Noble Crayfish (Astacus Astacus) in a Changing World – Implications for Management

Thesis for the degree of Doctor of Philosophy, Sundsvall 2012

NOBLE CRAYFISH (ASTACUS ASTACUS) IN A CHANGING WORLD – IMPLICATIONS FOR MANAGEMENT

Jenny K. M. Zimmerman

Supervisors:

Professor Thomas Palo

Professor Nils Ekelund

Department of Natural Sciences, Engineering and Mathematics Mid Sweden University, SE-851 70 Sundsvall, Sweden ISSN 1652-893X,

Mid Sweden University Doctoral Thesis 126 ISBN 978-91-87103-17-9 Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall framläggs till offentlig granskning för avläggande av filosofie doktorsexamen fredagen den 25 maj, 2012, klockan 10.15 i sal L 111, Mittuniversitetet Sundsvall. Seminariet kommer att hållas på svenska.

NOBLE CRAYFISH (ASTACUS ASTACUS) IN A CHANGING WORLD – IMPLICATIONS FOR MANAGEMENT Jenny Zimmerman

The picture of the front cover page illustrates the success of the reintroduction of noble crayfish to the River Ljungan. (Photo: Jenny Zimmerman 2011). The picture of the back cover page illustrates the noble crayfish in a changing world (Figure: Karin, Edit & Irma Zimmerman 2012).

Department of Natural Sciences, Engineering and Mathematics Mid Sweden University, SE-851 70 Sundsvall Sweden

Telephone: +46 (0)771-975 000

Printed by Kopieringen Mid Sweden University, Sundsvall, Sweden, 2012

NOBLE CRAYFISH (ASTACUS ASTACUS) IN A CHANGING WORLD – IMPLICATIONS FOR MANAGEMENT

Jenny K. M. Zimmerman

Department of Natural Sciences, Engineering and Mathematics Mid Sweden University, SE-851 70 Sundsvall, Sweden ISSN 1652-893X, Mid Sweden University Doctoral Thesis 126; ISBN 978-91-87103-17-9

ABSTRACT The noble crayfish (Astacus astacus) is critically endangered in Sweden. This is mainly due to the crayfish plague (Aphanomyces astaci), a lethal disease that, among other things, can be spread through the stocking of fish from contaminated water or contaminated fishing gear. The largest single propagation path is the illegal introduction of infected signal crayfish (Pacifastacus leniusculus). A conservation measure for crayfish is to re-introduce it to where it has a chance to survive, though a sustainable, locally regulated fishing can also serve as an indirect protection for the species. When the local inhabitants are allowed to keep their fishing culture and when fishing is acceptable, the incentive for illegal stocking of signal crayfish is low. However, it is important to avoid overfishing because the recovery, especially in the northern regions, can take several years. Therefore, it is important to know how crayfish respond long-term to fishing and environmental factors. Crayfish populations became extinct in the River Ljungan for unknown reasons in 1999. The water flow of the river has been used for activities such as fishing, timber transport and hydroelectric power since the 1500s, and the noble crayfish has been part of the fauna since the last century. The River Ljungan was known as one of Sweden's best fishing areas for crayfish and fishing became an important part of the local tradition. When the crayfish populations became extinct, a reintroduction program was a natural step, and crayfish are nowadays re-established in the river. From 1963 to 1990 the Swedish Board of Fisheries collected data from crayfish fishing in the River Ljungan to determine the economic damage to fishery owners caused by the construction of a power plant. After

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each season the fishermen reported the catch. In this thesis, the data was used to investigate which factors influence the long-term size of the crayfish catch and how the crayfish catches were affected by the power plant building. After re-introduction of the crayfish to the River Ljungan, the local fishermen monitored the population development in a simple, standardized way. To examine the validity of their measurements and to investigate the body growth of the individuals, a capture-recapture technique with a permanent marking of the crayfish was used. The crayfish catches were primarily impacted by the previous years' catch size, and a large catch the previous year resulted in a reduced catch the following year. A mild winter climate (NAO-index > -0.7) six years before the catch implied a large catch, whereas a high water flow during the autumn or spring (>95m3s-1) two years before the catch, implied a poor catch. Major habitat changes in the form of greatly reduced water flow (~90%) were negative for crayfish catches. The standardized method of fishing used by the local fishermen to monitor the development of the crayfish population was precise enough to detect population trends and this method can therefore be recommended to monitor future re-introductions of crayfish. Although the River Ljungan is located at the northern edge of the species' range, noble crayfish in the river presently have a body growth rate that is close to the maximum measured for crayfish (8 mm/moult for females and 10 mm/moult for males). Based on the results, the most important advice for sustainable fisheries in Ljungan and other northern rivers is to: Monitor the population trends, NAO-index and water flow in May and October. Use the results from the monitoring to determine the number of allowed fishing days and traps. Collect data about the catch size and efforts from legal fishing and use it to evaluate the sustainability of the fishing. Enhance the buildup of the harvestable cohort by -saving reproductive females -introduce a size limit of 10 cm -provide proper shelters for the non-harvestable cohort.

Keywords: Noble crayfish (Astacus astacus), CPUE, climate change, water regulation, body growth, reintroduction, sustainable fishing, monitoring.

SVENSK SAMMANFATTNING

Flodkräftan i en föränderlig värld – förutsättningar för skötsel Flodkräftan (Astacus astacus) är akut hotad i Sverige främst på grund av kräftpest (Aphanomyces astaci). Kräftpest är en dödlig sjukdom som bland annat kan spridas vid fiskutsättningar från smittade vatten eller med smittade fiskeredskap. Den enskilt största spridningsvägen är illegala introduktioner av smittade signalkräftor (Pacifastacus leniusculus). En bevarandeåtgärd för flodkräfta är att återintroducera den till lokaler där den har chans att överleva, men ett hållbart fiske med lokal styrning kan också fungera som ett indirekt skydd för arten. När lokalbefolkning tillåts att behålla sin fiskekultur och fisket är bra, blir incitamentet för illegal inplantering av signalkräfta lågt. Men det är viktigt att undvika överfiske då återhämtning, speciellt i nordlig miljö kan ta åtskilliga år. Därför är det betydelsefullt att veta hur flodkräftan svarar på fiske och omgivningsfaktorer i det långa loppet.

Flodkräftbeståndet dog, av okänd anledning, ut i Ljungan 1999. I Ljungans flöde har det fiskats, flottats timmer och utvunnits vattenkraft etc. sedan 1600-talet och under det senaste århundradet har flodkräftan varit en del av Ljungans fauna. På sin tid var det en av Sveriges bästa lokaler för flodkräfta. Kräftfisket blev en viktig del av den lokala traditionen, så när kräftbeståndet dog ut var återintroduktion en självklarhet och flodkräftan har åter sin hemvist i älven.

Under perioden 1963 till och med 1990 samlade Fiskeriverket in data från kräftfisket i Ljungan för att fastställa den ekonomiska skadan som fiskerättsägarna åsamkats i samband med ett kraftverksbygge. Efter varje säsong fick fiskarna rapportera hur fisket gått. I den här avhandlingen, har det materialet använts för att undersöka vilka faktorer som påverkar kräftfångstens storlek på lång sikt och hur kräftfångsterna påverkades av kraftverksutbyggnaden. Efter återintroduktionen av flodkräfta till Ljungan mättes beståndsutvecklingen på ett enkelt, men standardiserat sätt av de lokala fiskevårdsområdena. För att undersöka validiteten av deras mätningar och hur kräftornas individuella utveckling fortskred, användes fångst- och återfångstteknik, med permanent märkning av kräftorna.

Kräftfångsternas storlek påverkades främst av tidigare års fångststorlek; en stor fångst föregående år innebar minskad fångst

följande år. Ett milt vinterklimat (NAO-index > -0.7) sex år före fångst innebar bättre fångster, medan höga vattenflöden höst och vår (>95m3s-1) två år före fångsttillfället innebar sämre fångst. Större habitatförändringar i form av kraftigt reducerade vattenflöden (~90%) var negativt för kräftfångsterna. Den standardiserade metoden som fiskevårdsområdena använt för att mäta beståndsutvecklingen var tillräckligt precis för att påvisa populationsutvecklingen och kan därför rekommenderas också för uppföljning av andra återintroduktioner av flodkräfta. Trots att Ljungan ligger i norra kanten av flodkräftans utbredningsområde har flodkräftorna i Ljungan för närvarande en kroppstillväxt som är nära den maximala som uppmätts för flodkräfta (8 mm/ömsning för honor och 10 mm/ömsning för hannar).

Utifrån resultaten är de viktigaste råden för ett hållbart fiske i Ljungan och andra nordliga vattendrag att:

Övervaka kräftstammens utveckling, NAO-index samt vattenflöde i maj och oktober. Använda resultaten från övervakningen för att bestämma antalet tillåtna fiskedagar och burar. Samla in data om fångststorlek och hur många burnätter som faktiskt gjordes under säsongen. Använda data för att utvärdera fiskets hållbarhet. Stärka uppbyggnaden av den fångstbara storleken genom att -spara reproduktiva honor -införa en storleksgräns på 10 cm -tillse att det finns gömslen för kräftor av icke-fångstbar storlek.

TABLE OF CONTENTS ABSTRACT III SVENSK SAMMANFATTNING V Flodkräftan i en föränderlig värld – förutsättningar för skötsel v TABLE OF CONTENTS VII LIST OF PAPERS VIII 1 INTRODUCTION 1 1.1 Thesis background 1 1.2 Noble crayfish 2 2 THE PRESENT THESIS 6 2.1 Research area 6 2.2 Long-term studies 8 2.2.1 Catch data 8 2.2.2 Modeling population dynamics 8 2.2.3 Climatic impact 9 2.2.4 Water regulation 12 2.3 Field study 14 2.3.1 Experimental setup 14 2.3.2 Capture-recapture methods 14 2.3.3 CPUE versus capture-recapture methods 17 2.3.4 Body growth after reintroduction 18 3 GENERAL DISCUSSION 19 3.1 Methodologies to monitor noble crayfish 19 3.2 Management of noble crayfish 21 3.2.1 Reintroduction 21 3.2.2 Sustainable fishing 22 3.3 Future research 24 3.4 Noble crayfish in a changing world 25 4 REFERENCES 27 5. TILLKÄNNAGIVANDEN 39

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LIST OF PAPERS This thesis is mainly based on the following four papers, herein referred to as their Roman numerals:

Paper I Zimmerman J.K.M. and Palo R.T. 2012. Time series analysis of climate related factors and their impact on red listed noble crayfish population in northern Sweden. Freshwater Biology 57: 1031–1041.

Paper II Zimmerman J.K.M. and Palo R.T. 2010. Influence of water regulation and water flow on noble crayfish (Astacus astacus) catch in the River Ljungan, Sweden. Freshwater Crayfish 17: 141-144.

Paper III Zimmerman J.K.M. and Palo R.T. 2011. Reliability of catch per unit effort (CPUE) for evaluation of reintroduction programs – A comparison of the mark-recapture method with standardized trapping. Knowledge and Management of Aquatic Ecosystems. 401: 07.

Paper IV Zimmerman J.K.M. 2012. Body growth of noble crayfish (Astacus astacus) after reintroduction to a northern river (Ljungan, Sweden). Manuscript.

In paper I-II Jenny Zimmerman conducted most of the planning, data analyses and writing.

In paper III Jenny Zimmerman conducted most of the planning, field work, data analyses and writing.

In paper IV Jenny Zimmerman conducted all of the planning, field work, data analyses and writing.

Papers I-III are reprinted in this thesis with the permission of the publishers concerned.

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Till Mormor, Morfar, Farmor och Farfar – Ni gjorde detta möjligt!

1 INTRODUCTION

1.1 Thesis background Knowledge on the impact of climate change and how habitat alterations may affect organisms is needed when determining of conservation management objectives to achieve economically and ecologically sustainable actions. An evaluation of such actions is crucial, but often ignored in conservation projects (Seddon et al. 2007). The availability of reliable techniques that are inexpensive and easy to perform is essential and could increase any incitements for evaluation. Thereby, knowledge of ecological mechanisms that lead to successful or failed conservation measures could increase.

In Europe, reintroductions are common conservation actions for noble crayfish (Astacus astacus) (Šmietana et al. 2004, Sint & Füreder 2004, Taugbøl 2004b, Taugbøl & Peay 2004, Jussila et al. 2008, Edsman & Schröder 2009, Erkamo et al. 2010, Kozák et al. 2011 ). A conjecture is that this action will be used more frequently in the future to compensate for the loss of populations due to the increased spread of crayfish plague or to enhance the possibilities for weak populations in restored habitats. In Scandinavia where crayfish traditionally is important, a sustainable fishing of noble crayfish might serve as a tool for conservation of the species. The possibility for exploitation and economical as well as recreational benefits among local inhabitants may make them more eager to protect the species (Taugbøl 2004a, Jones et al. 2006).

In present thesis, the focus is mainly on factors that influence the sizes of adult noble crayfish populations in a northern river. This might give a clue how to manage noble crayfish in a northern climate. Research into the reintroductions of crayfish, especially noble crayfish to lotic ecosystems is limited, and the purpose of the present thesis has been to gain knowledge of the long-term climatic impact and habitat alteration on noble crayfish in the northern Swedish River Ljungan (I-II), methodology of evaluation after reintroduction (III), and noble crayfish body growth after reintroduction (IV).

1.2 Noble crayfish

In terms of commercial importance, crayfish are among the same order as shrimps, crabs and lobsters that belong to the class Malacostraca of the subphylum Crustacea (Ahyong & O’Meally 2004, Meland & Willassen 2007). Over 650 freshwater crayfish species are found worldwide and are indigenous to all continents, with the exception of Africa and Antarctica (Taylor 2002, Crandall & Buhay 2008).

Figure 1. The two crayfish species that is present in Sweden. The indigenous noble crayfish (Astacus astacus) is to the left and the introduced signal crayfish (Pacifastacus leniusculus) to the right. The noble crayfish is mainly recognized by the scratchy carapace (A) and by the red spot (B) at the joint between the two fingers of the claw. The most distinctive signs of signal crayfish are the smooth carapace (A) and the white-turquoise patch (B) on top of their claws at the joint between the two fingers of the claw. Figure: Linda Nyberg.

Two crayfish species are present in Sweden (Fig. 1) belonging to the Astacideae family. The indigenous noble crayfish (Astacus astacus, Linnaeus) is widely distributed in Eurasia and native to 28 European countries (Edsman et al. 2010) and the North American signal crayfish (Pacifastacus leniusculus, Dana), which was introduced to Sweden in the late 1960s (Fjälling & Fürst 1985). Nowadays the signal crayfish has

become more common than the noble crayfish (Bohman & Edsman 2011).

Crayfish, especially noble crayfish, have been consumed in continental Europe since the medieval times (Gherardi 2011). A search for crayfish recipes in Google gave 2 830 000 hits (Google 2012), indicating the importance of crayfish in food culture. Crayfish parties have been a part of Swedish folklore since the 16th century when crayfish was served by the Vasa dynasty at celebrations. However, the general public did not consume crayfish until the end of the 19th century but nowadays the “Kräftskiva” (crayfish party) is a well-known Swedish tradition that is celebrated annually each August (Swahn 2004) and the recreational catch of crayfish in Sweden is approximately 10 times larger than the professional catch (Fiskeriverket 2000, SCB 2011).

Crayfish are found in both lentic and lotic ecosystems and are used as indicator species for good water quality (Richardson 2012). Three factors are critical for crayfish; oxygen content, pH and temperature (Nyström 2002). The abundance of crayfish is also influenced by the presence of predators, water flow velocity, urban influence, proportion of arable land and sun exposure (Gherardi 2002, Schulz et al. 2006). At the northern limit of the distribution area for noble crayfish successful crayfish populations are more frequent in streams than lakes (Odelström & Johansson 1999, SCD 2010).

The body temperature of poikilothermic organisms varies with that of the surrounding environment (Rastogi 2007). This makes their physiological processes and thus their life cycles temperature dependent (Giller & Malmqvist 1998). Crayfish is poikilothermic and consequently, its growth rate and activity pattern are governed by temperature (Abrahamsson 1983, Westin & Gydemo 1986, Bubb et al. 2004, Nyström 2002, Salkonen et al. 2010).

During the autumn, a decreasing temperature and shortened days induce the reproduction cycle for Astacideae. Noble crayfish in northern environments begins with moult in July, followed by mating in September-October (Abrahamsson 1983, Zimmerman, personal observation). The maturity age and size are individual and postponed in cold climate compared to warmer (Abrahamsson 1972, Reynolds 2002). Females breed every second or third year depending on the availability of high quality foods and temperature (Reynolds 2002, Skurdal et al. 2010, Zimmerman, personal observation). The embryonic development

of noble crayfish eggs demands a period of time below 5˚C for high survival (Reynolds 2002, Skurdal & Taugbøl 2002), with the hatching of juveniles occuring in July (Abrahamsson 1972). A cold spring might delay hatching and thereby cause less growth and difficulties for juveniles to survive the winter (Gydemo & Westin 1989, Pursiainen & Erkamo 1991, Reynolds 2002, Skurdal & Taugbøl 2002).

Crayfish spend most of their time in shelters and an ideal habitat is heterogeneous with the bottom substrate consisting of stones, gravel and silt, tree roots and driftwood (Gherardi 2002, Nyström 2002). The shelters serve as refuges from predators or from currents during floods. Juvenile crayfish prefer shallow hard-bottom areas with rocks, vegetation or both (Hamrin 1987, Skurdal & Taugbøl 2002). Substrate availability (shelters) sets the upper limit of the population size in lakes and streams, though for streams, there are also ecological top-down processes, i.e. the abundance of predator fish seems to be more important for crayfish abundance than substrate availability (Stenroth 2005).

Noble crayfish live long and can reach an age of 20 years (Edsman et al. 2010). Mortality declines with increasing body size. Almost 95% of the juveniles are probably lost during their first summer due to predation, cannibalism or incomplete moult (Ackefors et al. 1989, Skurdal & Taugbøl 2002). Adults are most vulnerable to predation during moult when the carapace is soft and not protective.

Perhaps one of the most important mortality factors is diseases. This could be viral, bacterial, parasites or caused by fungus (Evans & Edgerton 2002). Among diseases the Crayfish plague (Aphanomyces astaci Schikora) is the most fatal for European crayfish since whole populations of crayfish can be obliterated within several weeks (Evans and Edgerton 2002, Edsman et al. 2010). The infection is mainly spread through illegal introductions of infected crayfish, but other human activities like the stocking of fish from contaminated water, fishing with contaminated fishery tools and fishing nets or boats can act as agents (Evans & Edgerton, 2002, Edsman & Schröder 2009).

The food web structure of crayfish is complex since their feeding habits change depending on age and situation. They could be regarded as prey, predators, omnivores or detrivores and eat algae, periphyton, aquatic vascular macrophytes, invertebrates, vertebrates, fish, fish eggs, carrion and detritus (Usio 2000, Nyström 2002, Dorn & Wojdak 2004,

Stenroth 2005, Setzer et al. 2011). A lack of alternative food generates cannibalism and in certain situations crayfish is a top-predator (Nyström et al. 1999, Nyström 2002,). The consumption of detritus from land-based material adds nutrition and organic material to the water. Fish, birds and mammals are predators of adult crayfish, whereas invertebrates and fish eat juvenile crayfish (Skurdal & Taugbøl 2002, Stenroth 2005).

Crayfish are keystone engineers because they bioturbate its environment, i.e. they displace and mix sediment particles by excavating, lifting and jostling small stones with their walking legs, scrape and flick with the abdomen or telson when walking or swimming and thereby move sediments from the bottom to the water column (Parkyn et al. 1997). Crayfish influence lower trophic levels directly via selective consumption of macrophytes and invertebrates, especially “slow” preys such as snails (copepoda) (Nyström et al. 1999, Dorn & Wojdak 2004). In a long-term perspective, high densities of crayfish may cause destruction of the macrophyte community (Abrahamsson 1966). Indirect effects, so-called trophic cascade effects, arise through the crayfish consumption of snails, resulting in increased periphyton biomass due to the reduction of predating snails, or the consumption of midges (Tanopodinae), the predatory invertebrates, leading to an indirect increase of its prey (Chironomidae), or when crayfish prefer Chara macro algae and Cladophora spp. macrophyton as food, thus shifting the species composition to blue-green algae that is less favourable to the crayfish (Abrahamsson 1966, Parkyn et al. 1997, Nyström et al. 1999, Usio & Townsend 2002, Dorn & Wojdak 2004, Stenroth 2005).

Crayfish compete for food, shelter and females (Nyström 2002). Interspecific competition occurs, for example, between fish and crayfish when they compete for the same food resources. Trout (Salmo trutta) was more effective in foraging and the abundance of crayfish was therefore low when the trout was present (Olsson et al. 2006). Interspecific competition between crayfish species is important when the distribution of the crayfish species is determined. Most NICS (non indigenous crayfish species) are more aggressive and have a higher capacity for population increase than ICS (indigenous crayfish species). The large capacity for a NICS population increase is derived from a higher individual growth rate, earlier sexual maturity and a higher per capita egg production that makes them invasive and outrivals ICS (Gherardi 2002).

One-third to one-half of the crayfish species in the world are threatened by population decline or extinction (Taylor 2002, Crandall & Buhay 2008). The current status of the noble crayfish is alarming. Present day the catches of noble crayfish in Europe are below 1/10 of the catches in the 19th century (Skurdal & Taugbøl 2002) and known Swedish populations have decreased by 97% during the last century with an accelerating trend since the 1960s (Bohman et al. 2006). The species is classified as vulnerable on the IUCN Red list of Threatened Species (Edsman et al. 2010) and as critically endangered on the Swedish Red List (Gärdenfors 2010).

The most immediate threat to European crayfish is the introduction of NICS (Taylor 2002, Holdich et al. 2009). Crayfish plague is mainly spread through illegal introductions of signal crayfish (Edsman et al. 2010, Gärdenfors 2010). In addition, crayfish are sensitive to degraded habitats. Therefore, impoundment or channelization of streams, drainage of wetlands, urban development, increased siltation and changes in water chemistry along with over-fishing and climate change are factors that threaten populations (Taylor 2002, Holdich et al. 2009). 2 THE PRESENT THESIS

2.1 Research area The research area in the River Ljungan is centred around 62˚31´59”N and 15´27”0˚E, a 60 km long stretch of the river (Fig. 2). The western border of the study area was situated immediately upstream Holmsjön, with power plant at Ljungaverk along the eastern border. The River Ljungan flows from the Scandinavian Mountains in the west to the Baltic Sea in the east and stretches about 400 km with a catchment area of 12,851 km2 (Thoms-Hjärpe 2002). The River Ljungan has been influenced and exploited by man since at least the sixteenth century until today. Fisheries, sawmills, watermills and the transport of timber dominated before small hydroelectric power plants were built and energy- demanding factories, such as nitrate and chlorate plants, were constructed near the river close to the electric power in the early twentieth century (Nordberg 1977). The expansion of modern hydroelectric power took place from the 1950s to mid-1970s and today the river is regulated along its entire length and the effects of the dams are pronounced throughout the river ecosystem (Giller & Malmqvist 1998).

Noble crayfish were probably introduced to the river in the early 1900s (Odelström & Johansson 1999). Local folklore tells that it happened on two separate occasions. The first was in 1910 when the managing director of the Alby Chlorate Plant planned a crayfish party, but lost a corf with crayfish from southern Sweden into the river. The second occasion is a similar occurrence and interestingly with some support for these stories in the genetic variation of the crayfish in the River Ljungan, showed similarity to crayfish populations with Southern and Western Swedish origin (Edsman et al. 2002, Gross et al. 2012). The “introduction” was successful and the crayfish fishing was extraordinary. It was not unusual with 50 crayfish per trap (Ahl 1957).

Figure 2. The research area along the River Ljungan. The large ellipse (I) indicates the river stretch where the analysis of climatic and weather impact on crayfish catches was performed (paper I). The small ellipses (1, 2, 3) indicate Sections 1, 2 and 3 that together with the catch areas a, b, c, d and e (orange circles) were analysed with regards to impact of a new water flow regulation scheme (paper II). The red circles A, B, C, D and E indicate the areas where capture-recapture experiments were performed (paper III & paper IV). The direction of water flow is indicated by small arrows in the major reservoirs.

In the 1960s, the crayfish in the river were subjected to research. The population structure, growth and fecundity were investigated and compared with other Swedish sites (Abrahamsson 1965, Abrahamsson 1972). Fishing was a tradition in the region until the crayfish became extinct for unknown reasons in 1999. Soon after extinction the local fishery owners associations, the local municipality, the county board and the Swedish board of fisheries decided to run a reintroduction program for the crayfish. The location of the reintroduction sites and the numbers of introduced crayfish at these sites were determined by the fishery

owners associations. So far, more than 80,000 adult crayfish have been reintroduced to the river at 35 sites (Ånge kommun 2010).

2.2 Long-term studies

2.2.1 Catch data A prerequisite for long-term studies is the comprehensive time series of reliable quality. These are difficult to achieve, but from the fishing of crayfish in the River Ljungan data was collected by the Swedish Board of Fisheries from 1963 to 1990. Fishing took place in August-September and the fishermen reported the site, the efforts used and the catch size after each season. The number of fisherman reports totaled about 10,000 during the period and the data was transformed to catch per unit effort (CPUE), i.e. number of captured crayfish per trap and night. The CPUE was used for two purposes in this thesis; to investigate the impact on crayfish catches by climatic factors (I) and the decreased water flow after the power plant was built (II).

2.2.2 Modeling population dynamics In conservation management and fishing, predicting future population development is important. Population modelling can be a useful tool to determine when actions are needed to preserve a species or to the size of fishing quotas for sustainable fishing.

The population size of a species at a certain future time Nt+1, is generally described by the sum of the total population size (Nt) at the current time (t), the births (B), deaths (D), immigrants (I) and emigrants (E) (eqn 1). The population size is then a consequence of the population success of survival (B - D) and the number of individuals that have been permanent to the site (I – E) during the actual time.

(eqn 1) (Gotelli 2001) N t 1 N t B D I E

This model is simple, but reality is complex and many factors act upon the population. For example, the number of females that are mature, successfully mated, and the quantity of spawned eggs determine the amounts of births (B). The number of births is then a consequence of the female physical status and the amount of newborn crayfish is then a result of how successful the female has been in taking care of the eggs during maturation and how well she has defended the eggs from predators. However, senescence, diseases, accidental events and

predation including cannibalism and fishery are setting the amount of deaths (D). The physical status of the crayfish determines the “death risk”, e.g. a weak and starved crayfish runs a greater risk of being consumed by a predator/infection than a healthy one. The population rate of migration (I+E) is determined by several factors. The low presence of populations in the vicinity at the sites and the high availability of unoccupied sites imply a fast overall migration rate (Gotelli 2001), whereas favourable physical and biological conditions, such as site area, availability of critical habitats (e.g. shelters and food), and the absence or scarcity of predators and competitors, inhibit emigration (E).

A logistic model can describe the population growth with density- dependent birth and death rates, assuming that the availability of resources at a site does not vary over time and that every future individual added to the population imply a decreased per capita growth (eqn 2).

dN N rN 1 (eqn 2) (Gotelli 2001) dT K

In other words, the change in population size (dN) over time (dT) is a function of a site specific constant (r), the population size (N) and the carrying capacity (K) of the site. (1 – N/K) corresponds to the unused proportion of available resources, where r is the population success of survival (B-D, eqn 1). This means that a population will stop growing when either r or N equals 0, as well as when N = K. The population is in equilibrium when dN/dT=0 i.e. N = K, when the population growth reaches carrying capacity. This model can be used to predict the size of a crayfish population at a certain site before and after changes in environmental factors (K) and changes in survival (for example, the time a population needs to recover after a disease or increased fishing). However, a more complicated model incorporating climatic factors, and weather and population density dependent factors was used to investigate the cause of crayfish catch fluctuations in the River Ljungan (I).

2.2.3 Climatic impact The size of the annual crayfish harvest for adults longer than 9 cm in the River Ljungan fluctuated throughout the years. A prerequisite for the catch is that the crayfish reach the harvestable size and are actively

seeking food during the harvest occasions. The size of the harvest is therefore a consequence of the size of the harvestable crayfish cohort and the behavior of the crayfish. The size of the harvestable crayfish cohort is the result of a long-term process over several years, whereas the behavior is short-term. As mentioned before, the crayfish is a poikilothermic organism and, hence, the life cycle and behavior are largely governed by temperature. Crayfish catches in a southern Swedish lake were shown to respond to winter temperatures with time lags of several years (Olsson et al. 2009).

Regional weather is partly a consequence of large scale climate. The North Atlantic Oscillation (NAO) directs primarily the winter weather pattern in the northern hemisphere, and when the NAO index is high, winter in northern Europe is humid and warm, whereas low index implies the opposite weather pattern (Hurrel & Deser 2009). Climate impact is difficult to predict because it affects the life cycle of crayfish, both directly and indirectly and in different ways, at different scales and at different stages. In paper I the influence of both large scale climatic and regional weather patterns in terms of the NAO index, air temperature and water flow on crayfish catches was investigated.

CPUE was assumed to reflect population size and the time series of crayfish catches from the River Ljungan was compared pair-wise with cross correlation analysis (CCA) using the NAO index, air temperature and water flow time series from measurement stations in the vicinity of the river. When large coefficients appear in a time lag, you may suspect that the time series are dependent (Huitema & Laraway 2007). Significant correlations were therefore used to develop two separate linear regression models (Model 1, Model 2) that predicted the magnitude of crayfish catches based on the climatic and weather variables. Model 1, a modified Ricker Model, predicted the catch growth rate from population density dependent factors and environmental variables, and Model 2 predicted the magnitude of catch from the environmental variables only (Ricker 1954, Kölzsch et al. 2007, Olsson et al. 2009). The most parsimonious models that were significant (p>0.05) in all variables were chosen from a stepwise approach with a forward and backward selection of variables and determined with Akaike´s Information Critera (AIC) and the relative importance of regressors for the models (Burnham & Anderson 2002, Grömping 2007). The predictive power of the models was tested with correlations to two independent datasets from the river.

The two models gave almost the same results, but with a better fit to data for Model 1. The models showed that crayfish catches were influenced by the NAO index with a time lag of 6 years and by water flow in May and October with a time lag of 2 years. Model 1 also included density dependent factors with a time lag of 1 year (i.e. the magnitude of catch current year was dependent of the previous catch size).

Weather and climatic factors influenced the size of crayfish catches in the River Ljungan, but the single most important factor was the magnitude of the annual harvest. This implies that the population is affected by intrinsic competition but effects of fishing cannot be ruled out. The population might need at least a season to replace the captured cohort with a large body size (i.e > 9 cm) by a smaller cohort after a large catch.

Figure 3. Tree diagram of Model 1 and Model 2, show implications of the sizes of previous catch (CPUE), water flow in May two years before catch and NAO index six years before catch, on the crayfish catches in the River Ljungan. The size of the crayfish in the figure corresponds to the size of the modeled catch i.e. a large crayfish means that the catch is large and vice versa. The tree models are built up from a process of several binary splits. An unordered factor is divided into two non-empty groups and the process will stop when the terminal nodes are too small or too few to be split (Ripley 2011). It is noteworthy that during the study period between 1963 and 1990 CPUE was below 6.1 seven times, the average water flow in May was below 95 m3s-1 nine times and winter NAO-index was below -0.69 eight times.

In addition to intrinsic competition and harvesting, winter climate and water flow during the spring and autumn seem to play a role in population dynamics and act on different cohorts of the population (Fig. 3). The NAO signal with a time lag of 6 year might be related to survival of eggs or juvenile stages; a positive NAO index 6 years before harvest implied larger catches. A high water flow seems to be negative for crayfish, where a large water flow in May or October two years before harvest implied smaller catches. The results imply that the size of the crayfish catch is mainly governed by the size of the harvestable cohort and that the behavior of crayfish (i.e. timing of molt) is of minor importance in the long-term.

Global climate change is forecasted with high NAO index and raised precipitation in northern Europe (IPCC 2007, Hurrel & Deser 2009). Based on the models in paper I, the climate is forecasted, thus providing a prediction of the effects on noble crayfish in northern environments. This prediction shows that the effects of climate change might be non- existent to slightly negative on crayfish catches and therefore on crayfish populations.

2.2.4 Water regulation The regulation of water for hydroelectric power leads to large-scale ecological effects, with the loss of river ecosystems and fragmentation of habitats and aquatic organism populations (Dynesius & Nilsson 1994). Former rapids become inundated or lost, and the river becomes more or less similar to a lake. To satisfy electrical demands, water regulation is at different time scales with long-term dampening of flow seasonality and short-term daily fluctuations (Malmqvist et al. 2009).

The life strategies of many organisms are adapted to natural flow schemes with low flow during the winter, a flow peak in the spring and high flow in the summer. A long-term regulation with an even water flow throughout the year might alter the timing of events and have a negative impact on these organisms (Bunn & Arthington 2002). The diversity of macro invertebrates located downstream from power plants is also negatively affected by sudden flushes that occur when the reservoirs are filled and when a rain fall makes it necessary to open the dam gates (Malmqvist et al. 2009).

The effects of short-term regulation include changes in discharge, water depth, temperature and increased siltation (Cushman 1985). A rapid lowering and rising of the water level increase the risk of stranding

and catastrophic drift of invertebrates, while increased siltation might destroy refuges and prevent production of vegetation (Bunn & Arthington 2002, Bruno et al. 2010). An effect of lowered water depth is that shallow shelters become stranded forcing the crayfish to seek new shelters into deeper water. This unintended movement exposes the animals to predators and thus increases the predation risk and mortality (Hamrin 1987, Tulonen et al. 2010).

A new water regulation scheme was introduced in a section of the River Ljungan in 1976, when the water from this river stretch was redirected to a power plant located on land. The consequences for crayfish in the river in relation to this water regulation are discussed in Paper II.

CPUE was assumed to reflect population size and catch data was sorted by geographical position (sections 1, 2 and 3; catch areas a, b, c, d and e; Fig. 2) and catch date before or after the building of the power plant. Differences in the mean annual values for water flow and median values of annual crayfish catch between the sites and between the times before and after the building of the power plant were confirmed with the Kruskal-Wallis test, followed by the Dwass-Steel-Chritchlow-Fligner- test. In Section 1, the differences between CPUE within the catch areas were compared to determine if these were associated with changes in water flow.

The most upstream section of the study area (section 1) had the lowest annual water flow already before the water from this river stretch was redirected to the power plant on land. But the difference was even more pronounced after this measure when the annual water flow was lowered by about 90% in this stretch of river and the other sections were unaffected.

The median CPUE after the new water regulation scheme decreased in Section 1 and decreased slightly in Section 3, with Section 2 unaffected. Within Section 1, CPUE decreased significantly in the upstream catch areas A, B, C and D with no significant difference in catch area E.

The building of a new power plant with water from the river redirected to the plant, inferred a new water regulation scheme in Section 1. This implied a low and even water flow with occasional peaks compared to the scheme with a maintained flow and seasonal variation

that persisted for Section 2 and 3 during the study period. The new scheme was negative for the crayfish in Section 1 and the effects were most pronounced immediately downstream from the water intake to the bypass tunnel. For Sections 2 and 3 these effects were negligible.

The regulation of water flow combined with the reduced water flow decreased the species richness of invertebrates, which was not the case where only regulation occurred (Giller & Malmqvist 1998). This might perhaps explain the reduced crayfish catches in Section 1, where the water flow was decreased and regulated after 1976, while the water flow in Sections 2 and 3 only were regulated.

2.3 Field study

2.3.1 Experimental setup Five reintroduction sites were chosen for capture-recapture experiments (A, B, C, D and E; Fig. 2). In total, 120 baited traps, evenly allocated on 24 lines, were positioned over-night along both sides of the river bed (Fig. 7). Covering a total area of 1 hectare, the experimental setup stretched downstream 100 metres from the reintroduction sites (A, B, C, D and E; Fig. 2). The distance between the traps was 10 metres, which is within the range that crayfish can spot and be attracted by bait (Acosta & Perry 2000, Cukerzis 1989 and Fedotov 1993, cited by Kholodkevich et al. 2005).

At each site 700 animals were caught, marked individually with Passive Integrated Transponders (PIT-tags) and released at the same spot during the second weeks of June, August and September from 2007 to 2010 (Fig. 4). Data were collected using a protocol that noted the length, moult phase, fecundity and existent injuries.

2.3.2 Capture-recapture methods Population trends are essential knowledge in the decision-making of management actions and evaluation of conservation measures (IUCN 1998, Vié et al. 2009). There are plenty of methods to choose between; from simple counts and indices to more complicated capture-recapture methods (Seber 1986, Williams et al. 2002).

A capture-recapture method consists of several capture occasions. At the first occasion, all animals are marked and released at the same spot as for capture. During subsequent occasions unmarked animals are marked and the relationship between the marked and unmarked

Figure 4. A male noble crayfish released to the River Ljungan, after marking with the PIT-tag. In a few minutes the crayfish will locate a shelter to avoid exposure to predators, such as perch (Perca fluviatilis) and pike (Esox lucius). Photo: Jenny Zimmerman. animals is used to estimate the animal abundance. The simplest capture- recapture model is the Lincoln-Petersen estimation, which only requires two sampling occasions (Petersen 1896, Lincoln 1930). It is a so-called closed population model, which assumes a constant population size during the study. This implies neither events of birth and death nor the occurrence of any immigration and emigration between sampling occasions (Pollock et al. 1990). The Lincoln-Petersen estimation assumes equal catchability during sample occasions. With several recapture occasions other closed population models considering heterogeneity, temporal and behavioral effects of trapping can be used (Pollock et al. 1990).

In long-term studies, the assumption of constant population size is not realistic and open population models considering events of birth and death are used instead. The population abundance can be estimated with the Jolly-Seber model from the probability to catch an individual (Jolly 1965, Seber 1965). A prerequisite for this model is that all individuals caught during previously sampling occasions are marked and released to the study area. The Cormack-Jolly-Seber model, however, gives survival rates based on the probability that an individual has survived until the sampling and only the capture history of each individual is

therefore needed for estimation (Pollock & Alpizar-Jara 1985, Pollock et al. 1990). Pollock (1982) presented the robust design, where sampling occurs on two temporal scales; several short-term investigations where closed models can be used and a long-term investigation that applies to open models.

Common to capture-recapture models is the assumption that marks are neither lost between capture occasions nor unobservable at recapture (Pollock et al. 1990). This requires synchronization between the time duration of the marking method and the choice of capture-recapture model. Temporary methods such as paints (Fig. 5), attached streamers and radio isotope marking last for a limited time and can be used if the study period is short. Semi-permanent methods like tags, neck collars and radio telemetry, or permanent tattoos, branding, passive integrated transponders (PIT-tags, Fig. 6) or visible implant fluorescent elastomer (VIE) are more convenient for long-term studies (Beausoleil et al. 2004). The moulting behavior of crustaceans makes long-term marking difficult. Paints and fluorescent marking only last until the next moult, whereas branding or piercing may last for several moults (Abrahamsson 1965, Brandt & Schreck 1975 and references herein, Guan 1997). Only recently after the development of PIT-tags, VIE and coded microwire tags (CWT) could crayfish be individually, permanently marked (Haddaway et al. 2011).

In papers I-III CPUE was used as an index of population size, in paper III the CPUE index was compared with capture-recapture models based on PIT-tags and in paper IV capture-recapture models were used for population estimations and to follow individuals over several years.

Figure 5. Noble crayfish marked Figure 6. Noble crayfish marked temporarily with paint. The mark will be permanently with PIT-tag. The PIT lost when the crayfish moults next time. tag has a unique code that is read Photo: Jenny Zimmerman with a reading device. Photo: Renée Lindblom

2.3.3 CPUE versus capture-recapture methods There is a need for scientifically-based, accurate and cost-efficient methods to monitor and evaluate reintroduction programs. The monitored indicators should be measurable, precise, consistent, and sensitive to the target (IUCN/SSC 2008). CPUE is an index that has been used for population estimations for fish (Maunder et al. 2006) and crayfish (Skurdal & Taugbøl 2002, Westman et al. 2002, Olsson et al. 2009, Erkamo et al. 2010). But the accuracy of this population estimation method has been questioned (Harley et al. 2001, Goodyear et al. 2003, Maunder et al. 2006, Laloë 2007). Local fishermen have used CPUE calculated from the standardized trapping of crayfish in the River Ljungan since 2004 to evaluate the success or failure of the reintroduction program. The aim of paper III was to investigate if their efforts were enough to reliably evaluate the program.

The relationships between CPUE using different efforts (15 traps/hectare and 120 traps/hectare) and population estimations were investigated at five sites (A, B, C, D and E; Fig. 2). The population estimations were made with log linear closed population models and followed Pollock´s robust design. The Mann Whitney U-test was used to compare the differences in CPUE, and the Pearson correlation test and linear regression were used to investigate the relationships between log transformed CPUE and the estimated population sizes.

In all, CPUE with low and high efforts and population estimations with capture-recapture models showed an increasing trend during the study. The variation of CPUE was larger with low effort and the values of CPUE were larger compared to high effort. CPUE was positively correlated to the population estimations and the CPUE with a high effort had a greater correlation to the abundance than CPUE with low effort.

CPUE might be considered suitable for monitoring only if it is properly standardized (Hockley et al. 2005). The number of traps is obviously important for reliable estimates regarding the abundance and population trends from CPUE. The more traps, the more accurate the result (III). However, when the effective radius from the traps overlap; i.e. there are more than one trap within the range that crayfish can spot and be attracted by bait, further traps will not improve the accuracy (Peay 2004).

The trapping procedure performed by the fishermen in the River Ljungan was standardized and performed with the same equipment, at

the same time of the year (late-August) and at the same spots from year to year. These actions seem sufficient for a qualitative estimation of population trends and to handle the limitations of CPUE concerning the bias of size and sex on the estimation caused by the trapping or by crayfish moulting behavior (Abrahamsson 1966, Rabeni et al. 1997, Westman & Pursiainen 1982, Dorn et al. 2005, Price & Welch 2009). The minimalistic monitoring design used by the fishermen is a less accurate, though it is a reasonable method for estimating population trends. The accuracy can be improved by increasing the number of traps. The simple design that can be done by amateurs makes the method inexpensive compared to capture- recapture methods. Therefore, the method is valuable for the evaluation of reintroduction programs.

2.3.4 Body growth after reintroduction The body growth of crayfish and other crustaceans over several seasons has been studied very little due to their behavior, which makes simple marking techniques impossible to apply. A marking on the crayfish shell is lost simply through moulting. In recent years the development of new marking techniques have made it possible to reliably follow individuals in the wild for several years (Bubb et al. 2002, Camp et al. 2011, Haddaway et al. 2011).

In the harsh climatic conditions with low temperatures that prevail in waters at high latitudes, such as the River Ljungan (62˚N), the rate of physiological processes and digestion decrease and cause a shorter growth season. This can influence organisms into a less somatic growth, longer life span and delayed maturation compared to conspecifics at lower latitudes (Momot 1984, Karasov & Martinez del Rio 2007). A result of delayed maturation is that the build-up of populations may be slow and sensitive to disturbance.

The body growth of noble crayfish in relation to water temperature and other physical and chemical properties was investigated at the sites A, B, C, D and E (Fig. 2). Differences in size structures and body growth rates were confirmed between sites with the Kruskal Wallis test followed, by the Conover-Inman posthoc test. Linear growth models were developed from recapture data. The powers of the models were tested with Pearson's product moment correlation tests, between predicted values from the models and values from measurements of animals that were measured two or three years earlier.

Most animals moulted once a year with length increments close to the maximum that is reported for the species (Skurdal & Taugbøl 1994). The male growth was larger than the female growth and the size of the length increment did not differ among the sites, which was apparently not associated to differences in habitats or water temperatures.

Body growth measurements were conducted on noble crayfish in the River Ljungan during the 1960s (Abrahamsson 1972), when the length increments were 31 % smaller for females and 19 % smaller for males compared to today. Obviously, other conditions were prevalent in the river 50 years ago, such as more fishing and a different water regulation scheme (I, II). As well, the water temperatures were lower and the noble crayfish population was denser. Therefore, it cannot be precluded that these differences in climate and population density positively enhance the body growth rate as observed presently in the River Ljungan.

From paper IV it is concluded that it is possible to individually follow noble crayfish in the wild for several years using PIT-tags and that noble crayfish have the ability to grow equally as fast in the northern edge of its distribution area as in southern locations (<60˚N).

3 GENERAL DISCUSSION

3.1 Methodologies to monitor noble crayfish Two methodologies were used in this thesis to monitor and investigate noble crayfish. The investigations at the population level were made with CPUE as the population index and estimations of population abundance from capture-recapture methods based on PIT- tags. PIT-tags were used to identify crayfish at the individual level.

The CPUE index was sufficient for long-term population studies in relation to climate (I), for the evaluation of population alterations after a large-scale habitat change (II) and for the evaluation of population trends after reintroduction (III). In addition, the capture-recapture method based on PIT-tags was suitable for estimations of population trends (III).

The accuracy of the CPUE-index seems adequate provided that crayfish behavior is considered. Investigations should be performed when the crayfish are actively entering baited traps, i.e. in between moult and mating season. Paper III also showed that it is possible to increase the accuracy of the CPUE-index by increasing the number of

traps. The accuracy of estimates from the capture-recapture method seemed to gradually increase (III). This was most likely a consequence of the increased proportion of marked crayfish in the study area and several marking occasions are needed to achieve an improved accuracy from capture-recapture methods.

The CPUE-index was standardized in that the yearly trapping occasion occurred during the same season and the numbers of traps and their locations were the same throughout the years (III). The use of the CPUE-index can be expanded with analyzes of size structure, fecundity and proportion of injured crayfish among the years, if measurements of length, maturity of eggs and injuries are included in the investigation protocol.

In addition to estimations of population trends, population growth rates, survival rates and population densities, permanent marking with PIT-tags also provides an opportunity to follow the performance of an individual crayfish. Marking lasts for at least 3 years in noble crayfish (IV). Depending on which measurements are made during capture occasions, information about body growth (IV), fecundity, moult frequency, history of injuries, migration pattern, etc. can be achieved. The PIT-tag is implanted inside the crayfish body and is therefore not recognized without a reading device. This is an advantage because the appearance of marked animals does not differ compared to unmarked and their behavior or survival is not affected (Bubb et al. 2002). However, a disadvantage is that special equipment to recognize a marked crayfish is needed. This is unsuitable in fishing areas, where an obvious risk is that fishermen will remove marked crayfish by mistake.

The CPUE method only requires baited traps compared to the capture-recapture method, which also needs PIT-tags and a reading device. Besides the extra equipment needed for the capture-recapture method, it is also based on multiple capture occasions. Overall, this makes the CPUE method easier to perform and less expensive compared to the capture recapture method based on PIT-tags. However the costs of the capture-recapture method can be decreased if the PIT-tags are replaced with a less expensive marking method.

Table 1. A brief comparison between the CPUE-method and the capture- recapture method based on PIT-tags (CRM).

CPUE CRM Population abundance X Population trends X X Population density X Population growth rate X X Population survival X Migration rate X Individual performance X Tags and reading equipment are needed X Cost Low High Effort Low High Method handiness Easy Complicated The choice of method has to be adapted to the situation, with Table 1 perhaps supporting this. A comparison of the most important properties of the CPUE-method and the capture-recapture method based on PIT- tags show that if the need is only to know crayfish population trends, the best choice is the CPUE method because it is easy to perform, with less effort and less cost than the capture recapture approach (Table 1). Otherwise, the capture-recapture method should be used.

3.2 Management of noble crayfish

3.2.1 Reintroduction The reintroduction of noble crayfish to the River Ljungan is so far successful. The body growth of individual crayfish is remarkably high (IV) and the sub-populations show an increasing trend (III) , though it is difficult to know exactly why. However, the reintroduced crayfish were from the same county and had the same genetic origin (Gross et al. 2012). It could therefore be expected that the crayfish were well suited to the seasonal climatic variations of the region. Many specimens were reintroduced. Thus providing a boosted start for population development (Taugbøl & Peay 2004)

Still, one of the most important measures in the project has been the involvement of the local inhabitants in deciding the reintroduction sites and in the practical work with introduction and monitoring (Ånge kommun 2010). To ensure the long-term nature of the project, the locals

were trained in management and the local papers annually reported the progress of the project to the general public (Zimmerman 2011).

3.2.2 Sustainable fishing It may seem contradictory to argue for the fishing of an endangered species. However, in Scandinavia noble crayfish have long been a part of the culture. The risk that future generations of local population will not have the opportunity to experience recreational fishing is a strong incentive to conserve the species. This commitment was manifested in the voluntary work to reintroduce noble crayfish to the River Ljungan by the local inhabitants (Ånge kommun 2010, Zimmerman 2011). To maintain this kind of commitment among locals, they must have the privilege to retain their fishing traditions. Hence there is a need for local management and sustainable fishing.

Fishing affects the sizes and size structures of populations and as indicated in paper (I), recovery and buildup of the population after a large catch in the River Ljungan required at least a season. The time needed depends on the recruitment rate, which in turn is a consequence of body growth rate and fecundity. A high body growth rate implies that the time between birth and sexual maturity as well as the time needed to reach a harvestable size may be shortened. Females will have the ability to reproduce additional times during their lifetime and juveniles will rapidly reach adult size. The result will be a faster build up of the adult cohort. This process might be enhanced with increased water temperature, low population density or a combination of these factors (IV).

Sustainable fishing is a challenge, though some lessons can be learned from the fisheries industry. A common strategy is to harvest down to a very small population size and expect recovery. This destroys ecological interactions when the dominant predator fish is suppressed and its ecological niche is occupied by other organisms, making recovery difficult. For example, cod (Gadus morhua) in Alaska has not yet recovered after the population collapse in the 1990s (Frank et al. 2007). Overfishing is especially dangerous for species with a slow process of recruitment, e.g. noble crayfish in northern environments, and should be avoided (Momot 1984).

The strategy called maximum sustainable yield (MSY) aims to maximize the population growth and thereby increase the recruitment to

the harvestable cohort. It is based on the logistic model described in section 2.2.2, where the maximum population growth rate occurs when the population size is half of the carrying capacity of the habitat (Gotelli 2001). The size of the carrying capacity must be determined, from which the fishing quotas are drawn up. MSY may be hazardous because the value of the carrying capacity is not constant, but varies with changes in habitat and without updated quotas there is a risk of overfishing. The consequence of a change in carrying capacity was shown in paper II, where a large-scale habitat change implied dramatic decreased catches of noble crayfish in a section of the River Ljungan.

Hence, the regulations for the fishing of crayfish have to be adapted to the population size and be robust to environmental changes. Regulating the numbers of traps is important, but the most effective recovery measure after overfishing seemed to be a shortening of the legal fishing period (Skurdal & Taugbøl 2002). The use of size limits, no- take zones and avoiding reproductive females have been suggested as tools for a sustainable harvest (Jones et al. 2006).

Figure 7. The goal with a sustainable, recreational fishing of noble crayfish is to have an acceptable catch from year to year and to maintain an interest for the species among the public. The equipment at this photo was used for the trapping of crayfish for capture-recapture estimations and for the standardized fishing, performed by the fishermen, to monitor the population development after reintroduction of the noble crayfish to the River Ljungan. Five traps were attached to the blue line, with a snap hook. The yellow boxes contained pelleted, bait. Photo: Anna Broich.

The aim of reintroducing noble crayfish to the River Ljungan is to reestablish the species to the river and then establish a recreational fishing (Fig. 7) (Ånge kommun 2010). Paper (I) showed that the most important factor determining the size of the catch was the size of the previously year´s catch. Hence, when the crayfish stock is large enough, there will be additional pressure on the population from fishing besides the environmental pressure. To avoid overfishing and realize a sustainable fishing of crayfish in the River Ljungan, my suggestions to the fishermen are:

Monitor the population trends, NAO-index and water flow in May and October.

Use the results from the monitoring to determine the number of allowed fishing days and traps.

Collect data about the catch size and effort from legal fishing and use it to evaluate the sustainability of the fishing.

Enhance the buildup of the harvestable cohort by -saving reproductive females -introduce a size limit of 10 cm -provide proper shelters for the non-harvestable cohort.

3.3 Future research The more knowledge gained, the more questions are raised, and my research has raised my curiosity with regards to

Flow effects on the survival of different cohorts of crayfish. What are the biological mechanisms that imply a decreased crayfish catch two years after a large water flow in May or October?

The body growth rate of crustacea is the result of length increment per moult and the number of moults (Hartnoll 1982). The extreme length increments of noble crayfish found in the River Ljungan lead to a number of issues. Are large length increments common in harsh climates? What is the breaking point regarding the profitability with several moults instead of one? Will the crayfish in the River Ljungan moult twice a year in the future? If so, will the extreme length increments remain?

The reintroduction of noble crayfish to the River Ljungan has gone well, but why? Is it the massive introduction with numerous specimens? How many crayfish need to be inserted for success? Does it matter where you put them? What is the migration from the release sites? What affects the migration?

3.4 Noble crayfish in a changing world The noble crayfish population in the River Ljungan was established at almost the same time as when the first cars were registered in Sweden. Since then, automobiles in Sweden have increased to over 4 million, society has undergone revolutionary changes and the River Ljungan has received profound alterations (SCB 2012). A century ago the river was free flowing; today the water flow is regulated both daily and seasonally in separated reservoirs. The climate has changed. This has been

Figure 8. The present distribution areas for the noble crayfish (blue dots) and the signal crayfish (red dots) in Sweden. The ellipse in the right figure shows recently made illegal introductions of signal crayfish to water courses in the northern Sweden. These introductions are an immediate threat to the noble crayfish populations and have a low probability for success (Sahlin et. al 2010). Figure: Patrik Bohman, SCD, Swedish University of Agricultural Sciences, Institution of Freshwater Research.

manifested with an increased NAO index during the winter since the 1960s (I) and increased water temperature during the summer (IV) with a prolonged growth season as a result. Not much is known about the crayfish population in the River Ljungan from the early 20th century, but the population has been subject to fishing since the 1950s. A sudden extinction occurred for unknown reasons in 1999. However, in the early 21st century the noble crayfish population was reintroduced and is today re-established in the river (III). During the century when noble crayfish were found in the River Ljungan they have become an important element of the local culture. The local inhabitants are concerned about the species and have been working to reestablish the species for future conservation. The future survival and development of this crayfish population in particular and noble crayfish in northern environments in general is unpredictable. Global climate change is forecasted to raise the NAO index, summer seasonal temperatures and precipitation in northern Europe (IPCC 2007, Hurrel & Deser 2009, Kjellström et al. 2011). This will probably enhance body growth (IV), perhaps with an increased number of moults, while the population development will probably be more or less stable or slightly decreased (I). Still, there is the increasing threat of illegal introductions of signal crayfish that in recent years have appeared in several water courses in the vicinity of the River Ljungan (Fig. 8). An introduction of signal crayfish may eradicate the noble crayfish population in the river within a few weeks. It is our responsibility to conserve this species for future generations and thus the future of the noble crayfish is determined by us.

“We do not inherit the Earth from our parents but we borrow it from our children.”

(Native American proverb)

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5. TILLKÄNNAGIVANDEN Det här projektet har varit ett samarbete mellan Ånge kommun och Mittuniversitetet. Jag riktar ett alldeles speciellt tack till Fredrik Westin som gjorde detta möjligt. Under projektets gång har det varit fruktsamma samarbeten med både ”Återplantering av flodkräfta till Ljungan” och Leader-projektet ”Förstudie för Älvens Hus”. Jag vill också passa på att tacka för stipendier från Makarna Öhrns fond, Helge Ax:son Johnsons stiftelse, Stiftelsen Markussens studiefond, samt från IAA18 organizing committe, som har möjliggjort att jag kunnat resa och få internationella kräftkompisar.

Det här med att doktorera har varit fantastiskt skoj och jag hade inte fixat det utan hjälp. Jag vill rikta ett stort tack till ALLA som bidragit till att jag fått en bra doktorandtid och till att mitt projekt kunnat genomföras!

Tack alla som bidragit med data; Sötvattens-laboratoriet och Eva Sers, Vattenregleringsföretagen, EON, Ljungandalens forskarverkstad, fiskevårdsområdena speciellt Borgsjö Mellersta och Viken för långa dataserier, samt Åsa och Johan på Ånge kommun för att ni sammanställt data när jag behövt det. Ett särskilt tack till Magnus Fürst som förklarade hur fångstdata i Ljungan kommit till och till Bertil Eriksson som samlat in data och dessutom visste att det fanns data ända sedan 1963 och vart de gick att hitta! Alla assistenter genom åren ska få en stor kram; Anna - den första, Gustav - den största, Tita - den riktiga kräftfiskaren, Carina - den mesta, Kim - den hungrigaste och Tom - den starkaste. Praktikanter, examensarbetare och andra som hjälpt till är Jonas - legitimerad kräftmålare, Rocio - utbildad roddare, Reneé – fotograf och Tobias – känd från TV. Ett tack också till Johan som sprut-målade flöten (som jag sedan rationaliserade bort). Jag hade inte kunnat drömma om det engagemang som finns i bygden för flodkräftan. Tack Permascand AB, Ånge Judoklubb och Karlsro Flyers för att vi fick tak över huvudet.

Det har varit kul att få lära känna alla fiskevårdsområden och ingenting kommer någonsin vara omöjligt för er. Ni har ställt upp för mig på ett otroligt sätt, vare sig det handlat om roddhjälp, utlåning av båtar, burar, flöten, eller om att lyssna på min svada. En del av er har jag haft lite extra mycket att göra med genom åren; Kjell – organisatören och stöttepelaren, Berit – kvinnan som har hand med både människor och kräftor, Åke och framlidne Lennart Nordin; ni lärde mig fiska kräftor på riktigt. Kurt och Elsy - det var gudomligt att efter fyra timmars ösregn få

torka upp i er varma bastu. Sören – alltid beredd att låsa upp bommen, Kenth-abborrdödaren, Håkan – sambandscentral för kräftutsättningar och Janne – det du inte vill veta om kräftor det finns inte!

Jag hade inte klarat mig utan mina speciella kräftmentorer Anders, Ivar, Tommy och Lelle. Inga frågor har varit för små för era öron. Ljunga Frukostklubb lärde mig att frukost intas på tåget i trevligt sällskap, medan tonhallsgänget diskuterade högt och lågt under lunchen. Jag tror att våra nya fotbollsregler med stildomare kommer att göra succé. FIFA here we come! NAT, denna trivsamma arbetsplats, tog emot mig med öppna armar. Vad hade fikarummet, expeditionen och labben varit utan Ingrid, VIK/VER, Torborg och Håkan? Jag har gillat den osannolika mixen på NAT och i doktorandgruppen. Till min nuvarande rumskompis Sara - jag hoppas att vi haft lika villkor på rummet och att du och hararna kommer fortsätta att trivas ihop! Jag tänkte också passa på att tacka vaktmästeri, städ och bibliotek för god service.

Vad glad jag blev när du Nils läste och gav kommentarer på kappan. Thomas – tack för att du trott på mig från vårt första möte. Vilka intressanta diskussioner vi har haft. Jag har uppskattat att du vågat släppa ut mig på djupt vatten och att du inte blev arg när jag körde sönder bilen…

Ett stort tack också till mina föräldrar, svärföräldrar, syskon med familjer för att ni ”älskat” kräftor och statistiska analyser under de senaste åren. En extra kram till min mormor för att du alltid bryr dig om. Ni som står mig allra närmast; Irma, Edit, Karin och Erik har sett att jag under de här fem åren blivit äldre, klokare och mer förvirrad än någonsin. Jag har glömt saker, varit på fel ställe och fastnat framför datorn. Men jag är så innerligt glad över att ni finns och nu går vi mot nya djärva mål - eller som det står på vår dörr – we interrupt this family for football season!

Johannisberg 11 april, 2012