A decrease, however, was found in the Marsdiep tidal inlet that more or less coincided with changes in the coastal zone (Fig. 2.4.13.3). The absence of synchronicity with the other areas suggest the presence of a local driver in the coastal zone that is reflected in the fyke catches in the Marsdiep tidal inlet. Van der Veer et al. (2015) found a correlation between time series of decreasing fish numbers and increasing volumes of sand nourishments, for example, but it is unclear if this relationship is causal or not. As was suggested for Smelt (Erwin Winter, IMARES, pers. comm.), variations in stocks of Flounder might also or additionally be driven be large freshwater discharges from Lake IJssel and Ems. This is in line with the positive correlation in bottom-trawl catches of the North Sea and the Ems estuary (Fig. 2.4.13.2).
Fig. 2.4.13.3 Correlations between times series of normalized abundance of European Flounder between adjacent basins as determined by means of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative (red) correlation, colour intensity the (absolute) size of the correlation coefficient.
49 2.5 Conclusions Wadden Sea (including the Ems estuary) suggest a large-scale driver within the marine waters of the From a historical perspective (100-2000 yrs), study area. Such possible large-scale drivers include long-term stock dynamics of the fish species under atmospherically driven dispersal by ocean currents and consideration were driven by overfishing, pollution change in hydrology and exploitation. Within Lake and habitat destruction, including the interruption of IJssel, the effects of these large-scale climate-driven the river continuum by barriers (Chapter 1). Within influences may have been obscured by local impacts, a shorter time frame (< 50 yrs), the synchronies in including the influence of the Afsluitdijk in 1932 and abundances suggest an influence of additional factors, changes in fishery pressure and restocking hereafter. from large-scale climate variation to local import from freshwater systems (Fig. 2.5), depending on the For European Anchovy, the lack of correlations species. between basins and adjacent marine waters (North Sea, North Sea coastal zone) suggest the absence of For River Lamprey and Sea Lamprey, a common driver. Since Anchovy visits the study in synchronisation patterns were difficult to assess as summer whilst most of the surveys were performed the result of possible misidentifications and low in autumn, however, possible existing correlations catchabilities, in particular for Sea Lamprey. For River might have been missed. It cannot be excluded that the Lamprey, abundances in the Ems estuary might be climate-driven increase of Anchovy stocks in the North driven by spawning success in the “Gasterense Diepje”, Sea results in time in larger stocks in the study area. a branch of the “Drentsche Aa” river. For Atlantic Herring, the synchronisation in stock For European Eel, synchrony in time series from dynamics in all tidal basins suggests the existence of North Sea (including the coastal zone) and the a local driver at the scale of the Dutch Wadden Sea.
Fig. 2.5 Potential long-term and short-term drivers (as derived from synchrony in time series of surveys) of stock sizes of migratory fish in the Dutch Wadden Sea.
50 The correlation structure of the stock dynamics of For Three-Spined Sticklebacks, most of the the tidal basins with the coastal zone of the North Sea sampling gear used in the surveys was not suited to get further suggests that the central and eastern part of reliable estimates of local densities of this species. This this zone are part of the synchronisation area as well. species is better caught by means of 1x1 m lift nets, as Possible local drivers include competition for food been used during a monitoring study at relatively small with ctenophores, parasitic infections and predation by freshwater discharge points along the mainland coast fish, birds and seals. of the Wadden Sea area. Results of this study will be discussed in Chapter 3. For Allis Shad and Twaite Shad, only synchronisation patterns of the latter could be For European Smelt and European Flounder, analysed. Stock dynamics of Twaite Shad appear to synchronisation appears to occur between the North be synchronised in the western and eastern part of Sea and the westernmost (Marsdiep) and easternmost the Dutch Wadden Sea and the adjacent coastal zone. (Ems estuary) tidal basins of the Dutch Wadden Sea. As has been observed for the Scheldt estuary, stock Both basins are characterised by a strong salinity dynamics of Twaite Shad in estuarine tidal basins gradient, suggesting freshwater discharges as a such as the Marsdiep and the Ems might be driven by driving factor. Possible mechanisms include a stock- environmental conditions at their freshwater spawning recruitment relationship, with high adult numbers grounds. Improvement of these conditions might then offshore resulting in high numbers of larvae and attribute to larger stocks in the Wadden Sea. juveniles inshore. An alternative driver might be supply of fish to the Wadden Sea and North Sea from For Atlantic Salmon and Sea Trout, the catchability lakes and rivers via freshwater discharges, as has been of the fishing gear used during the surveys was observed for Smelt from Lake IJssel to the Marsdiep generally too low to get reliable information on tidal basin. variation in local densities. Whilst Atlantic Salmon uses the Wadden Sea only as a corridor to the open Summarized, local stocks of all targeted fish species ocean, Sea Trout is roaming the coastal areas and are influenced by large-scale long-term drivers estuaries to feed and local stocks might therefore such as fisheries, habitat destruction and climate. benefit from available resources. For some species, however, local short-term drivers such as re-introductions, freshwater discharges and For Houting, fish numbers caught were too low environmental quality of the freshwater spawning for further analysis. Incidental catches of this species areas might obscure the influence of these drivers. If throughout the study area suggested, however, that so, then improvement of local conditions might result offspring of successful reproduction of introduced fish in in strengthening of local stocks, within the boundaries River IJssel was caught in the Marsdiep tidal basin in the set by the large-scale long-term drivers. same year. This implies that successful re-introduction programs of diadromous fish in rivers might contribute to local stocks in the Wadden Sea area.
51 3. FINDING OPTIMAL LOCATIONS FOR FISH MIGRATION FROM MARINE TO FRESH- WATER SYSTEMS
3.1 Introduction study area within a day; fish fish-1 d-1). This implies that, for example, a relatively high passing efficiency In order to aid in identifying optimal areas and (with many fish migrating from the sea to freshwater locations for fish migration from the Wadden Sea systems) may result in relatively low abundances and to adjacent freshwater systems, the following basic densities of fish in front of the fish passage (Zwanette information is provided in this chapter. First, an Jager, ZiltwaterAdvies, pers. comm.). Unfortunately, overview of the existing options for fish passing such information is not available. Fish densities might, (including natural estuarine gradients) is presented therefore, only be weakly related to fish supply (Erwin as potential locations for optimisation within present Winter, IMARES, pers. comm.). conditions. Second, the density of migratory fish Furthermore, finding optimal locations requires a at a larger (tidal basin) scale is given as an index of quantitative assessment of fish passing efficiency of potential supply of migratory fish for local options. existing fish passages in the area. Despite the general When fish supply is high, a large number of fish is high costs of building these structures, however, no able to migrate to inland freshwater systems. Third, comparable data to perform a quantitative evaluation the spatial variation in density of migratory fish at of the construction of fish migration facilities were the seaside of existing discharge stations is statistically available. So far, only one study (at Roptazijl) analysed. These local-scale densities are compared to compared the density of fish measured in front of the large (tidal basin) scale spatial variation in densities fish passage with the number of fish that goes through and discussed with regard to local conditions (e.g. (Allix Brenninkmeijer, Altenburg & Wymenga, freshwater discharge). pers. comm.). Interpretation of the results is not easy, however, because different fishing gears were used This analysis has some strong limitations, which should at both sides of this fish passage. In another study (at be taken into account when interpreting the results. Nieuwe Statenzijl), passage efficiency was measured The supply of fish (number of fish that can potentially by means of mark-recapture experiments with dyed migrate within one day; fish d-1) in a fish passage Glass Eel, but results are not yet available (Peter sampling point is actually the product of density (e.g., Paul Schollema, Waterschap Hunze en Aa’s, pers. number of fish per m2), area (m2) and the turnover comm.). More details on these studies are given in the rate of fish (net flux of fish entering and leaving the discussion of this chapter.
52 3.2 Estuarine gradients in the Dutch • Culverts – characterised by an abrupt salinity Wadden Sea gradient, caused by relatively small inputs of freshwater into the sea with an almost freshwater Within the Dutch Wadden Sea area there are 65 environment during low tide and a nearly full saline locations that can be considered as estuarine gradients environment during high tide; as originally put forward by De Boer & Wolff (1996) • Locks – characterised by an abrupt salinity gradient and four more locations, which are planned or under on either side of the ship locks. Leakage of freshwater construction. These gradients vary from tidal creeks into the sea or salt water into the lake does not in saltmarshes to large freshwater sluices that discharge neutralize abrupt salinity gradients; over 300 m3 s-1 on average at an annual basis. The • Salt Water (SW) inlets – found at De Bol on the discharge of freshwater is a reflection of the size of the island of Texel and in Polder Breebaart. Both are inland potential habitat available. We have actualised enclosed brackish areas, where seawater is taken in and revised the list of estuarine gradients by De during high tide; Boer & Wolff (1996) and grouped them according to • Fresh Water (FW) seepages – at several island (Figure 3.1, Table 3.1): locations seepage of freshwater is resulting in estuarine gradients near the coast. These areas can be • Sluices – characterised by an abrupt salinity gradient, flooded with salt water during storm tides; caused by relatively large discharges of freshwater • Creeks – open connections between freshwater and into the sea at low tide, resulting in an almost seawater characterised by a gradual salinity gradient, freshwater environment during low tide and a nearly caused by freshwater (rainwater) coming from the full saline environment during high tide; dunes and marshes, and by saltwater flowing in at • Pumps – characterised by an abrupt salinity gradient, the seaside during high tide. These include island caused by inputs of freshwater into the sea during saltmarsh creeks that are fed by water from the pumping, resulting in a temporary freshwater Wadden Sea, with the exception of De Slufter that is environment; fed by North Sea water. • Estuaries – open connection between a river and the sea.
Fig. 3.1 Locations of estuarine gradients in the Duch Wadden Sea.
53 Many of the estuarine gradients are freshwater sluices, pumps, culverts or locks, some of which are equipped with facilities to enable fish to migrate from the sea to the freshwater system, or to prevent damage to fish when migrating from freshwater to the sea through discharge facilities. Some pumping stations are not suitable for anadromous migration upstream, but downstream migration may be possible for small fish that migrate to the sea (George Wintermans, WEB, pers. comm.). Within the Wadden Sea, there are five main types of fish passage facilities installed, tested or planned (Table 3.1):
• Tide gates or tidal flaps in culverts, or small openings or slots made in medium-scale to large-scale locks; • Fish friendly discharge sluice and ship lock management (FFM). The effectivity varies depending on the way the sluices or locks are operated and on the amount of seawater that is allowed to flow landwards; • Pumps used for fish bypasses (usually with low capacity pumps, and relatively low numbers of fish passing) or fish-friendly pumps (usually with a high pump capacity); • Fish ladders (“cascade”), siphon spillways (“vishevel” in Dutch; using freshwater discharges) or Eel gutters; • Large-scale fish passage constructions that include a collection basin for the inflow of salt water with the incoming flood tide.
Table 3.1 Names, characteristics and freshwater discharge (m3 s-1) of estuarine gradients in the Dutch Wadden Sea at the islands of Texel (TX), Vlieland (VL), Terschelling (TS), Ameland (AM), Schiermonikoog (SC), Rottumerplaat (RP), Rottumeroog (RO) and the coastal mainland of the Provinces North- Holland (NH), Fryslan (FR) and Groningen (GR). Information on estuarine gradients is an update of the overview published by De Boer & Wolff (1996), information on fish passages as supplied by George Wintermans (WEB) and Allix Brenninkmeijer (Altenburg & Wymenga).
54 3.3 Fish densities in tidal basins of The Dutch survey area of the Demersal Fish Survey the Dutch Wadden Sea (DFS) comprises the coastal zone north of the Wadden Sea islands (DFS404), the Wadden Sea (from west to A tidal basin is defined as a water body in a semi east: DFS610, DFS612, DFS616 -619), and the Ems- enclosed coastal area that is subject to tides. Dollard estuary (DFS620). The DFS areas within Tidal basins are useful for comparative research the Wadden Sea not fully match the tidal basins as because they are logical units from morphological, proposed by Kraft et al. (2011). Within this report, hydrodynamic, and ecological perspectives (Kraft et tidal basins are therefore based upon the DFS areas, al. 2011). Within the Dutch Wadden Sea, Kraft et al. i.e. Marsdiep (DFS610), Eijerlandse Gat (DFS612), (2011) distinguished ten tidal basins, i.e. Marsdiep, Vlie (DFS616), Borndiep (DFS617), Zoutkamperlaag Eijerlandse Gat, Vlie, Amelander Zeegat, Pinkegat, (DFS618), Lauwers/Schild (DFS619) and Ems-Dollard Zoutkamperlaag, Eilander Balg, Lauwers, Schild, and (DFS620), (Fig. 2.2). Ems-Dollard (from west to east).
Fig. 3.2 Normalized densities of migratory fish (from top to bottom: European Eel, Twaite Shad, Herring, Houting, Anchovy, River Lamprey, Smelt, Flounder & Sea Lamprey) as determined by means of the Demersal Fish Survey (DFS) between 1970 and 2013.
55 The Demersal Fish Survey (DFS), which started in period (September-October) (Tulp et al. 2015). 1970, was able to fish for most years in most of these Although densities varied from year to year (see tidal basins and the adjacent Dutch coastal zone. Chapter 2 for a more elaborate discussion on On average, fish densities showed strong variations this), some species show more or less stable spatial between years and basins with many low values and distribution patterns in time (Fig. 3.2). Smelt, for only a few high values (see Chapter 2). To reduce the example, generally showed higher densities in strong influence of the extreme values on the overall the larger tidal basins of the western Wadden Sea patterns, annual fish densities (Ft; numbers per ha) (Marsdiep, Vlie) compared to the eastern part of were rescaled to normalised fish densities (Ftn) for each the Dutch Wadden Sea (“O. eperlanus” in Fig. 3.2). species, according to: Anchovy is most commonly found in the Marsdiep
Ftn = (Ft−min(Ft) / max(Ft)−min(Ft)) tidal basin (“E. encrasicolus” in Fig. 3.2). Most other where min(Ft) and max(Ft) are the lowest and highest species show temporal variation as well as spatial values within the data set. patterns. Houting (“C. oxyrhinchus” in Fig. 3.2) was observed once in the Marsdiep tidal basin in 2010 As result of the sampling gear, the DFS results are (see Chapter 2). Flounder, for example, shows high considered to reflect the densities of demersal fish, densities in the central part of the Dutch Wadden Sea such as Flounder, best and less those of the other (Borndiep, Zoutkamperlaag) during the beginning target species (Ingrid Tulp, IMARES, pers. comm.). of the study period (1970-1974), but densities were Furthermore, one should consider the timing of relatively high in the Marsdiep tidal basin between occurrence in the Wadden Sea in relation to the survey 1994 and 2004 (“P. flesus” in Fig. 3.2).
56 3.4 Fish densities near small Recent data (2012-2015; Period II) could be freshwater sluices discharging into compared with older data (2001-2003; Period I) on fish densities derived by similar method at the same the Wadden Sea locations. Sampling was performed by volunteers, and coordinated by WEB in 2012-2015 (Wintermans 3.4.1 Introduction 2014) and by WEB and RIKZ in 2001-2003 (Wintermans & Jager 2003). As part of the project ‘Making way for fish in the Wadden Sea area”, the northern angling federations The catch data were used to explore the spatial and regional water authorities initiated a four-year variation of the density of migratory fish species along monitoring study (2012-2015) on the density of the small sluices and fish passages. The statistical migratory fish at the seaside of relatively small sluices, analysis was restricted to those species (Table 3.4.1), pumps and fish passages along the mainland coast of which were consistently sampled. Flounder was, for the Wadden Sea. During these surveys, 15 out of the example, excluded because larvae were not counted 65 transitions between sea and freshwater systems in at each station. Eel was subdivided into Glass Eel and this area were consistently sampled during late winter Yellow Eel. Herring and Sprat were lumped, because and next spring. Sampling was performed by means not all volunteers could identify these fish up to species of 1x1m lift net with a funnel of 75 cm and a mesh level. Species-specific fish densities (Catch Per Unit size of 1 mm and took place within 2 hrs around local Effort; CPUE) was calculated as the total number of high tide, with 3 to 5 hauls per sampling day, during fish of a particular species caught divided by the total 4 to 5 months per year (February - June). Freshwater number of hauls taken during that year. discharges were always zero during sampling.
CODE NAME LATITUDE (°N) LONGITUDE (°E) 01_DH1 Den Helder Helsdeur 52.946 4.788 02_DH2 Den Helder Oostoever 52.933 4.793 04_ROZ Roptazijl 53.210 5.439 05_ZWH Zwarte Haan 53.309 5.626 06_LO1 Lauwersoog discharge 53.411 6.191 07_LO2 Lauwersoog shiplock 53.405 6.188 08_NPZ Noordpolderzijl 53.433 6.582 09_SPP Spijksterpompen 53.413 6.873 10_DZ1 Damsterdiep 53.329 6.925 11_DZ2 Ems channel discharge 53.328 6.927 12_DZ3 Duurswold 53.327 6.930 13_DZ4 Ems channel shiplock 53.320 6.940 14_TEZ Termunterzijl 53.302 7.038 15_FIE Fiemel 53.299 7.073 17_NSZ Nieuwe Statenzijl 53.231 7.208
Table 3.4.1 Overview of small freshwater discharge sluices that were analysed for spatial variation in densities of European Eel, Herring (lumped with Sprat), Smelt and Three-Spined Stickleback. NB: The originally planned sampling stations “Harlingen” (nr. 03) and “Polder Breebaart” (nr. 16) were excluded from the analyses, because sampling was performed differently than at the other stations.
57 3.4.2 Statistical analyses fish passages). This can either be done visually via a boxplot of the residuals versus locations or by applying After data exploration (Zuur et al. 2010), data a linear regression model in which the residuals are were analysed according to the following stepwise modelled as a function of the categorical covariate procedure for each species (and for Eel for both age location. A significant covariate effect indicates that groups as well). there are residual patterns, which means that the Poisson or negative binomial GLM is faulty. In that Step 1: Effect of Period case a Poisson or negative binomial GLMM that takes The analysis was started by applying a Generalized into account the location effect should be applied. Linear Model (GLM) with the log-transformed
i N µi σ catching effort (number of hauls) as an offset (so that M2: ( 2 ) E = µ in essence we model the ratio) with only Period as an Re(siduals ~i ) i, µ = β +Passage effect, assuming a Poisson distribution of the numbers i Residuals i of fish caught per sampling event (Zuur et al., 2009): The results1 give information on how much of the variation in the residuals (in %) can be explained by Catch Poisson µ M1a: i ( i ) the differences between fish numbers at the different E Catch = µ ( ~i ) i locations. µ = β + β × Period + Effort ( i ) i i log 1 2 log( ) The results show, assuming a Poisson distribution, Step 3: Interaction between Period and if the number of fish caught significantly differed Location for the two periods or not. The fit of the model was If fish numbers differed for the locations per period, checked for overdispersion (i.e. the observed variance there might be an additional need to add a so-called is higher than the variance of a theoretical model) and “interaction” term, i.e. highest density of fish was underdispersion (i.e. less variation in the data than observed at other locations during in the early 2000s predicted by the model). than in the early 2010s. The existence of such a If overdispersion or underdispersion occurred, then difference was checked by means of comparing the the GLM was fitted assuming a negative binomial fits of two different Generalised Linear Mixed Models distribution of the numbers of fish caught per sampling (GLMM’s), assuming a Poisson or negative binomial event: distribution of the data. GLMM’s allow for the analysis of grouped data and complex hierarchical structures Catch NB µ k M1b: i ( i ) in data (e.g., different catches at the locations for the E Catch = µ ( ~i ) i , different periods). One GLMM model included Period µ = β + β × Period + Effort ( i ) i i as fixed term and Fish Passage as random effect (M3), log 1 2 log( ) The negative binomial distribution is a valid choice whilst the other model included Period as fixed term, when the variation in the data is larger than allowed Fish Passage as random intercept and a random slope for by a Poisson distribution. for Period (M4): The results show, assuming a negative binomial distribution, if the number of fish caught significantly Catch NB µ k differed for the two periods or not. If overdispersion M3: i ( i ) or underdispersion still existed, then this could be E Catch = µ ( ~i ) i , attributed to another distribution of the data then µ = β + β × Period + Effort + a ( i ) i i i Poisson or negative binomial, or as the result of a a N 1 2 loi g σ Passage log( ) spatial effect (i.e. differences between numbers of fish 2 caught at the different locations). ~ (0, 1, ) C atch NB µ k M4: i ( i ) E Catch = µ Step 2: Effect of Location ( ~i ) i , µ = β + β × Period + Effort + a + b × Period As a next step, the Pearson residuals of the Poisson ( i ) i i i i i a N 1 2 loi g σ Passage log( ) or negative binomial GLM (model 1a or 1b) were 2 b N 1, i ~ (0,σ Passage ) checked for the presence of a pattern related to the 2 ~ (0, 2, ) different sampling locations (freshwater sluices and
58 The accuracy of the fit of these models was checked Step 5: Identification of significantly different by comparison of additional models assuming Poisson stations distribution of the data and by taking the presence of To include uncertainties in anticipated performance catches with no individuals of the particular species indices (e.g., fish densities at fish passages), the (zero-inflated data sets) into account. statistical analysis of the best model (model 3 or 4, The best model (M3 versus M4) was identified by with or without spatial correlation) was performed means of the Akaike Information Criterion (AIC), a by means of Markov Chain Monte Carlo (MCMC) measure of the relative quality of statistical models for methods. a given set of data (Akaike 1980). The AIC deals with The Markov Chain refers to a random process that the trade-off between the goodness-of-fit of the model undergoes transitions from one state to another on a and the complexity of the model. If the difference in state space, where the probability distribution of the AIC between the models is smaller than 2, then the next state depends only on the current state and not most simple model (in this case M3) is considered to on the sequence of events that preceded it (e.g., rolling be the best one explaining the data. a dice). Monte Carlo refers to running a computer simulation to simulate this random process many Step 4: Spatial and temporal correlation times (e.g., as rolling a dice time after time) and see Significant differences between fish caught at different the probability as the number of a particular outcome locations can be the result of local conditions at the divided by the number of simulations performed (e.g., small freshwater sluices or fish passages or reflecting the number of times that you rolled a five divided by a wider spatial pattern (e.g., more fish in the western the total number of rolls). than in the eastern part of the Dutch Wadden Sea). Significant differences between fish caught at different The Deviance Information Criterion (DIC), an periods can be the result of the periods themselves or estimate of expected predictive error, is a measure reflecting a dependency between years (e.g., if fish is of how well the model fits the data (lower deviance abundant in one year it may also be abundant in the is better). The outcomes of the model allow for next and vice versa). Bayesian post-hoc test to identify which locations are significantly different from each other with respect The existence of such spatial and temporal correlations to anticipated fish densities. These results can be used was analysed by means of interpretation of variograms, to group locations according to their commonalities i.e. graphs that visualise the autocorrelation of a variable (i.e., no significant differences in densities between such as the density of fish at sluices and passages in locations). relationship to space (km) or time (year). The number of unique years (n=7), however, was not sufficient to Statistical analyses were performed by Alain Zuur analyse the existence of temporal correlation. It was (HighStat) using R and relevant packages (lmer). therefore assumed that temporal correlation, if present, Interpretations of the results were checked by George was captured by the random effect of location (the term Wintermans (WEB). “Passage” in models 3 and 4).
59 3.4.3 Results applied (identifying the most simple model as best European Eel (Anguilla anguilla) model, which was Model 3). This implies that fish densities over locations showed a more or less similar Glass Eel stage pattern during the two periods. The variogram did Summary results stepwise analysis not show the presence of spatial correlation in the data • No significant effect of Period set. The parameter values for Intercept and Period of • Significant effect of Location (60% of variation the fit of Model 5 were comparable to those of Model explained) 3 and the DIC was relatively low, indicating a good • No spatial correlation predictive power of this model for the differences in • Densities high at ROZ, ZWH, NPZ & NSZ, and densities of Glass Eel between locations. lower at other stations • Locations fall into 13 density groups In conclusion, the densities of Glass Eel were highest at the fish passages of Roptazijl (ROZ) followed by The best model for Glass Eel with regard to testing the Zwarte Haan (ZWH), Noordpolderzijl (NPZ) and effect of Period was the model that assumed a negative Nieuwe Statenzijl (NSZ), whilst lowest densities were binomial distribution of the data (lowest AIC for found at Den Helder2 (DH2) and Fiemel (FIE) (Fig. Model 1b compared to Model 1a; see Table 3.4.2.1), 3.4.2; Table 3.4.2.2). This spatial distribution was with a non-significant effect of Period on fish densities similar for both periods and not due to larger spatial near fish passages (p > 0.05). The overdispersion of patterns (Fig. 3.4.2; Table 3.4.2.2). Locations were this model (d = 1.88) might be due to a spatial effect. significantly different with regard to density of Glass The residuals of Model 1b showed a strong spatial Eel and could be divided into 13 groups with no effect (p < 0.001) and explained more than 60% of overlap between the first three groups (that included the variation. The AIC of Models 3 and 4 were not the four locations with highest densities of Glass Eel) different enough (difference < 2) to be used as a and the remaining ten locations where densities were criterion for best model, so the parsimony rule was significantly lower (Table 3.4.2.2).
STEP MODEL PARAMETER Value P-VALUE REMARKS 1 MODEL 1A AIC 459197 MODEL 1B AIC 1480 Better model among M11 and M1b Intercept 2.181 ± 0.277 < 0.001 Period 0.202 ± 0.367 0.582 Dispersion 1.88 2 MODEL 2 SS FishPassage 114.44 < 0.001 Multiple r2 0.602 3 MODEL 3 AIC 1372 Best model Intercept 0.601 ± 0.507 0.236 Period -0.045 ± 0.252 0.859 Dispersion 0.79 MODEL 4 AIC 1369 4 VARIOGRAM Spatial correlation Not visible 5 MODEL 5 Nr of iterations 50000 Intercept 0.685 ± 0.596 Period -0.045 ± 0.255 Size 0.828 ± 0.109 Sigma site 2.108 ± 0.483 DIC 1342
Table 3.4.2.1 Summary of results of stepwise statistical analyses of European Eel (Glass Eel stage). Less than 1% of this data set consisted of zero counts.
60 RANK NAME A B C D E F G H I J CODES GROUPS Table 3.4.2.2 Grouping of sampling 1 04_ROZ A I locations for European Eel (Glass 2 05_ZWH AB II Eel stage) as derived from a Bayesian 3 08_NPZ BC III post-hoc test. Locations are ranked 4 17_NSZ C IV from high to low fish densities as 5 10_DZ1 D V derived from Model 2. Green blocks 6 14_TEZ DE VI indicate similarity (non-significant 7 01_DH1 DEFGH VII differences) within a column. 8 11_DZ2 DEFGH VII 9 09_SPP DEFGHI VIII 10 07_LO2 EFGHI VIII 11 13_DZ4 FGHI IX 12 12_DZ3 GHI X 13 06_LO1 HIJ XI 14 15_DEF IJ XII 15 02_DH2 J XIII
01_DH1 02_DH2 04_ROZ 05_ZWH 150
100
50
0 06_LO1 07_LO2 08_NPZ 09_SPP 150
100
50
0 10_DZ1 11_DZ2 12_DZ3 13_DZ4 150
100
CPUE European Eel (glass eel) 50
0 14_TEZ 15_FIE 17_NSZ 150
100
50
0 PeriodI PeriodII PeriodI PeriodII PeriodI PeriodII
Fig. 3.4.2 Boxplots of densities (CPUE) of Glass Eel (Anguilla anguilla) as caught by lift nets at various freshwater sluices and fish passages along the mainland of the Dutch Wadden Sea during 2001- 2003 (Period I) and 2012-2015 (Period II).
61 Yellow Eel stage This implies that the Period effect on fish densities Summary results stepwise analysis at sampling locations did not change over time, i.e. between the two periods. The variogram showed a • A significant effect of Period weak presence of spatial correlation in the data set. • Significant effect of Location (47% of variation The parameter values for Intercept and Period of the explained) fit of Model 5 were comparable to those of Model • The Period effect did not change over time 3 and the DIC was relatively low, indicating a good • No spatial correlation predictive power of this model for the differences in • Densities high at ROZ, NSZ, ZWH & NPZ, and densities of Yellow Eel between sampling locations. lower at other stations • Locations fall into 5 density groups In conclusion, the densities of Yellow Eel were highest at Roptazijl (ROZ) followed by Nieuw Statenzijl The best model for Yellow Eel with regard to testing (NSZ), Zwarte Haan (ZWH) and Noordpolderzijl the effect of Period was the model that assumed a (NPZ), whilst lowest densities were found at Den negative binomial distribution of the data (lowest Helder2 (DH2) and Fiemel (FIE) (Fig. 3.4.3; Table AIC for Model 1b compared to Model 1a; see Table 3.4.3.2). The densities were higher during the first 3.4.3.1), with a significant effect of Period on fish period compared to the second period, but similarly densities near freshwater sluices and fish passages distributed over the locations and could not be (p < 0.001). This model was slightly underdispersed attributed to larger spatial patterns (Fig. 3.4.3; Table (d = 0.79), indicating that the Period effect would 3.4.3.2). Locations were significantly different with be even more significant if the heterogeneity in the regard to density of Yellow Eel and could be divided data would be fully covered. The residuals of Model into 5 groups with no overlap between the first 1b showed a strong spatial effect (p < 0.001) and three groups (that included the four locations with explained almost 47% of the variation. The AIC of highest densities of Yellow Eel) and the remaining Models 3 and 4 was different enough (> 3) to be ten locations where densities were significantly lower used as a criterion for best model, being Model 3. (Table 3.4.3.2).
STEP MODEL PARAMETER Value P-VALUE REMARKS 1 MODEL 1A AIC 1848 MODEL 1B AIC 365 Best model Intercept -2429 ± 0.454 < 0.001 Period -1516± 0.604 0.012 Dispersion 0.79 2 MODEL 2 SS FishPassage 37.49 < 0.001 Multiple r2 0.468 3 MODEL 3 AIC 307 Best model Intercept -6.128 ± 1277 < 0.001 Period -1250 ± 0.402 < 0.001 Dispersion 0.47 MODEL 4 AIC 310 4 VARIOGRAM Spatial correlation Weakly visible 5 MODEL 5 Nr of iterations 300 000 Intercept -6.498 ± 1770 Period -1.259 ± 0.417 Size 0.809 ± 0.226 Sigma site 4.839 ± 1.895 DIC 279
Table 3.4.3.1 Summary of results of stepwise statistical analyses of European Eel (Yellow Eel stage). More than 67% of this data set consisted of zero counts.
62 RANK NAME A B C D CODES GROUPS Table 3.4.3.2 Grouping of sampling 1 04_ROZ A I locations for European Eel (Yellow 2 07_NSZ A I Eel stage) as derived from a Bayesian 3 05_ZWH A I post-hoc test. Locations are ranked 4 08_NPZ A I from high to low fish densities as 5 14_TEZ B II derived from Model 2. Green blocks 6 11_DZ3 BC II indicate similarity (non-significant 7 11_DZ2 CD IV differences) within a column. 8 01_DH1 CD IV 9 06_LO1 CD IV 10 07_LO2 CD IV 11 09_SPP CD IV 12 10_DZ1 CD IV 13 13_DZ4 D V 14 15_DEF D V 15 02_DH2 D V
01_DH1 02_DH2 04_ROZ 05_ZWH 1.5
1.0
0.5
0.0 06_LO1 07_LO2 08_NPZ 09_SPP 1.5
1.0
0.5
0.0 10_DZ1 11_DZ2 12_DZ3 13_DZ4 1.5
1.0
CPUE European Eel (yellow eel) CPUE European Eel (yellow 0.5
0.0 14_TEZ 15_FIE 17_NSZ 1.5
1.0
0.5
0.0 PeriodI PeriodII PeriodI PeriodII PeriodI PeriodII
Fig. 3.4.3 Boxplots of densities (CPUE) of Yellow Eel (Anguilla anguilla) as caught by lift nets at various freshwater sluices and fish passages along the mainland of the Dutch Wadden Sea during 2001- 2003 (Period I) and 2012-2015 (Period II).
63 Hering/Sprat Model 3. This implies that the Period effect on fish Summary results stepwise analysis densities at sampling locations did not change over time, i.e. between the two periods. The variogram • No significant effect of Period showed no presence of spatial correlation in the data • Weak effect of Location (21% of variation explained) set. The parameter values for Intercept and Period • No spatial correlation of the fit of Model 5 were more or less comparable • Densities gradually declining from LO2 to DZ2 to those of Model 3 and the DIC was relatively low, • Locations fall into 7 (partly overlapping) density indicating a reasonably good predictive power of this groups model for the differences in densities of Herring/Sprat between locations. The best model for Herring/Sprat with regard to testing the effect of Period was the model that assumed In conclusion, the densities of Herring/Sprat were a negative binomial distribution of the data (lowest highest at the ship locks of Lauwersoog (LO2) and AIC for Model 1b compared to Model 1a; see Table declined gradually to the lowest densities as observed 3.4.4.1), with no significant effect of Period on fish at the discharge sluices of the Ems canal (DZ2) (Fig. densities near freshwater sluices and fish passages 3.4.4; Table 3.4.4.2). The densities similarly distributed (p > 0.9). This model was slightly overdispersed (d = during both periods and could not be attributed 1.79), indicating a possible spatial effect. The residuals to larger spatial patterns (Fig. 3.4.4; Table 3.4.4.2). of Model 1b showed a weak spatial effect (p = 0.055) Locations were significantly different with regard to and explained approximately 21% of the variation. The density of Herring/Sprat and could be divided into 7 AIC of Models 3 and 4 was different enough groups with large overlaps between groups of locations (> 3) to be used as criterion for best model, being (Table 3.4.4.2).
STEP MODEL PARAMETER Value P-VALUE REMARKS 1 MODEL 1A AIC 22129 MODEL 1B AIC 695 Best model Intercept -0.735 ± 0.422 0.082 Period -0.030± 0.560 0.957 Dispersion 1.35 2 MODEL 2 SS FishPassage 50.31 < 0.001 Multiple r2 0.218 3 MODEL 3 AIC 690 Best model Intercept -1.934 ± 0.599 0.001 Period 0.009 ± 0.615 0.989 Dispersion 0.71 MODEL 4 AIC 694 4 VARIOGRAM Spatial correlation Weakly visible 5 MODEL 5 Nr of iterations 50 000 Intercept -1.576 ± 0.680 Period 0.013 ± 0.651 Size 0.170 ± 0.030 Sigma site 1.720 ± 0.534 DIC 690
Table 3.4.4.1 Summary of results of stepwise statistical analyses of Atlantic Herring & Sprat. Almost 50% of this data set consisted of zero counts.
64 RANK NAME A B C D CODES GROUPS Table 3.4.4.2 Grouping of sampling 1 07_LO2 A I locations for Atlantic Herring & Sprat 2 14_TEZ AB II as derived from a Bayesian post- 3 17_NSZ ABC III hoc test. Locations are ranked from 4 05_ZWH ABCD IV high to low fish densities as derived 5 01_DH1 ABCD IV from Model 2. Green blocks indicate 6 15_DEF ABCDD IV similarity (non-significant differences) 7 13_DZ4 ABCD IV within a column. 8 09_SPP ABCD IV 9 10_DZI BCD V 10 04_ROZ BCD V 11 06_LOI BCD V 12 12_DZ3 CD VI 13 08_NPZ CD VI 14 02_DH2 CD VI 15 11_DZ2 D VII
01_DH1 02_DH2 04_ROZ 05_ZWH 12
9
6
3
0 06_LO1 07_LO2 08_NPZ 09_SPP 12
9
6
3
0 10_DZ1 11_DZ2 12_DZ3 13_DZ4 12
9
6 CPUE Atlantic Herring & Sprat
3
0 14_TEZ 15_FIE 17_NSZ 12
9
6
3
0 PeriodI PeriodII PeriodI PeriodII PeriodI PeriodII Year
Fig. 3.4.4 Boxplots of densities (CPUE) of Atlantic Herring (Clupea harengus) and Sprat (Sprattus sprattus) as caught by lift nets at various freshwater sluices and fish passages along the mainland of the Dutch Wadden Sea during 2001-2003 (Period I) and 2012-2015 (Period II).
65 Smelt (Osmerus eperlanus) to be used as criterion for best model, being Model Summary results stepwise analysis 3. This implies that the Period effect (which was not significant in the first place) on fish densities at • No significant effect of Period sampling locations did not change over time, i.e. • Weak effect of Location (21% of variation explained) between the two periods. The variogram showed a • Weak spatial correlation weak presence of spatial correlation in the data set. • Densities gradually declining from NPZ to DEF The parameter values for Intercept and Period of the • Locations fall into 6 (all partly overlapping) density fit of Model 5 were more or less comparable to those groups of Model 3 and the DIC was relatively low, indicating a reasonably good predictive power of this model for The best model for Smelt with regard to testing the the differences in densities of smelt between locations. effect of Period was the model that assumed a negative In conclusion, the densities of Smelt were highest at binomial distribution of the data (lowest AIC for the pumps of Noordpolderzijl (NPZ) and declined Model 1b compared to Model 1a; see Table 3.4.5.1), gradually to the lowest densities as observed at with no significant effect of Period on fish densities Fiemel (FIE) (Fig. 3.4.5; Table 3.4.5.2). The densities near freshwater sluices and fish passages (p > 0.5). similarly distributed during both periods and could not This model was overdispersed (d = 2.36), indicating be attributed to larger spatial patterns (Fig. 3.4.5; Table a possible spatial effect. The residuals of Model 1b 3.4.5.2). Locations were significantly different with showed a very weak spatial effect (p = 0.067) and regard to density of Smelt and could be divided into explained approximately 21% of the variation. The 6 groups with overlaps between groups of locations AIC of Models 3 and 4 was different enough (> 10) (Table 3.4.5.2).
STEP MODEL PARAMETER Value P-VALUE REMARKS 1 MODEL 1A AIC 28464 MODEL 1B AIC 1135 Best model Intercept -0.118 ± 0.216 0.586 Period -0.162 ± 0.286 0.571 Dispersion 2.36 2 MODEL 2 SS FishPassage 50.31 0.0069 Multiple r2 0.211 3 MODEL 3 AIC 1110 Best model Intercept -0.517 ± 0.335 0.122 Period 0.073 ± 0.269 0.786 Dispersion 1.16 MODEL 4 AIC 1124 4 VARIOGRAM Spatial correlation Weakly visible 5 MODEL 5 Nr of iterations 50 000 Intercept -0.414 ± 0.376 Period -0.074 ± 0.275 Size 0.719 ± 0.095 Sigma site 1.214 ± 0.307 DIC 1098
Table 3.4.5.1 Summary of results of stepwise statistical analyses of European Smelt. Less than 5% of this data set consisted of zero counts.
66 RANK NAME A B C D CODES GROUPS Table 3.4.5.2 Grouping of sampling 1 08_NPZ A I locations for European Smelt as 2 07_LO2 AB II derived from a Bayesian post-hoc test. 3 06_LO1 AB II Locations are ranked from high to low 4 04_ROZ AB II fish densities as derived from Model 5 17_NSZ AB II 2. Green blocks indicate similarity 6 09_SPP B III (non-significant differences) within a 7 01_DH1 B III column. 8 02_DH1 B III 9 14_TEZ BC VI 10 13_DZ4 BC VI 11 10_DZ1 BC VI 12 11_DZ2 BC VI 13 05_ZWH BC VI 14 12_DZ3 CD VII 15 15_DEF D VIII
01_DH1 02_DH2 04_ROZ 05_ZWH 20
15
10
5
0 06_LO1 07_LO2 08_NPZ 09_SPP 20
15
10
5
0 10_DZ1 11_DZ2 12_DZ3 13_DZ4 20
15 CPUE European Smelt 10
5
0 14_TEZ 15_FIE 17_NSZ 20
15
10
5
0 PeriodI PeriodII PeriodI PeriodII PeriodI PeriodII
Fig. 3.4.5 Boxplots of densities (CPUE) of Smelt (Osmerus eperlanus) as caught by lift nets at various freshwater sluices and fish passages along the mainland of the Dutch Wadden Sea during 2001-2003 (Period I) and 2012-2015 (Period II).
67 Three-Spined Stickleback (Gasterosteus (which was not significant in the first place) on fish aculeatus) densities at sampling locations did not change over Summary results stepwise analyses time, i.e. between the two periods. The variogram showed no presence of spatial correlation in the data • No significant effect of Period set. The parameter values for Intercept and Period of • Significant effect of Location (71% of variation the fit of Model 5 were comparable to those of Model explained) 3 and the DIC was relatively low, indicating a good • No spatial correlation predictive power of this model for the differences • Densities high at ZWH, ROZ & NPZ, and lower at in densities of Three-Spined Stickleback between other stations locations. • Locations fall into 9 density groups In conclusion, the densities of Three-Spined Stickleback were highest at Zwarte Haan (ZWH), The best model for Three-Spined Stickleback with Roptazijl (ROZ) and Noordpolderzijl (NPZ) and regard to testing the effect of Period was the model lowest densities were observed at as observed at the that assumed a negative binomial distribution of the ship locks of the Ems canal (DZ4) and Duurswold data (lowest AIC for Model 1b compared to Model 1a; (DZ3) (Fig. 3.4.6; Table 3.4.6.2). The densities Table 3.4.6.1), with no significant effect of Period on similarly distributed during both periods and could fish densities near freshwater sluices and fish passages not be attributed to larger spatial patterns (Fig. 3.4.6; (p > 0.6). This model was slightly overdispersed (d = Table 3.4.6.2). Locations were significantly different 1.50), indicating a possible spatial effect. The residuals with regard to density of Three-Spined Stickleback of Model 1b showed a strong spatial effect (p < 0.001) and could be divided into 9 groups with no overlap and explained more than 70% of the variation. The between the first group (that included the three AIC of Models 3 and 4 was not different enough (< locations with highest densities of Three-Spined 3) to be used as criterion for the best model, therefore Stickleback) and the remaining twelve locations where Model 3 (the most simple model) was considered to densities were significantly lower (Table 3.4.6.2). be the best model. This implies that the Period effect
STEP MODEL PARAMETER Value P-VALUE REMARKS 1 MODEL 1A AIC 515624 MODEL 1B AIC 1662 Best model Intercept 2.612 ± 0.234 < 0.001 Period 0.142 ± 0.311 0.647 Dispersion 1.50 2 MODEL 2 SS FishPassage 108.22 < 0.001 Multiple r2 0.715 3 MODEL 3 AIC 1555 Best model Intercept 1.534 ± 0.414 0.236 Period -0.045 ± 0.252 0.859 Dispersion 0.86 MODEL 4 AIC 1556 4 VARIOGRAM Spatial correlation Not visible 5 MODEL 5 Nr of iterations 50 000 Intercept 1.588 ± 0.467 Period -0.073 ± 0.200 Size 1.207 ± 0.162 Sigma site 1.723 ± 0.388 DIC 1524
Table 3.4.6.1 Summary of results of stepwise statistical analyses of Three-Spined Stickleback. For this species, there were no zero counts in the data set (i.e., these fish were caught in every haul).
68 RANK NAME A B C D E F G CODES GROUPS Table 3.4.6.2 Grouping of locations 1 05_ZWH A I for Three-Spined Stickleback as 2 04_ROZ A I derived from a Bayesian post-hoc test. 3 08_NPZ A I Locations are ranked from high to low 4 14_TEZ B II fish densities as derived from Model 5 01_DH1 BC III 2. Green blocks indicate similarity 6 10_DZ1 BCD IV (non-significant differences) within a 7 06_LO1 BCD IV column. 8 11_DZ2 CDE V 9 17_NSZ CDE V 10 15_DEF CDE V 11 09_SPP CDE V 12 07_LO2 DE VI 13 02_DH2 EF VII 14 12_DZ3 FG VIII 15 13_DZ4 G IX
01_DH1 02_DH2 04_ROZ 05_ZWH
100
50
0 06_LO1 07_LO2 08_NPZ 09_SPP
100
50
0 10_DZ1 11_DZ2 12_DZ3 13_DZ4
100
CPUE Three−Spined Stickleback 50
0 14_TEZ 15_FIE 17_NSZ
100
50
0 PeriodI PeriodII PeriodI PeriodII PeriodI PeriodII
Fig. 3.4.6 Boxplots of densities (CPUE) of Three-Spined Stickleback (Gasterosteus aculeatus) as caught by lift nets at various freshwater sluices and fish passages along the mainland of the Dutch Wadden Sea during 2001-2003 (Period I) and 2012-2015 (Period II).
69 3.4.4. Discussion Spatial correlation For most species, the spatial correlation was not Period significant. Only for Smelt, a weak spatial correlation European Eel (Yellow Eel) was the only group for was found, implying that there might be a larger (e.g., which a significant effect of Period on densities a gradual shift from west to east) than local (freshwater (Period II < Period I) was observed (Table 3.4.3.2; Fig. discharge point) spatial pattern in densities for this 3.4.3). This observation is in line with the observed species. Results of the lift-net surveys (Table 3.4.5.2) decline in densities of European Eel at tidal-basin and bottom trawl surveys during Periods I and II (Fig. scale between 1970 and 2013 in general (Fig. 3.3.1) 3.4.8) revealed, however, no obvious spatial pattern. and for the periods of lift-netting (2001-2003 and 2012-2015) in particular (Fig. 3.4.8). This decline is Stations observed for Eels at a large-scale, and attributed to Stations differed significantly in species-specific atmospherically driven dispersal by ocean currents and densities of fish, with highest densities found change in hydrology and exploitation (see Chapter 2). at Roptazijl and Zwarte Haan (4 out of 5), For Glass Eel and the other fish species, the effect of Noordpolderzijl and Nieuwe Statenzijl (3 out of 5), period was not significant, implying that densities at Lauwersoog shiplock (2 out of 5) and Den Helder the freshwater sampling points did not show an overall Helsdeur, Lauwersoog discharge, Spijksterpompen, increase or decrease from Period I (2001-2003) to Ems channel shiplock, Termunterzijl and Fiemel (1 Period II (2012-2015). out of 5) (Fig. 3.4.8). Annually averaged freshwater discharge rates vary strongly between these stations, Location i.e. with more than 40 m3 s-1 at the discharge sluices The Location effect was significant for all species, of Lauwersoog, between 2 and 11 m3 s-1 at Nieuwe explaining 21% (Herring/Sprat and Smelt), 47% Statenzijl, Den Helder Helsdeur, Den Helder (Yellow Eel), 60% (Glass Eel) and 71% (Three-Spined Oostoever and Termunterzijl, around 1 m3 s-1 at Stickleback) of the variation in local densities. This Roptazijl and Zwarte Haan, and less than 1 m3 s-1 at implies that densities, in particularly of Glass Eel and the shiplocks near Lauwersoog and the Ems channel Three-Spined Sticklebacks, were consistently low or (Table 3.1). high at particular sampling points.
Tidal MD VL BD ZL LS ED Basin
Fish Passage DH1 DH2 ROP ZWH LO1 LO2 NPZ SPP DZ1 DZ2 DZ3 DZ4 TEZ FIE NSZ
Glass eel Yellow eel
Herring Smelt SBckleback
Fig. 3.4.8 Locations at (potential) fish passages with highest densities of migratory fish species as determined by means of surveys in 2001-2003 and 2011-2015.
70 roptazijl caught in the fyke net). They found a multiplication At Roptazijl (Gemaal Ropta), highest densities of factor of 7 for the CPUE of Glass Eel and a factor 17 Glass Eel, Yellow Eel, Smelt and Three-Spined for Three-Spined Stickleback to calculate the numbers Stickleback were found (Fig. 3.4.8). Roptazijl is of fish inside. Furthermore they also found large positioned in the Vlie basin and discharges directly interannual differences and recommended determining into the relative deep water of the Kimstergat. the efficiency of fish passages with mark-recapture A location near deep water is advantageous for experiments with fish tags (3S-Sticklebacks) or fish the attraction of migratory fish. Moreover, the dyeing (Glass Eel). configuration of the breakwaters near Roptazijl is such that there is a deep and wave-sheltered basin in which zwarte haan fish can wait for a suitable moment to pass through, At Zwarte Haan (Gemaal Miedema), highest densities possibly increasing local densities (Fig. 3.4.9). In 2001, of Glass Eel, Yellow Eel, Herring and Three-Spined a siphon spillway (“vishevel”) has been constructed Stickleback were found. In contrast to the conditions at Gemaal Ropta (George Wintermans, WEB, pers. at Roptazijl, the freshwater at Zwarte Haan is not comm.). discharged in deep water, but on the tidal flat. Because the end of the discharge channel has silted up, there At Roptazijl, fish passage efficiency was measured is no connection with the sea during low tide (Fig. from 2002 to 2014 by means of a comparison on 3.4.9). The sluice discharges freshwater every 1 to 2 density of fish at the seaward-side measured in spring days, so there is locally only an intermittent freshwater with a 1x1 m lift net and the fish that passed the fish discharge. Local conditions can, therefore, not easily passage at the inside of Roptazijl in the same period explain the consistently high densities of fish caught using a fyke net (Brenninkmeijer et al., in prep.). by the lift nets at this point. Zwarte Haan is, however, As the result of the species-specific differences in under the influence of freshwater discharges from catchability of the fishing gear, this type of data is very Lake IJssel at Kornwerderzand (Duran-Matute et al. difficult to interpret. The authors have been looking 2014; Fig. 3.4.10). The large-scale gradient in salinity for a multiplication factor for the CPUE with the lift towards the mainland coast may attract fish that will net (the numbers of fish caught in 5 lift-net hauls) as eventually detect small-scale salinity gradients near a proxy for the numbers of fish inside (the numbers local freshwater sluices.
Fig. 3.4.9 Configuration of the freshwater discharge sluices at Roptazijl (left) and Zwarte Haan (right). Images from Google Satellite.
71 Recently (2015) a fish passage has been realised at The regional water authority Hunze en Aa’s is also Zwarte Haan. Fish are lured with freshwater into a studying the fate of fish further upstream in the water collection basin of 100 m3. This water then flows system (Peter Paul Schollema, Hunze en Aa’s, pers. into the polder by gravitational current under a mild comm.). A number of fish passage facilities is, and slope, so the fish do not swim against the flow. The will be, equipped with PIT antennas. This enables the efficiency of this mechanism has to be validated. It tracking of tagged fish that use Nieuwe Statenzijl and now seems that small Eels are trapped within the provides quantitative information on the fish passage culvert since they always swim against the current effectivity in the water system of the Westerwolde (Allix Brenninkmeijer, Altenburg & Wymenga, pers. river basin. This project receives funding from the comm.). Waddenfonds. noordpolderzijl den helder helsdeur Noordpolderzijl is located more or less at the Both freshwater sluices in Den Helder, i.e. Helsdeur tidal divide between the Lauwers and Ems tidal and Oostoever, do not seem to attract that many basins. The configuration of the freshwater sluice fish. Helsdeur is located in the back of Den Helder at Noordpolderzijl is comparable to Zwarte Haan Harbour. This layback position might explain a low and there is a high density in fish. The freshwater is attractiveness. The sluice Oostoever discharges directly discharged into a long gully that runs through the tidal into the Wadden Sea, in a shallow subtidal gully. Two flats. In 2013 a fish passage with a cat-flap has been of the four sluice openings are equipped with boosters constructed here. Due to regular dredging activities with a combined pump capacity of 28 m3 s-1. One of carried out in the Noordpolderzijl channel, the lift-net the four sluice openings has been equipped with a sampling data might underestimate the attractiveness fish passage, but due to malfunctioning this has been of this location without this disturbance (George removed. At present the Regional Water Authority Wintermans, WEB, pers. comm.). HHNK operates with fish-friendly sluice management in all four openings. It has breakwaters and a basin nieuwe statenzijl that make a good resting place. The problem seems to Eight freshwater sluices have been studied in the be the low frequency of discharge, so there is a lack Ems Dollard tidal basin. The one that has the highest of freshwater to lure and retain fish. Wintermans & density of fish is Nieuwe Statenzijl. In 2013, a cat- Dankers (2003) studied possibilities to optimise this flap as well as an eel-gutter have been constructed. functionality. In spring 2014 and 2015 extensive research has been committed by the regional water authority Hunze en lauwersoog discharge sluice & shiplock Aa’s (Peter-Paul Schollema) and Van Hall Larenstein The Lauwersoog sluice and ship locks discharge University of Applied Sciences (Jeroen Huisman). directly into deep water of the Wadden Sea and Besides standard sampling with lift nets and fykes, a in large flows. These are large-scale barriers so large-scale experiment with dyed glass eel was carried the sampling strategy at one spot with a 1x1 m lift out. A large number of dyed glass eels were released net is likely to underestimate the quantity of fish. on the seaside. Glass Eel was subsequently caught day Furthermore, with a discharge rate of more than 40 m3 and night at the seaside, in the eel-gutter and at the s-1, it is not unlikely that the fish abundance and supply cat-flaps during a one-week period. The results of this at the freshwater discharge sluices of Lauwersoog experiment will become available in the beginning of is relatively high compared to stations with similar 2016 and will shed light on the fish passage effectivity densities but lower discharge rates. of Nieuwe Statenzijl.
72 Fig. 3.4.10 Numerical model results for the average concentration of a passive tracer in the upper layer from Den Oever (left) and Kornwerderzand (middle) and average salinity in the upper layer (Duran-Matute et al. 2014).
3.5 Conclusions flow (amount and frequency) of freshwater to attract fish to the entrance location, (2) the closeness of the Due to the lack of data, a quantitative assessment of freshwater sluice to deeper waters that have a salinity the effectivity of existing fish passages in the Wadden gradient, and (3) the presence of a deep and wave- Sea was not possible. With regard to the costs of sheltered basin to allow fish to safely wait before building these structures, it is therefore strongly passing, and (4) good possibilities for survival, growth recommended to invest not only in the strengthening and migration in the freshwater systems. Present of fish migration (a.o. by construction of fish passing and future fish passage facilities should, therefore, facilities) but also in appropriate monitoring before pay attention to selecting and/or creating such local and after strengthening. Not only to quantitatively conditions, including optimal habitats before and after evaluate the activity such as the functioning of the passing. facilities, but also for modifications to optimize the investments made. Such programs should include The relatively high densities at locations that are under monitoring of (1) the numbers and characteristics (e.g., the influence of freshwater discharges at a larger than species, life phases, condition of fish passing, (2) the local scale, underlines the necessity of the presence of efficiency of the attraction flow, (3) the efficiency of large-scale salinity patterns to guide migratory fish the fish passage and, for larger fish passages, (4) the through the Wadden Sea. A variety of studies suggest habitat use of the local area (e.g., for acclimatisation to that migratory fish commonly use olfactory cues to freshwater conditions). locate streams from oceans or lakes (Creutzberg 1959; Dodson & Leggett 1974; Doving et al. 1985). Small Large-scale patterns in fish densities as determined by inorganic ions such as calcium (Bodznick 1978), means of demersal fish surveys for tidal basins do not larger organic compounds associated with microbial show a strong overlap with local densities of migratory decay (Tosi & Sola 1993), and pheromones (Nordeng fish near small freshwater sluices and fish passages. 1977; Li et al. 1995; Vrieze et al. 2011) have all been With the exception of Smelt for which a spatial suggested to serve as innately, or by imprinting during correlation was present but weak, the densities of the certain life stages, recognized cues. Salinity patterns species near the freshwater sluices showed no spatial in the Wadden Sea might, therefore, reveal preferred pattern. These results indicate that local densities pathways for migration of anadromous fish and aid in migratory fish at freshwater discharge points are in identifying optimal areas and locations for fish mainly driven by local circumstances. High migratory migration from the Wadden Sea to adjacent freshwater fish densities near freshwater discharge points appears systems. to be locally facilitated by (1) the presence of sufficient
73 4. NUMERICAL BUDGETS OF LOCAL STOCKS
4.1 Introduction (Pulliam 1988). Within a stable stock, N1 equals N0. Numerical fish budgets require actual and local In order to explore the most effective measures for information on standing stocks, and on rates of the strengthening of fish stocks in the Wadden Sea, recruitment, mortality and migration. Within our quantitative information on major inputs and outputs study area, this implies information is needed on (1) of fish are compared. This exercise was performed local stocks in the Wadden Sea and Lake IJssel, (2) for the Marsdiep tidal basin, Lake IJssel and the main recruitment and mortality (a.o., fisheries, predation) connections between these basins to identify the added rates, and (3) exchange by transport (or migration) of value of the planned Fish Migration River as projected fish between the study area and adjacent waters and by the Provincie Fryslan (2015) under the present exchange within the study area between these basins conditions. via the Afsluitdijk.
Stock dynamics describes the ways in which a given There is no quantitative information on the annual stock grows and shrinks over time, as controlled by exchange of fish between the Wadden Sea, other birth, death, and migration. The basic accounting tidal basins and the North Sea, nor on the exchange relation for stock dynamics is the BIDE (Birth, of fish between Lake IJssel and its surrounding rivers Immigration, Death, Emigration) model, according to: and streams. With regard to passing the Afsluitdijk, various pathways are considered, i.e. (i) via freshwater
N1 = N0 + B − D + I − E discharge sluices under regular discharge management, (ii) via freshwater discharge sluices and shiplocks under where N1 is the number of individuals at time 1, N0 is fish-friendly management, and (iii) via the projected the number of individuals at time 0, B is the number Fish Migration River (Fig. 4.1). Our annual budgets of individuals born (recruitment), D the number that are fully based upon values as derived from existing died (mortality), I the number that immigrated, and E information as supplied in cited reports, articles and the number that emigrated between time 0 and time 1 websites.
Fig. 4.1 Overview of inputs, outputs and local population dynamics (recruitment and mortality) that jointly define the dynamics in abundance of the local fish stocks in the Marsdiep tidal basin of the Wadden Sea and Lake IJssel.
74 4.2 Factors However, Flounder, Twaite Shad, River Lamprey and Smelt are reported as by-catch from shrimp fisheries 4.2.1 Standing stocks (Glorius et al. 2015). Data on annually averaged fishing mortality within the Wadden Sea of Flounder Marsdiep tidal basin (3.78 fish per ha fished), Twaite Shad (0.95 fish per The stock estimates were based upon beam trawl ha fished) and Smelt (6.05 fish per ha fished) were survey data (Demersal Fish Survey; WS_T610 in derived from Glorius et al. (2015). Tessa van der Table 2.2.1) and taken as average densities (number of Hammen (IMARES) supplied shrimp-fishing induced fish per ha) for the period from 2000 to 2014 (autumn mortality for River Lamprey (0.02 fish per ha fished). surveys). These densities were subsequently converted Total area fished per year (ha -1y ) was calculated by to stocks (number of fish per tidal basin Marsdiep) by multiplying the area fished by one fishing vessel per multiplication of the densities with the total sublittoral hour (ha h-1) with total fishing time per year (h -1y ) area (assumed to be 66% of the total Marsdiep area of of shrimp vessels as supplied by Van Overzee et al. 675 km2). (2008). The number of fish that die annually as the result of shrimp fisheries (fish-1 y ) is subsequently Lake IJssel calculated as the area-related fishing mortality (fish The stock estimates were calculated as average ha-1) times the area fished per year (ha -1y ). densities (number of fish per ha) as determined by the bottom trawl surveys (WFD; LIJ_TLIJ in Table 2.2.1) In Lake IJssel, both European Eel and Smelt are multiplied by the total surface area of Lake IJssel (1100 fished. For both species, average landings (tonnes) km2, i.e. excluding Markermeer). Because the beam for the period from 2000 to 2014 were multiplied by trawl surveys were generally performed in autumn, average weight of an individual Eel or Smelt (33.3 which is outside the migration window in spring, and 2.9 gram fresh-weight, respectively) as could be and sampling methods are not geared to many of the derived from the Lake IJssel surveys (WFD; LIJ_TLIJ target species in this report, stock values have to be in Table 2.2.1) by dividing the fresh-weight per ha by generally considered as an underestimate of actual the number of fish per ha. stocks. Predation mortality 4.2.2 Recruitment & Mortality Mammals - Predation by mammals was assumed only Recruitment and natural mortality to take place in the Wadden Sea (although it is known No data are available for local recruitment and from tagging studies that these species undertake mortality rates of fish species. For Lake IJssel, the extensive feeding trips to the North Sea) by Harbour productivity of Smelt in Lake IJssel was estimated to Seals (Phoca vitulina) and Grey Seals (Halichoerus grypus) be 3.1 kg kg-1 in the late 1990s (Mous 2000), but this with daily intakes of 4 kg (Härkönen et al. 1991) and value is based upon biomass and not upon numbers. 4.8 kg fish (Hammill & Stenson 2000), respectively. For the North Sea, natural mortalities of the target It was assumed that the diet of Grey Seals, which species range between 0.05 (European Sturgeon) and includes Flounder and Herring, was similar as that 2.96 (Three-Spined Stickleback) (Table 1.1). These described for Harbour Seals by Brasseur et al. (2004). estimates refer to late juvenile and adult phases and are Total numbers of seals in the Marsdiep tidal basin based upon maximum length and temperature (www. were taken from data as available 2002-2013 via fishbase.se). Natural mortality includes non-human IMARES websites (www.wageningenur.nl/nl/ predation, disease and old age. Therefore, these values Expertises-Dienstverlening/Onderzoeksinstituten/ are probably not representative for natural mortality of imares/show/..) for Harbour seal (…/Populatie- fish stocks in the Wadden Sea and Lake IJssel because Gewone-Zeehonden-in-de-Nederlandse-Waddenzee. of the different environmental conditions (e.g., htm) and Grey Seal (…/Populatie-Grijze-Zeehonden- temperature, predation) and life-phases of the fish in in-de-Nederlandse-Waddenzee.htm) and the these areas. WaLTER dataportal (www.walterwaddenmonitor. org/tools/dataportaal/). Total predation rate (numbers Fishing mortality per year) was subsequently calculated as the species- In the Wadden Sea, commercial fisheries target none specific daily uptake multiplied by the number of seals of the migratory fish species under consideration. and the number of days per year.
75 Since seals not only feed in the Wadden Sea but also in types of data were available (RIZA 2002). Information the North Sea (Brasseur et al. 2004), predation of fish on daily predation pressure on fish from Lake IJssel by seals is most probably overestimated. (460 gram per bird; Leopold et al. 1998) and diet (including European Eel and Smelt) was derived from Birds - Predation by birds in the Marsdiep tidal basin Van Rijn & Van Eerden (2001). and Lake IJssel was estimated for Great Cormorants Predation by birds is probably underestimated, because (Phalacrocorax carbo) only. For the Marsdiep tidal basin, also other birds than Cormorants feed on fish in the annual abundance indices for Cormorants (www. Wadden Sea and in Lake IJssel (e.g. terns, Dänhardt & compendiumofdeleefomgeving.nl) were back- Becker 2011). calculated to actual numbers using the information of the year 1992 where both types of data were available Fish - Predation by fish was only available for Lake (Leopold et al. 1998). Total numbers of Cormorants IJssel, where mainly Pike and Pikeperch were in the Marsdiep tidal basin were estimated as an consuming 49% of the annual production (3.1 kg area-weighted proportion (675 km2 of 2743 km2) kg-1) of Smelt between 1976 and 1994 (Mous 2000), of the average number of Cormorants in the Dutch resulting in a fish-induced mortality of 1.5 kg kg-1. Wadden Sea from 2000 to 2012. Information on daily For the stock budget of Smelt, it was assumed that the predation pressure on fish from the Wadden Sea (460 present proportion consumed by fish is similar to that gram per cormorant) and diet (including Flounder, period. In the Wadden Sea, predation of fish by fish is Smelt and Twaite Shad) was derived from Leopold et assumed to be moderate, because of the relatively low al. (1998). For Lake IJssel, annual abundance indices densities of top-predator fish in this area (being one for Cormorants in a wider area around Lake IJssel of the reasons that the Wadden Sea is thought to be a (www.compendiumofdeleefomgeving.nl) for the good nursery) (Zwanette Jager, ZiltwaterAdvies, pers. period from 2000 to 2011 were back-calculated to com.). actual numbers in the actual Lake IJssel area using the information during the period 1995-2001 where both
76 4.3 Stock Budgets For River Lamprey, the projected FMR migration is higher than observed under regular discharge The calculated budgets first of all illustrate the management conditions (REG), but the summed enormous gaps in data and knowledge that do not value for import from the Wadden Sea to Lake IJssel allow us to come up with confident results for most of of REG and FMR is of the same order of magnitude the considered species (Fig. 4.2- Fig. 4.11). as the export via the discharge sluices and to the There are no data on basic information, such as fishing mortality in the Wadden Sea (Fig. 4.2). Stock recruitment, mortality and rates of exchange between estimates appear comparable for the Marsdiep tidal the study area and adjacent waters. But even for those basin and the Wadden Sea, but probably much too budgets that are more or less filled in with regard to low as the result of the low catchability of this species the other factors, comparison is not fully possible as with bottom trawls (see Chapter 2). Assuming that the result of different sampling methods (e.g., beam River Lamprey shows more or less similar migratory trawls versus diet studies) and different times of the behaviour as Sea Lamprey (Griffioen et al. 2014b), year in which the information is gathered (e.g., import then its present passing efficiency would be 15%. via FFM to Lake IJssel in spring, and densities in the For Sea Lamprey, the projected FMR migration Marsdiep tidal inlet in autumn). Furthermore, some is higher than observed under regular discharge estimates are overestimated (e.g., predation by seals) management conditions (Fig. 4.3). But, as for River whilst others are most likely underestimated (e.g., Lamprey, no information is available on import rates if predation by birds). Interpretation of the budgets to fish-friendly discharge management (FFM) is applied. derive the added value of the FMR and other possible Of the 25 individuals that were tagged and released in measures (e.g., reduction of fisheries) to strengthen the Wadden Sea, however, 4 found their way to Lake local fish stocks should therefore be done with all these IJssel under regular discharge management conditions, limitations in mind. implying a passing efficiency of at least 15% under these conditions (Griffioen et al. 2014b).
Fig. 4.2 Budget for River Lamprey.
Fig. 4.3 Budget for Sea Lamprey.
77 For European Eel, migration by fish-friendly For Atlantic Herring, the projected FMR migration discharge management (FFM) appears to be much is higher than observed for regular and fish-friendly more effective than projected for the Fish Migration discharge management (Fig. 4.5). The FMR estimate River (Fig. 4.4). It must be noted, however, that the that is based upon densities at the seaside of the FFM surveys caught relatively more juveniles than discharge sluices, however, might be much too were caught by fykes (i.e., on which the estimates of high because Herring does not favour freshwater passing via the projected FMR are based). In Lake systems. This might also explain the relatively high IJssel, fisheries and predation by birds appear to be an numbers observed to migrate back from Lake IJssel important source of mortality for Eel. to the Wadden Sea, which might be washed in unintentionally as the result of FFM and tried to get back to the sea. Herring is on the diet of seals.
Fig. 4.4 Budget for European Eel.
Fig. 4.5 Budget for Atlantic Herring.
78 For Twaite Shad, the projected FMR migration For European Smelt, the most complete budget is higher than presently measured during regular could be made (Fig. 4.7). Fish-friendly management discharge management (Fig. 4.6). Again, the added strengthened upstream migration compared to regular value of fish-friendly discharge management is discharge management and is in the same order of not known here. Twaite Shad is one of the species magnitude as projected for the FMR. During regular accidentally caught during shrimp fishing, and on discharge management, the export of Smelt to the the menu of Cormorants. For this species, however, Wadden Sea is comparable to the import into Lake facilitation of migration to Lake IJssel is not considered IJssel, and relatively high compared to the stock. to be of added value to its stock size (Griffioen et al. This is in line with observations by Phungh et al. 2014b). (2015) that almost half of the Smelt stock in the western Wadden Sea originated from Lake IJssel and Markermeer. In Lake IJssel, fisheries and predation by fish and birds appear to be an important source of mortality. In the Wadden Sea, Smelts are caught by shrimp fishers and eaten by cormorants.
Fig. 4.6 Budget for Twaite Shad.
Fig. 4.7 Budget for European Smelt.
79 For, the projected FMR migration is in the same order For Houting, the projected FMR migration is in the of magnitude as measured during regular discharge same order of magnitude as measured during regular management (Fig. 4.8). Export seems, however, larger discharge management (Fig. 4.8). This is in line with than the import (Fig. 4.8). This is in line with the the result of tagged Houting, of which 3 out of 5 results of tagged Sea Trout, which were released in (60%) of the individuals released in the Wadden Sea the Wadden Sea in 1996-1999 and found their way to found their way to Lake IJssel through the discharge Lake IJssel: five out of nine (59%) that were tagged in sluices (Griffioen et al. 2014b). Export seems in the Den Oever and 28 out of 61 (46%) that were tagged same order of magnitude as the import (Fig. 4.9). in Kornwerderzand migrated into Lake IJssel (Bij de Vaate et al. 2003). In a recent study by Griffioen et al. (2014b), the one and only tagged Sea Trout found its way to Lake IJssel through the discharge sluices.
Fig. 4.8 Budget for Sea Trout.
Fig. 4.9 Budget for Houting.
80 For Three-Spined Stickleback, the projected FMR For European Flounder, the projected FMR migration is higher than the migration as determined migration is in the same order of magnitude as under regular and fish-friendly discharge management measured during regular and fish-friendly discharge (Fig. 4.10). During regular discharge management, the management (Fig. 4.11). During regular discharge export of Three-Spined Stickleback to the Wadden management, however, the export of Flounder to the Sea is comparable to the import into Lake IJssel Wadden Sea is higher than the import into Lake IJssel (Fig. 4.10). (Fig. 4.11). Flounder is eaten by cormorants and seals, and induced to fishing mortality.
Fig. 4.10 Budget for Three-Spined Stickleback.
Fig. 4.11 Budget for European Flounder.
81 4.4 Discussion & Conclusions It is not clear if and, if so, how much the additional import of fish into Lake IJssel will strengthen local Compared to regular discharge management, fish- stocks, because of the yet unknown survival rate of the friendly discharge management appeared to enhance fish after entering. Fykenet programs show that Sea the migration or transport of at least European Eel, Lamprey and Sea Trout use Lake IJssel as a corridor Smelt and Three-Spined Stickleback (Fig. 4.12) to more upstream rivers and tributaries during part of within one order of magnitude. This is underlined the year (Winter et al. 2014). Tagged and subsequently by observations during previous testing of discharge released Sea Trouts have been recorded back in the management to enhance fish migration in the early river IJssel, indicating that they do survive (Peter Paul 1990s, which resulted in an increase of Flounder Schollema, Waterschap Hunze en Aa’s, pers. comm.). (Winter 2009) and, possibly, Herring (Chapter 2) in The number of fish that migrate yearly from Lake Lake IJssel. IJssel via the discharge sluices to the Wadden Sea is comparable (European Eel, Flounder, Houting) or an Migration as projected for the Fish Migration River order of magnitude higher (Herring, River Lamprey, is comparable as observed for Sea Trout and Flounder Sea Lamprey, Sea Trout, Smelt) than the estimates of during regular discharge management (Fig. 4.12). the local stocks in the Marsdiep tidal basin (Fig. 4.2- Compared to fish-friendly discharge management, Fig.4.11). It cannot be excluded that this fish migrates projected migration via the FMR is higher for back to Lake IJssel. The high proportion of Smelt in Herring, Smelt and Three-Spined Stickleback, but the Wadden Sea that originated from Lake IJssel and lower for Eel (Fig. 4.12). Markermeer indicates, however, that these freshwater lakes are an important source for the Wadden Sea stocks of this fish species.
Fig. 4.12 Indication of relative effectiveness of measures to strengthen migratory fish populations in the Marsdiep tidal basin (Wadden Sea) and Lake IJssel by means of reduction of fisheries (red) or sluice management (blue) as derived from local stock budgets. The number of stars indicates relative order of magnitude, e.g. within a row, a measure of 3 stars is 1000 (103) times as much effective as another with 1 star (101). REG = regular discharge management, FFM = fish- friendly sluice management; FMR = projected fish migration river.
82 Fisheries (catch and bycatch) appear to be an efficiency) (Bunt et al. 2012). With regard to finding important source of mortality, both in the Wadden the entrance, a choice has to be made to migrate either Sea (Flounder, River Lamprey, Smelt, Twaite Shad) towards the FMR, the ship locks or the discharge and in Lake IJssel (European Eel, Smelt) (Fig. 4.2-Fig. sluices (Winter et al. 2014). Once within the FMR 4.11). For these species, the mortality as the result complex, fish can decide to just stay there (e.g., to of commercial fishing (catch, bycatch) appears to forage) or pass the FMR, including several potential be in the same order of magnitude as the projected barriers (e.g., the breach in the Afsluitdijk and the migration via the Fish Migration River. Furthermore, sluice doors between the FMR and Lake IJssel). Once although it is not allowed at present to land Eel by the fish has successfully passed the FMR, it runs the recreational fishers in Lake IJssel, illegal fishing risk of being flushed back to the Wadden Sea again might still occur (Van der Hammen et al. 2015). In via the discharge sluices (Winter et al. 2014). Because the Marsdiep tidal basin, the by-catch from shrimp the FMR is designed as one river consisting of several fishing on diadromous fish is estimated to be almost parts that are aligned after each other, a bottleneck for 2.5 million fish per year, comprising of Twaite Shad fish in one part will strongly limit the overall passing (213 000 fish per year), Smelt (1 400 000 fish per year), efficiency. In addition, when taking species-specific Flounder (840 000 fish per year) and River Lamprey needs and behaviour into account, an optimal situation (4500 per year). Reduction of fishing activities in Lake for one fish species might be limiting to another. IJssel and the Wadden Sea might, therefore, be an alternative or additional measure to strengthen local An overview of efficiencies of riverine fishways fish stocks within the study area as a whole. indicated that, for nature-like fishways (with 21 evaluations), both the attraction and passing efficiency For the estimations in this chapter, a total passing varied between 0 and 100% (Bunt et al. 2012). On efficiency of 50% (see 4.2.3) for the proposed Fish average, the passing efficiency was higher (mean=70%, Migration River complex was assumed in accordance median=86%) than attraction efficiency (mean=48%, with Provincie Fryslan (2015). Based on a very limited median=50%) (Bunt et al. 2012). Taking both aspects small-scaled number of tagging experiments and into account, the mean of the total passing efficiency several assumptions, however, the passing efficiency of nature-like fishways would then be 50% x 70% at Kornwerderzand under present regular discharge = 35%. When considering all types of fishways conditions appears already to be around 15% for (technical and nature-like), variations in attraction River Lamprey, 25% for Sea Lamprey and 50% or efficiency appeared to be mostly related to variation in more for Sea Trout, Atlantic Salmon and Houting entrance location, near-field hydraulic conditions and (Bij de Vaate et al. 2003, Griffioen et al. 2014b, Ben entrance configuration. Although passing efficiency Griffioen, IMARES, pers. com.). And, compared to was positively correlated to naturalness and negatively regular discharge management, fish-friendly discharge to slope, it could not be excluded the performance management appeared to at least double the passing of nature-like types was largely attributable to slope efficiency for European Eel, European Smelt, Three- rather than to any other intrinsic benefit of their Spined Stickleback and Flounder (Vriese et al., 2015, design (Bunt et al. 2012). Bunt and co-authors (2012) this chapter). Taking such species-specific variation conclude that more work is needed on the design at present into account, an average value of 50% most of nature-like fishways before they can be reliably probably does not correctly addresses future passing prescribed, as they often are, for passing a broad efficiencies under FMR conditions for all target variety of species. species. Although the Fish Migration River has the potential Winter et al. (2014) give an extensive overview of to be of added value, present data and knowledge do the complex and multifactorial configuration of the clearly not justify the projected passing efficiency of FMR that affects the passing efficiency. They indicate 50% for all fish species. Species-specific questions that migratory fish need to make many choices, and that remain include (1) the conditions of fish species need to be very motivated to pass through. Fish has (e.g. life phase, size, physiology, condition) under to be able to find the entrance of a fishway such as which they are motivated to migrate from the sea to the FMR (i.e. attraction efficiency), and subsequently freshwater, (2) species-specific aspects of attraction to successfully pass the fishway itself (i.e. passing efficiency (e.g. entrance location, near-field hydraulic
83 conditions and entrance configuration), (3) species- of fishing activities in the Wadden Sea and Lake specific aspects of passing efficiency (e.g. currents, IJssel is most probably beneficial for stocks of River salinity gradient, bottom structure), and (4) habitat Lamprey, European Eel, Twaite Shad, European requirements of fish when passing (e.g. for feeding and Smelt and European Flounder. Fish-friendly discharge acclimatisation to freshwater conditions). To gather management (FFM) at a structural base would be such knowledge, a dedicated Fish Migration Testing beneficial for European Eel, European Smelt, Three- Facility would be needed with access to natural Spined Stickleback and Flounder. The Fish Migration seawater and freshwater. To our knowledge, such River could have added value to FFM, but only when a testing facility does not presently exist, but these optimally designed. Existing data and information are conditions could be created at Kornwerderzand. presently, however, not sufficient to support design recommendations. By making the design adequate for Summarized, the size of fish stocks in the Wadden testing facilities, however, this necessary knowledge Sea area is generally driven by a combination of could be gained, which is not only relevant for natural factors (e.g. predation by fish, birds and seals) strengthening migratory fish in the Wadden Sea area and by human-induced drivers (e.g. fisheries and but also in other coastal systems with comparable man- supplied options to pass via the Afsluitdijk). Reduction made barriers world-wide.
84 5. POTENTIAL MEASURES AND COSTS
5.1 Introduction Furthermore, fish abundance in the Wadden Sea area is considered to be of main importance for fish- From a historical perspective, large-scale and long- eating birds such as Great Cormorants (Phalacrocorax term anthropogenic pressures such as fisheries, carbo), Common Terns (Sterna hirundo) and Eurasian pollution and habitat destruction have impacted fish Spoonbill (Platalea leucorodia) (Dänhardt & Becker stocks in coastal ecosystems including the Wadden 2011; El-Hacen et al. 2014) and for seals (Brasseur et Sea to historic lows (Wolff 2000; Jackson et al. 2001; al. 2004). The importance of particularly Herring, Lotze et al. 2005; Limburg & Waldman 2009). Twaite Shad, Smelt and Flounder as a food source Although improvement of local conditions might for local birds and mammals (Chapter 4) underlines result in strengthening of local stocks, this can only the importance of strengthening local fish stocks to occur within the boundaries set by the large-scale enhance these natural values of the Wadden Sea as long-term drivers. Large-scale regulations are also well. taken and prove to be successful, as shown for the recovery of Herring in the North Sea following a In this Chapter, an overview is given of potential fishing ban (Dickey-Collas et al. 2010). Acting locally measures (including indications of efforts and costs, if while fish stocks are still hampered by large-scale available) to strengthen local stocks of migratory fish anthropogenic pressures might still be worthwhile, in in the Wadden Sea, based upon the information from anticipating (and possibly even stimulating) further the previous chapters and within the boundaries of the international measures to strengthen fish populations Wadden Sea area covered by the Waddenfonds, i.e. the globally. Wadden Sea, the islands, the tidal inlets, the North Sea coastal zone (up to 3 nautical miles from the coastline) For all species addressed in this study, the Wadden and the municipalities at the mainland bordering the Sea is (or was) an essential area to fulfil a particular Wadden Sea (including a part of Lake IJssel). Measures phase in their life cycle (Zijlstra 1978; Van der addressed include the reduction of fisheries, the Veer et al. 2000, 2015). For the diadromous species provisioning of suitable habitats (such as brackish zones Atlantic salmon, Allis shad, Sea Lamprey, and and freshwater spawning grounds) and the facilitation European Eel, this shallow coastal sea functions as a of fish migration. corridor between freshwater systems and the open sea. Flounder, Herring and Anchovy use this area as a nursery, and as spawning grounds (Anchovy). Sea Trout and Twaite Shad use it for feeding, whilst juvenile European Smelt, Houting and River Lamprey migrate from freshwater to the Wadden Sea to grow and mature. Improving environmental conditions for fish recruitment, growth, survival and migration in the Wadden Sea area might counteract the observed declines in local fish stocks.
85 5.2 Fisheries 5.2.2 Lake IJssel In this freshwater lake, there is direct fishing on 5.2.1 Wadden Sea European Eel (but not for fish smaller than 28 cm, Annual fishing mortality in the Marsdiep tidal basin and not allowed between September and November; as derived from reported by-catch of shrimp fisheries www.sportvisserijnederland.nl/vispas/visserijwet- (individuals per ha fished; Glorius et al. 2015; Tessa en-regels/binnenwater/paling.html) and a recently van der Hammen, IMARES, pers. comm.) and fishing (possibly temporarily) abandoned fishery on Smelt. pressure (ha y-1; Van Overzee et al. 2008), adds up Landings of European Eel were, on average, almost 6 to almost 2.5 million fish per year (see Chapter 4). 000 000 fish and on Smelt almost 200 000 000 fish per This by-catch includes Twaite Shad (213 000 fish -1y ), year during the last 15 years (De Boois et al., 2014; De Smelt (1 400 000 fish -1y ), Flounder (840 000 fish Graaf & Deerenberg 2015; see Chapter 1). y-1) and River Lamprey (4500 y-1). Total numbers of diadromous fish caught in the entire Dutch Wadden A proportion of the stocks of these fish species in Lake Sea would add up to more than 4 million fish per year. IJssel migrate (or are being transported) to the Wadden If this fishing mortality limits local fish stocks, then a Sea via the sluices, i.e. approximately 16 000 Eels and reduction of by-catch could be strengthening these fish 22 600 000 Smelt per year (see Chapter 4). The Eels species and their predators (birds, seals). Reduction of are probably silver eels on their way to the open ocean the fishing efforts is considered to increase additional for reproduction, but Smelt might add to the local natural values of the Wadden Sea (e.g. benthic stocks of the Wadden Sea. For Smelt, for example, organisms and biogenic structures such as mussel beds almost half of the population (45%) in the Marsdiep (Ramsay et al. 1998; Jongbloed et al. 2014), whilst the tidal inlet originated from Lake IJssel and the adjacent yield might remain stable (Temming & Hufnagl 2015). Markermeer (Phung et al. 2015). Strengthening of Turenhout et al. (2016) recently published an overview the fish population in Lake IJssel, e.g. by means of on economic aspects of shrimp fisheries. They structural reduction of fishing efforts, can therefore calculated that, on average between 2012 and 2014, be expected to also to strengthen fish stocks in the 23% of landed shrimp by Dutch shrimping vessels was Wadden Sea. caught in the Wadden Sea. In 2014, 3446 tonnes of Wadden Sea shrimp was landed. The direct revenues For European Eel, the annual market value of were 19.6 million Euro in 2013 and 11.2 million Euro fish caught in Lake IJssel en Markermeer was in 2014. In 2013, the subsequent processing of the approximately 1.5 million Euro (10% silver eel; 90% shrimps created an added value of 9.2 million Euro. larger eel) between 2003 and 2012 (Kees Taal & Additionally, the feasibility of possible alternative Wim Zaalmink, LEI Wageningen UR, presentation sources of income for fishermen are explored in case 2/12/2013). For Smelt, the annual landings of Smelt shrimp fishing in the Wadden Sea would be no longer amounted 2 to 3 million kg in the 1980s and 1990s, possible. but this was considered an unsustainable yield. According to fishermen the maximum sustainable yield is 0.8 million kg per year. Assuming a market price for Smelt of 0.50 Euro per kg, the economic revenue for Smelt fishery would then be 400 thousand Euro per year.
86 5.2.3 Coastal zone 5.3 Brackish water zones For several migratory fish species, long-term variation in the coastal zone north of the Wadden Sea islands 5.3.1 Introduction appears to be different than that in the adjacent North Within the Wadden Sea area (within the Waddenfonds Sea and tidal basins of the Wadden Sea (Chapter 2). boundaries), most estuarine gradients are presently This might be due to natural circumstances typical for characterised by an abrupt salinity gradient. In former this area, but impacts of fisheries or coastal protection days, Anchovy, Herring, Smelt and Flounder used the cannot be excluded (Van der Veer et al. 2015). large open brackish water bodies with tidal influence Sand nourishments, for example, usually take place at in this area to spawn, to grow and to mature (Redeke the foreshore in water depths of -5 to -8 m NAP (De 1939). According to Redeke (1939), Anchovy spawned Ronde 2008), an area that has special significance as in the northern funnel-shaped part of the area (with nursery habitat for fish (Teal & Van Keeken 2011). salinities of 10 ppt). After hatching of the eggs, larvae The recovery time for benthic communities, a moved (by drift and active migration) to the central potential food source for fish, is 4 to 6 years (Baptist part of the basin. Herring deposited its eggs spawned et al. 2008). Shoreface nourishments from Texel on the stems and branches of hydroids, especially to Ameland occurred on average and at different Laomedea gelatinosa, which grew abundantly on subareas, however, once every 3 yrs between 2004 and the sand banks near the western and southern coasts 2014 (Rijkswaterstaat 2014). Impacts might be reduced of the former Zuiderzee (with salinities of 5 ppt). when natural sediments are used, and when foreshore Herring larvae and juveniles subsequently fed on the sand is nourished outside the seasonal windows with diatoms and copepods that occurred in this brackish high fish abundances in the coastal zone (Annelies De environment, until seaward migration in autumn. Backer, ILVO, presentation 28/1/2016). Foreshore Smelt was generally found in the less brackish parts, nourishment started only in 1993, sand was supplied and this fish species migrated upstream to the IJssel onto beaches before that time. Measures could, and other, smaller, rivers to spawn. Juvenile Flounder therefore, include adaptations of the timing (outside entered the former Zuiderzee in early spring, feeding nursery and migration windows) and location (beach) mainly on benthic fauna, i.e. small polychaetes during of sand nourishment. the first weeks after arrival, and then switching to Corophium volutator and Nereis diversicolor. In de Zuiderzee, Flounder stayed approximately 3 to 4 years until they were mature.
In general, strongly developed salinity gradients appear to favour the estuarine recruitment of marine fish species (Schlacher & Wooldridge 1996). In addition to protection and improvement of such areas at present (e.g. small creeks and the Ems estuary), restoration of former gradients may aid in strengthening fish stocks in the Wadden Sea area. Basically, there are two alternative ways to create such large habitats with estuarine gradients, i.e. input of seawater into freshwater systems or (continuous) input of freshwater into marine waters. On the mainland bordering the Dutch Wadden Sea, several large freshwater bodies exist that potentially could be turned into brackish ecosystems, i.e. Lauwersmeer, Amstelmeer and Lake IJssel. Alternatively, the southern part of the Marsdiep tidal basin could become more permanently under the influence of freshwater discharges.
87 5.3.2 Lauwersmeer operated such that a brackish zone persists along this 3 This freshwater lake was formed when a tidal inlet km length that extends to the brackish water into the to the former Lauwerszee was closed off in 1969. deep parts of the Amstelmeer, which creates favourable Full restoration of tidal brackish water dynamics conditions to, for example, Three-spined Sticklebacks is considered not feasible due to ecological and in this area. This species spawns in inland waters with technological constraints (Raad voor de Wadden aquatic vegetation, and overwinter in large freshwater 2008). Because of subsidence of the mainland due to bodies or at sea. A wintering population can be found peat settling, the (formerly natural) brooks in the area in the Amstelmeer (George Wintermans, WEB, pers. can no longer drain by gravity into the Wadden Sea. comm.). In addition, the Lauwersmeer is considered crucial for the freshwater management of the northern provinces. A fish passage from the Wadden Sea towards The migration of fish from Lauwersmeer to streams in Amstelmeer via the Balgzandkanaal appears feasible at Drenthe is dependent on water level management and the freshwater discharge sluice at Oostoever near Den future plans might hinder migration. Furthermore, the Helder (Wintermans & Dankers 2003). Although a natural spawning habitat for Zuiderzee-Herring and fish passage facility has been realised in the discharge Anchovy is coarse sand whilst at present the bottom sluices Oostoever, this is not working properly at the of Lauwersmeer consists of soft muddy sediments. moment (George Wintermans, WEB, pers. comm.). Restoration and maintenance of the bottom of this The Amstelmeer is already connected with Lake lake would require another major operation unless full IJssel via the water system south of Wieringen and tidal dynamics can be restored. the discharge sluice Stontelerkeer near Den Oever. Since the water level of Lake IJssel is higher than The Marnewaard located in the northeastern part the Amstelmeer, the flow direction is towards of the Lauwersmeer area, however, appears to be the Amstelmeer. The water system Oostoever- suitable to be further developed as a brackish zone Balgzandkanaal-Amstelmeer-Stontelerkeer offers (Raad voor de Wadden 2008). This would require a good opportunities for a small fish migration river. newly built sluice in the sea defence at the (former) When the sluices at Oostoever will be opened Vierhuizergat connecting with the Nieuwe Robbengat at flood tide, this can bring in salt water and fish in the Wadden Sea (Raad voor de Wadden 2008). A into the Balgzandkanaal. Brackish conditions with rough cost estimate can be obtained by comparing reduced tidal amplitude can thus be created in the with a similar project near Delfzijl (Oterdum), Balgzandkanaal. An existing sill at the end of the canal where building a new sluice including fish passage prevents further transport of salt into the Amstelmeer. facility and brackish transfer zone was estimated to be The Amstelmeer has rather brackish water at the approximately 3 million Euro (Verhoogt et al., 2014). bottom, but its waters are still used for irrigation. Moreover, by opening the sluices at Stontelerkeer, 5.3.3 Amstelmeer fresh water from the IJsselmeer can flush out the salt In this freshwater lake, the restoration to full tidal water in the Amstelmeer. An additional advantage estuarine conditions with brackish gradients might is that a larger freshwater current can be maintained be possible but only with large adaptations to existing over a longer period at Oostoever, thereby attracting infrastructure. There is, however, an opportunity migratory fish to this sluice. to create a salinity gradient including a brackish zone in the Amstelmeer and Balgzandkanaal, the Flushing with fresh water from Lake IJssel is currently channel that connects the lake with the Wadden Sea not part of the operational water management by (Wintermans & Dankers 2003). The bottom layer the responsible water board. Realisation is thought of water of the Amstelmeer and Balgzandkanaal is to be feasible for only several Euros per day (Sas & already brackish due to saltwater seepage from the Wanningen 2014). Measures to increase the capacity of neighbouring Wadden Sea (Rijkswaterstaat 1979). The the fish passage at Stontelerkeer are necessary as well. Balgzandkanaal has a length of 8 km and the borders No estimate for the construction costs is available at of the first 3 km of this channel do not have any users this moment. that are dependent on freshwater. The sluices might be
88 5.3.4 Lake IJssel 5.3.5 Wadden Sea Because Lake IJssel is a main source of drinking Alternatively to creating brackish environments in water for The Netherlands, salt intrusion is kept to a these freshwater systems, it might be considered minimum. In 2015, a fish passage facility was created to establish a large and permanently brackish zone near the discharge sluices of Den Oever. This facility at the seaside of the Afsluitdijk. At present, this will be operational together with fish-friendly sluice area is already brackish during and after discharge management at the sluices and locks of Den Oever of freshwater from Lake IJssel (Fig. 3.4.10). The and Kornwerderzand. To overcome potential drinking discovery of eggs of Anchovy north of the sluices of water problems from the saline Wadden Sea water Kornwerderzand in 1994 (Boddeke & Vingerhoed coming into the lake, a salt-water return system 1996) and the observed increase in Anchovy stocks in will be installed at both locations (Den Oever and NW European waters since 1995 (Beare et al. 2004), Kornwerderzand). Here the relatively heavy seawater underlines the potential of this area for development is taken in at the bottom near the sluice and pumped into a brackish water zone. In addition, the coarse back into the Wadden Sea (Staatscourant 2014; sediments in this area might provide ample spawning www.rijkswaterstaat.nl/water/projectenoverzicht/ grounds. verbeteren-vismigratie-afsluitdijk/index.aspx). However, when discharges are limited (e.g., during This new system may even allow for an extension high tide and in periods of drought) this area becomes of the brackish zone in Lake IJssel, in particular if saline again. Due to expected accelerated sea level rise this area is semi-enclosed by means of a wall. First it will become more difficult to discharge freshwater calculations of such a design suggest that even without using gravity at low tides. To compensate for sea additional pumping, salinity in Lake IJssel can be level rise, Rijkswaterstaat plans to install pumps at controlled by management of freshwater discharges Den Oever in order to maintain the present drainage (Marcel Klinge & Coen Kuiper, Witteveen+Bos, capacity of Lake IJssel (mirt2016.mirtoverzicht.nl/ presentation 5/3/2015). The advantages of having a mirtgebieden/project_en_programmabladen/527. brackish water zone in Lake IJssel are threefold, i.e. (i) aspx). Such discharge pumps could potentially migratory fish such as Flounder (Jager 1998) that enter offer a continuous outflow of freshwater into the Lake IJssel via the discharge sluices will get more time Wadden Sea. Average discharges of Den Oever and to adapt to freshwater conditions than at the present Kornwerderzand add presently up to 530 m3 s-1, situation, (ii) unintentional transport of marine fish implying that high and continuous pumping capacity in Lake IJssel can be corrected by return flushing, and is required in order to create a continuous brackish (iii) unintentionally flushing of freshwater fish might zone. In particular during dry periods, such amounts be reduced as the increase of salinity towards the of freshwater might not be available for discharging sluices might act for them as an early warning system. into the Wadden Sea (Theo van der Kaaij, Deltares, Total costs were estimated to be between around 40 pers. comm.). Limiting the additional supply of million Euro, excluding VAT (Marcel Klinge & Coen freshwater to the Wadden Sea during the periods with Kuiper, Witteveen+Bos, presentation 5/3/2015). surplus of freshwater, however, is in line with natural Because The Netherlands is highly depending on salinity variations in estuaries and might be sufficient Lake IJssel as a source for drinking water, such to aid in strengthening local fish stocks by supplying measurements can only be considered if the restriction brackish conditions during their main migration and of intrusion of salt water into this freshwater lake can spawning periods. be guaranteed.
89 5.4 Freshwater habitats River Lamprey is known to spawn in the Gasterensche Diep, a tributary to the Drentsche Aa (Riemersma & Once fish has passed from the sea to the freshwater Kroes 2004, Brouwer et al. 2008; Winter et al. 2013, system, it should be able to reach its final destination. Hofstra 2014; Schollema 2015) and can find suitable In general, this implies that connectivity between all habitat in the IJssel and Vecht tributaries. Studies tributaries, polder ditches and brooks are important into River Lamprey in the Drentsche Aa show that to population sizes of migratory fish. In addition, not the availability of spawning and nursery habitat fish might require particular conditions in freshwater is currently limiting, but migration is. The biggest habitats, e.g. to spawn. problem is in the long stretches of almost stagnant water in the canals between the sea sluices and the Twaite Shad is a species that spawns on gravel beds Drentsche Aa. In years with wet winters and high or coarse sand of tidal rivers near freshwater tidal freshwater discharges towards sea, the resulting interfaces. It is presently unknown if and, if so, increased flow velocities attract and accommodate where Twaite Shad spawns in the Dutch Wadden Sea the landward migration of River Lampreys (Peter regio. Suited conditions for spawning can, however, Paul Schollema, Waterschap Hunze en Aa’s, pers. probably be found in (side channels of) the upstream comm). For River Lamprey, both safeguarding of the Ems, a river presently characterized by high fluid mud habitat quality and the connectivity between various concentrations and low oxygen levels during summer. tributaries in the northeastern part of The Netherlands Since fine sediment, either in suspension or when and the sea are important for strengthening their deposited, can have multiple negative impacts on fish populations in the Wadden Sea region. (Kemp et al. 2011), the water and habitat quality of the river Ems needs to be improved drastically before becoming an important spawning ground. Sea Trout, Atlantic Salmon and Allis Shad spawn in upstream parts of river basins, mainly in the Rhine and accessible via the IJssel (De Groot 2002). It is questionable if sufficient habitat is available for a spawning population of Sea Trout in small Dutch streams such as the Drentsche Aa (Peter Paul Schollema, Waterschap Hunze en Aa’s, pers. comm.). Sea Lamprey spawns on gravelly substrates in upstream parts of large rivers and might find suitable habitat in (side-channels of) larger rivers such as the Ems and IJssel. Such local habitat improvement in small streams can relatively easily be achieved by re-meandering without the need for introducing gravel in these systems (Peter Paul Schollema, Waterschap Hunze en Aa’s, pers. comm.).
90 5.5 Fish passages More expensive solutions are pumps. Pumps can be used for fish bypasses (usually with low capacity 5.5.1 Introduction pumps) or the pumps themselves are fish-friendly Over the past years, many studies have listed (usually with a high pump capacity). The construction bottlenecks for fish migration in The Netherlands (e.g. costs for pumps are highly dependent on the capacity Jager 1999, Wanningen & Van Herk 2007, Wymenga and the ‘hydraulic head’, i.e. the difference in surface & Brenninkmeijer 2007, Kroes et al. 2008). These elevation between the two water bodies. The costs for studies stress the importance of unhindered fish pumps range between 50 and 900 thousand Euro. migration for sustainable populations of migratory Fish passages can, in some cases, also be realised species. To facilitate migration via potential barriers, without pumps in the form of fish ladders, sliding a suite of technical solutions is available. The doors or siphon spillways (“vishevel” in Dutch). The construction costs for the realisation of fish passages construction costs are dependent on the design and differ widely and are dependent upon type and scale scale and fall in a similar range as for pumps, i.e. 45 to (Table 5.1 and references herein). 657 thousand Euro.
Relatively simple and low cost solutions for tidal The most expensive are fish passage constructions that barriers are tide gates, tidal flaps or valves in small- include collection and return flow for the inflow of salt scale barriers, or small openings or slots made in water with the incoming flood tide, of which the costs medium to large-scale locks. These can be realised for range between 2.9 and 5 million Euro. The high costs 5 to 27.5 thousand Euro. Another low-cost solution is (52 million Euro) of the Fish Migration River are formed by fish-friendly sluice management, which can related to its size and its complexity. be realised in medium to large-scale barriers for 5 to 50 thousand Euro.
Ref. Type of passage Barrier Scale Costs (Euro) Remarks Gates, valves, slots 1 Adjustable tidal flap tidal small 5,000 1 Self regulating tide gate tidal small 27,500 Range of 25 to 30 thousand Euro
Fish-friendly lock management 1 Fish-friendly lock management tidal medium to 5,000 Software costs large 2 Fish-friendly lock management inland small 10,000 For defining a protocol 2 Fish-friendly lock management tidal medium to 37,500 Software costs; 25 to 50 thousand Euro large Fish ladders, siphon spill ways, sliding doors 2 DeWit fish ladder inland small 45,000 2 Manshanden siphon spillway fish tidal small 155,000 Range of 130 to 180 thousand Euro passage 3 Fish bypass at pumping station tidal medium to 500,000 large 2 Siphon spillway fish passage tidal small 510,000 1 100 m long culvert with two sliding tidal small 657,000 doors Fish-friendly by-pass pumps Table 5.1 Estimated costs of 1 Fish bypass at pumping station tidal small 140,000 Pump capacity 7.25 m3/min 1 Fish-friendly screw pump inland medium 130,000 Pump capacity 2x 25 m3/min, Head 1.0 m various types of fish passages. 1 Fish-friendly screw pump inland medium 200,000 Pump capacity 2x 32 m3/min, Head 0.8 m Sources: 1 Fish-friendly pumping stations by inland small 250,000 Pump capacity 18 m3/min, Head 1.05 m 1 Heemstra & Veneberg Venturi system 1 Archimedes screw pump with return inland small 318,000 Pump capacity 8.3 m3/min, Head 2,68 m (2012); flow and lights 2 www.scheldestromen.nl/ 1 Fish-friendly screw pump inland medium 335,000 Pump capacity 2x 60 m3/min, Head 1.5 m 1 Fish-friendly screw pump inland medium 677,000 Pump capacity 2x 40 m3/min, Head 1.95 m actueel/nieuwsberichten 1 Fish bypass at pumping station inland small 900,000 Pump capacity 1.2 m3/min, Head 4.35 m /@215422/innovatieve; Fish passage with salt water return flow system 3 Verhoogt et al. (2014); 4 Fish bypass at pumping station with tidal medium to 2,900,000 brackish basin large 4 Heuer et al. (2011); 5 Siphon spillway fish passage with tidal large 4,500,000 Range of 4 to 5 million Euro 5 Brenninkmeijer & brackish basin Fish Migration River Wymenga (2007); 6 Fish migration river tidal large 52,000,000 6 Ecorys (2015)
91 5.5.2 Small passages 5.5.3 Passing at the Afsluitdijk Based on the comparison of fish densities near A study by Arcadis (2014) on preferred alternatives freshwater outlets at the mainland into the Wadden of fish-friendly lock and sluice management (FFM) Sea, it appears that high densities near freshwater at the Afsluitdijk gives a qualitative estimate of discharge points are locally facilitated by (1) the its costs (Table 5.2). The implementation of fish- presence of sufficient flow (amount and frequency) of friendly management mainly requires adjustments freshwater to attract fish to the entrance location, (2) in the working protocols because of the number and the closeness of the freshwater sluice to deeper waters complexity of additional actions controlling the sluices that have a salinity gradient, and (3) the presence and locks. Some alternatives require large changes in of a deep and wave-sheltered basin to allow fish to control actions and additional staff might be necessary. safely wait before passing, and (4) good possibilities For some alternatives technical adjustments in the for survival, growth and migration in the freshwater control panel for the lock and sluice operation are systems (see Chapter 3). desired (Table 5.2).
The relatively high densities at locations that are under To estimate the potential added value of a Fish the influence of freshwater discharges at a larger than Migration River (FMR), the prospected numbers local scale, underlines the necessity of the presence of fish passing the Afsluitdijk were compared with of large-scale salinity patterns to guide migratory those being observed during fish-friendly discharge fish through the Wadden Sea. Present and future fish management (FFM) of sluices and shiplocks (Table passages should, therefore, pay attention to creating 5.3). It must be noted that this comparison is based on such conditions at basin-wide and local scale, including a relatively small data set (FFM) and on abundances optimal habitats for migration through the Wadden on fish in front of the Afsluitdijk of which 50% of all Sea and for retention areas before passing. fish species was assumed to pass the FMR, and that no distinction was made between fish of different size and/or age classes (see Chapter 4).
Afsluitdijk discharge sluices Alternative Open both doors Open both doors Use sluice doors Use sluice doors at large water level at small water level separately as fish with variable difference (10 – 20 difference (< 10 cm) lock opening cm) Costs Minor adjustment to Minor adjustment to New action; extra New action; extra sluice control sluice control personnel needed, personnel needed, control panel control panel adjustments desired adjustments desired Afsluitdijk ship locks Alternative Use small openings Use one open door Use two open doors in both lock doors and small openings in the other Costs Few extra personnel Some extra Many extra needed, control personnel needed, personnel needed, panel adjustments control panel no adjustments desired adjustments desired
Table 5.2. Qualitative assessment of costs for fish-friendly lock and sluice management at the Afsluitdijk (Arcadis 2014).
92 Under these assumptions, the FMR appears to be The total initial one-time investment cost of the more effective than FFM for Herring and Three- FMR is 52 million Euro, i.e. 18 million Euro Spined Stickleback (Table 5.3). For Smelt, Sea Trout (including VAT) for the passage in the Afsluitdijk, and Flounder, the numbers of fish passing as the result and approximately 34 million Euro (excluding VAT) of fish-friendly sluice management (FFM) appear to for the facilities at the northern and southern side of be comparable to that as projected for the FMR (Table the Afsluitdijk including 1 million for maintenance 5.1). Because Smelt has a migrating and a landlocked and management. If annual costs for maintenance population (Tulp et al. 2013, Phung et al. 2015), and management range between 1% (FMR specific; improving connectivity between both populations by Meinard Bos, DNA, pers. comm.) and 2% of the either method might increase population size of Smelt investment costs (general investments Afsluitdijk; in the Wadden Sea. For European Eel, fish-friendly Ecorys 2015), then this would be approximately 500 sluice and shiplock management (FFM) would bring in 000 thousand to 1 million Euro per year. These costs much more fish in than the FMR. Since the facilities will be annually budgeted. are already in place, fish-friendly management via discharge sluices and shiplocks appears to be a highly cost-effective measure that can contribute to the restoration of the Eel population on Lake IJssel.
REG (x103) FFM (x103) FMR (x103) Ratio Ratio REG:FFM:FMR FFM:FMR River Lamprey 0.12 no data 2.4 1: nd : 20 no data Sea Lamprey 0.06 no data 0.9 1 : nd : 14 no data European Eel 0.6 105094 608 1 : 17516 : 1015 17 : 1 Herring 800 200 111800 4 : 1 : 559 1 : 559 Twaite Shad no data no data 1203 no data no data Smelt 400 9700 19400 1 : 24 : 49 1 : 2 Sea Trout 0.06 no data 0.09 1 : nd : 2 1 : 2 Houting 2 no data no data no data no data Stickleback 400 1100 30100 1 : 3 : 75 1 : 27 Flounder 20 50 60 1 : 3 : 3 1 : 1 Total 1623 116114 163174 1 : 72 : 101 1 : 1
Table 5.3 Numbers of fish potentially passing annually through the Afsluitdijk with regular discharge management (REG), fish-friendly discharge management (FFM) and as projected for the fish migration river (FMR; assuming 50% total passing efficiency). Ratios are given in rounded numbers (see Chapter 4 for details on estimates).
93 5.6 Concluding remarks To estimate the added value of a new fish passage, monitoring should be performed at all other Construction of brackish zones within freshwater surrounding fish passages before and after a new systems should be preceded by feasibility studies, passage is constructed. Otherwise, it will not be including the behaviour of salt water and possible possible to distinguish between additional fish passing consequences for other use of the water (e.g. or fish passing through the new facility (and no longer agricultural, drinking). To come up with an optimal through the old facilities). This would require an design, additional measurements on flows, tides, integrated monitoring scheme for migratory fish in the salinity gradients and fish passage rates under present Wadden Sea. With standard techniques, equipment conditions might be required. If a construction of a and protocols, and addressing the preferred pathways more permanently brackish zone at the northern side of migratory fish species at various spatial (e.g., of the Afsluitdijk is considered, potential impacts of freshwater discharge points, tidal basins) and temporal pumps on migrating fish from freshwater to the sea (tide, season, year-to-year) scales. Incorporating the should be kept to a minimum (e.g. by means of using migratory behavior of fish into hydrodynamic models fish-friendly pumps) and an inventory of possible could produce biophysical models of fish movements effects of additional freshwater discharges on local through the Wadden Sea. Such models would allow species and habitats in this zone should be made (c.f. for building scenarios for optimizing investments to Smit et al. 2003). strengthen (migratory) fish stocks in the Wadden Sea under various environmental conditions such as the Unfortunately, the efficiency of existing fish passages creation of large brackish zones, climate change and could not be quantified due to lack of data. For present sea level rise. and future passages, measurements on fish abundances should be performed in a comparable way before and after such facilities, e.g. by means of comparable fishing gear and/or mark-recapture experiments. To evaluate efficiency and to develop knowledge on optimal designs, monitoring programs should include monitoring of (1) the numbers and characteristics (e.g., species, life phases, condition) of fish passing, (2) the efficiency of the attraction flow, (3) the efficiency of the fish passage and, for larger fish passages, (4) the habitat use of the local area (e.g., for feeding or acclimatisation to freshwater conditions).
All potential actions to strengthen fish stocks in the Wadden Sea area, in particular the ones that are long-term and costly, should be considered and ranked in the light of climate change. The supply of Anchovy, European Eel and Herring to our waters appears to be driven by large-scale climate-induced currents and increasing water temperature. At a more local scale, Smelt migrates to deeper and cooler places (such as old gullies or sand mining pits in Lake IJssel, or to the open North Sea) when confronted with water temperatures higher than 20°C (De Leeuw et al. 2008). The strength and locations of estuarine gradients will be determined by the supply of freshwater and by wind-driven currents (Duran Matute et al. 2014). Sea level rise will hamper the future natural flows of freshwater by gravity into the sea.
94 ACKNOWLEDGEMENTS
We acknowledge:
Loes Bolle (IMARES) Meinard Bos (Provincie Fryslân) Allix Brenninkmeijer (Altenburg & Wymenga) Erik Bruins Slot (Provincie Fryslân) Ingeborg de Boois (IMARES) Wilco de Bruine (LINKit Consult) Martin de Graaf (IMARES) Jan Doornbos (Provincie Fryslân) Dennis Ensing (Agri-Food and Biosciences Institute Northern Ireland, IRL) Eelke Folmer (Ecospace) Ben Griffioen (IMARES) Henk Heessen (IMARES) Zwanette Jager (Ziltwater Advies) Guus Kruitwagen (Witteveen+Bos) Romuald Lipcius (Virginia Institute of Marine Science, USA) Roef Mulder (Provincie Fryslân) Gualbert Oude Essink (Universiteit Utrecht) Jaap Quak (Sportvisserij Nederland) Peter Paul Schollema (Waterschap Hunze en Aa’s) Ingrid Tulp (IMARES) Tessa van der Hammen (IMARES) Theo van der Kaaij (Deltares) Henk van der Veer (NIOZ) Hans van Hilten (Waddenfonds) Herman Wanningen (PRW) Erwin Winter (IMARES) George Wintermans (Wintermans Ecologen Buro) Alain Zuur (Highland Statistics) Joey Zydlewski (University of Maine, USA) for supplying data, supplying information, data analyses, reviewing or a combination of these much appreciated contributions to this report.
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