ICES 1990 POSTER C.M.1~/L:71 impact on the lake ecosystem of both fishery and cormorants has never been quantified siinultaneously in 1 2 A.D.Buijse ), M.R. van Eerden ), W.Dekker) & W.L.T.van the case of IJsselmeer. DensenI). Atrophie model for IJsselmeer (The IJsselmeer is one of the most important baunts for Netherlands), a sballow eutrophie lake. waterbirds and is thereby a wetland of international importance (van Eerden and Zijlstra 1986). Conceming I) Department of Fisb Culture and Fisheries the -eating species a year round usage occurs, both by Agricultural University Wageningen, P.O.Box birds from colonies of breeding (gulls, teros and 338,6700 AH Wageningen, Tbe Netberlands cormorants) as well as by wintering and migratory species 2) Directorate Flevoland, P.O.Box 600, 8200 AP (mergansers, black tern (Chlidonias niger». Lelystad, Tbe Netherlands Tbe aim of this study was to depict the major 3) Netherlands Institute for Fisheries Research, routes for the energy flow in the IJsselmeer ecosystem, in P.O.Box 68,1970 AB, Umuiden, Tbe Netherlands this way elucidating especially the impact of both fisheries and birds on the ecosystem. The study has been extended ABSTRACT to include all information available conceming the energy flow in the fish oriented part of this ecosystem. IJsselmeer is a 182,000 ha shallow eutrophie freshwater body with an average depth of 4 m. MATERIAL AND METHODS Commercially important fish species are the exploited eel (Anguilla anguilla), the predators pikepereh (Stizostedion Studyarea lucioperca) and perch (Perca fluviatilis) and the sbort- eving smelt (Osmerus eperlanus). Tbe situation of IJsselmeer and the marshes, Important fish-eating birds are the cormorants where the cormorants breed, is presented in Fig. 1. (Phalacrocorax carbo), grebes (Podiceps cristatus), IJsselmeer is since 1975 partitioned into a 112,000 ha mergansers, gulls and terns. Other important fish species northern and 70,000 ha southern part. in the eeosystem are (Gymnocephalus cernua),

bream (Abramis brama) and roach (Rutilus rutilus). The "E fishery eonsists mainly of a fyke net fishery for ee! and spawning smelt arid a gill net fishery for pikeperch and percb. Tbe trophic relations between phytoplankton, zooplankton, fish, birds and the fishery were estimated over the period 1983-1987. Tbe zooplanktivorous smelt, produeing 130 kg fresh wt.ha·l.yr,l, appeared to be a key species within the food web. Smelt was mainly eonsumed by perch (59 kg.ha' I l.yr· ). Birds eonsumed 48 kg.ha'l.yr,1 of fish, wbereof 42% is smelt. The impact of the fishery consisted of overexploitation of eel and pikepereh stock and of catching large amounts of 0- and l-group fish as bycateb in the fyke net fishery. The discarded bycatch was utilized • gulls and terns.

INTRODUCTION

IJsselmeer, with its 182,000 ha, is one of the larger freshwater lakes in Europe. Due to the nutrient input from the River IJssel, branch of the River Rhine, and its shallowness it has a high productivity. It serves many funetions e.g. fisheries, recreation, drink-water' supply, transport and rest and forage area for birds. Some of these functions are eonflieting witb each other e.g. fisheries and birds are using the same food souree. Fishermen are blaming the cormorants for the decline in the eel-catches. The cormorants, which forage •.• r,.. on Usselmeer, come mainly from two colonies in three Fig. 1. The situation of Usselmeer and the marshes nearby marshes: Lepelaarsplassen, Naardenneer and Oostvaarders~n (1). Lepelaarsplassen (2) and Oostvaardersplassen. These colonies are unique by size in Naardenneer (3). Europe and are therefore of international importance. The Tbe water is supplied for 70% through the River IJssel. Since only the produetion of predators is known Tbe phosphorus load amounted 7 and 1.4 g.m,2.yr'! in the a transfer efficieney (ratio of the biological produetion of northem and southem part respectively. The nitrogen load predator and prey) of 20% from fish"to fish (Pauly 1986) is about 10 limes as higher (Berger and Sweers 1988).. and 10% from macrofauna and zooplankton to fish is Total phosphorus levels averaged over the penod 1983-86 taken. 028 mg.l'! and 0.15 mg.l'! in the northem and southem part respectively. In the southem part phosphorus is Birds mainly adsorbed by susPended silt and c1ay. Hence only 10% of phosphorus is directly available for groWth of Birds were counted by means of aerial sUrVeys on algae. From time to time growth of algae can be a monthly basis over the entire lake. FIsh predation by phosphorus-limited (Berger et a1. 1986). Mean deplh of birds is investigated by exainining pellets of gulls and the northem and southem part is 4.5 and 3.6 m. In this cormorants and stomaebs of drowned mergansers and study data for northem and southem part are averaged grebes. Some data on fish consumption bY. birds were (weighed for surface area), since information for birds and collected in other years thciD. the period 1983-87. This is fisheries could not always be specified per lake part. due to the fact that abundaDce of a eertain bird species on IJssehDeer is influenced by the av3ilability of prey species. Phytoplankton FIshery . Diatoms (Melosira spp., Asterionella formosa) are fomid especially early in the year. Tbe most abundant Tbe imPact of the fishery is estimated on the basis green algae are Scenedesmus spp.. Microscystis aeruginosa of registered landing statistics. Age and size eomposition is the most abundant blue-green alga. In some years a are based on market sampling programmes. Surveys bloom of Oscillatoria agardhii is found (Berger and revCa1ed the amountof bycateb, cOInposed of 0- and 1­ Sweers 1988). Phytoplankton concentrations were group fish, in the fyke net fishery. This bycateb is measured at 3 stations in the northem and 3 stations in discarded and is thought to be the main food souree for the southem of IJsselmeer. Data were available for the gulls and tems. period 1983-1986. Chlorophyll a concentrations were converted to fresh weight as folIows: 1 mg chlorophyll a RESULTS = 100 mg dry weight and 1 g fresh weight =:= 0.15 g dry weight. Primary production data were not available. So Tbe estimated biomass, production and data according to Berger and Sweers (1988) for 1976 were consumption are presenled in FIg. 2 and Table 1. Tbe used: daily biomass gross produetion averaged 14% of thiekness of the line is an indication for the amount of the total biomass, whieh results in a yearly gross P:B ratio of energy Oow. Solid boxes and lines represent quantified 51.1. " biomass and traDSrer estimates. Dotted boxes and lines represent gtiestimaled prCdalor-prey relationships. Zooplankton Food conversion factors from phytoplankton to zooplankton were not available. For eutrophie systems it No zooplankton data were available for the is not known whether the major portion of the transfer is period 1983-1987. So the zooplankton data from a lake­ direct from phytoplankton to zooplaDkton or indirect via wide survey during June, August and September 1987· detritus and bacteria (Vijverberg pers. conim.). 1989 were used. Sampling was carried out on 22 stations PlanktivoroUs fish consumed about.10% of the with a Friendinger or Sehlinder-Patalas sampier. zoopIaDkton production. Smelt has the highest production • P:B ratios are based on estimates made by (130 kg.ha-!) of all fish species. Piscivorous f!sb and birds Vijverberg et al. (in press) in Tjeukemeer, whieb is utilise 77% of this produetion. Perch has the highest situated in the northem part of the Netherlands and has produetion of the piscivorous f!sb (13.9 kg.ha'!). ci similar zooplankton community. Daphnia hyalina. . Fish predation by birds is occurring all over the spp., Chydorus sphaericus and cyc1opoids are the lake, but is especially prominent in the neighbourhood of main species. On a yearly basis P:B ratio for the species resting places or eolonies. On an aimual basis cormorants eombined was 52.7 yr'!. . consume about 22, gulls and tems 14, mergansers 7 and grebes 6 kg of fish.ha-!. Sme1t, ruffe and small perch are Fish iniportant foUrage for most bird spedes. Cormorallts take larger prey as weIl, like adult perch and roach. Nowadays, Fish stock biomasses were estimated during the consumption of eel by birds has beconie marginal. November surveys, using a 20 mm bottom trawl. Area Although in low abundanee (0.01 kg.ha-t ) red·breasted swept c1ear was used to calculate abundanee per hectare. mergansers (Mergus serrator) consumed the largest For smelt volume swept clear was used, since itwas the amount of eel (0.7 kg.ha't). Cormorants took this specles only species, whieh was c1early not eonfined to demersal only in less than 1% of their diet (0.1 kg.ha'!). layers only. , •

which are thought to be of great importance, have to be The impact of the fishery consisted mainly in further quantified. removing large piscivorous eel, perch and pikeperch. The fyke net fishery for spawning smell is found to be of CONCLUSIONS minor importance compared to the consumplion by perch and birds. 1. Smelt lS a key-species 10 the IJsselmeer No information is yel available for biomass, ecosystem: It is the main consumer of 1 production and consumption estimales of macrofauna zooplankton (1298 kg.ha-J.yr- ) and the main food (Dreissena polymorpha, Gammarus cigrinus, source (42%) for fish-eating birds and also the integer, chironomids and Oligochaetae) and macrofauna main prey species for the commercially important eating birds (lufted duck (Aythya fuiigula), scaup duck eel, perch and pikepereh. Management strategies (A.marila), pochard (A.ferina), goldeneye (Bucephala should take this into consideration. clangula) and coot (Fulica atra). These transfer rOUles,

x 0.01

birds

COR

RU

ZOOPLANKTON

x 10

Fjg. 2 Biomass .nd coOSumptiOD of the major compoocDts of the various lJ"ophic levels in the Usselmeer ecosyscem. Solid bcu:es and lines are quanlified estimate$. Thc sizc cf the boxes aod Ihe thicmess oe Ihe lines depiet lhe amount oe biomass or consumplion by a cenain componenl. With lhe multipliers al lhe right. biomass ""n be adjusled 10 the scale of middle panel. Dolled ooxcu are thought co be imponanl but not quantificd. Dolted üoes are guestim:1tcd transfer

rOUles. Data and most abbrcyjalions are given in Table 1: DR '01; Dreissena fKJlymorpha: PO ­ pochard; TD ~ lufled duck; GE : goldeneye; SO - ,,,,,ur duck; er - coot .) nie consurnplion of in venebrates and zooplankloo by perch is quaolified, bUI not for roach and brcam. .. .' \ "

2. Perch is the most important piscivorous !ish Berger, C. arid Sweers, H.E., 1988. Tbe Usselmeer and its species and cormorants are the most important phYtoplankton - with special attention to the fish-eating birds as the amount of fish consumed suitability of the lake as a habitat for Osdllatoria is concemed. agardhii Gom.• J. Plankton Res. 10: 579-599. 3. Estimates of !ish stock biomass could be under­ estimated, since trawl efficiency is assumed to be Eerden, 1\l.R. van and Zijlstra, 1'.1.,1986. Natuurwaarderi 100%. Comparison of trawl survey indices and van het Usselmeergebied. Prognose.van emge cohort analysis of the commercial eatch for perch natuurwaarden in het Usselmeergebied bij aanleg and pikeperch did not show an wider-estimation van de MarkerWaard (Nature values of the by the survey indices. This is thought to be due to USselmeer area. Prognosis of some of the nature the fact that sampling is carried out at places, values in the Usse1Jlleer area after reclaimation of which are abounding in !ish. the Markerwaard). Flevobericht no. 273. 4. This study was meant as an mterdisciplinary Directorate Flevoland. 88 p. (in Dutch) approach by quantifying production and consumption in the same units (kg.ha-I fresh Pauly, D., 1986. A simple method for estimating the food weight). It serves as an overview which consumption of flSh population from growth data information is yet available and what important and food conversion experiments. V.S. Fish. Bul!. information is stilliacking. 84: 827-840. 5. Although data presented do not represent ultimate and detailed estimates, this study Vijverberg, J., Boersma, M., Densen, W.L.T. van, nevertheless scaled the eXPerienced problems Hoogeboezem, W., Lammens, E.H.R.R. and (cormorants eating ee!, and human overfishing on Mooij, W.M., 1990. Seasonal variation in the the smelt) to more realistic proportions. iriteractions between piscivorous fish, planktivorous fish and zooplankton in a shallow REFERENCES eutrophie lake. In: Bonin, DJ. and Golterman, HL. (Eds.). Fluxes between trophic levels and Berger, C. et al., 1986. De kans op blauwalgenbloeien in through the water-sediment interface. de randmeren van de Markerwaard (Tbe Hydrobiologia 000: 000-000. possibility for blooms of blue-green algae in the border lakes of the Markerwaard). Flevobericht no. 268. Directorate Flevoland. 90 p. (in Dutch)

Table 1 Biomass, production and consumption estimates of the major compo':!ents of the various trophic levels in the IJ'sselmeer ecosystem. Biomass data are 1.n kq fresh wt.ha-·: production and consumption data are in kq fresh wt.ha-·.yr-l. eH .. chironomids: GA .. Gammarus spp. : NE .. Neomysis integer

Im CQlSlJ!PTI:Il (kq.ba-1.yr-')

Ill:WOI ettE BIOOSS Ill:mtIQI ct:&1PfION %0 ar Cl Ilt SI 111 RO Ii .-m. P-IIL !-Pt P-Pt pp (kq.ba-·) Ikq.ba-1.yr·llkq.ba-·.yr-'l alqae AL 1928 mn %ooplanktlll %0 181 5723

SEIt 0S!ImS ererl!nlS S! 66.1 129.1 ma.' l~a.o mfe !jyIloc!maJlIS ee!ll!n RD '.3 U roadl Rytj!lS rnillS RO 15.0 U tIUI !lmis brnB Ba '.5 3.1 lICll'"pisci'lOl1l1IS eel Ar!rJilla !!lauill! .-m. u 1.9 pisci"lOl1l1IS tel P-tEl. 2.1 0.7 3.5 3.5 DlIl1'isci'lOlO1l!i pert:b ma llwhtilis HE 2.5 5.3 52.7 21.2 3.7 12.6 12J pisciVOlO1l!i pertIl HE lU 13.9 67.5 51.5 9.l piteperdl StilllStedirn heitrem IP 2.0 2.0 10.2 U 1.0 D.1 0.1 0.1

betriJr;1Uli !.arJs arg~tatDs RG 0.0165 1.5 o.e 0.5 0.3 U. tlaci:-beaded qull Ln ridjmys RlC 0.0715 lU 1.1 l.D 1.0 qnaur tla::Hacied qull !.arJs !lr!lllS GII: 0.0071 0.1 0.0 0.2 0.2 0.0 little qull Lw; 'l!!!!1IS LG 0.0010 0.2 0.2 ClDllI qull l.m!UM ce 0.00l! 0.2 0.1 0.0 0.0 0.0 Ctla tem St!ma ßirmh rn: MJIl 0.7 U 0.0 Q.1 tlael: tmI C!lIidccuas njgu lTE 0.De53 1.1 1.1 0.0 0.0 9lUt msted 9rebe pgtim c;is13tl!S CI 0.0618 5.7 U 0.' 0.1 0.1 0.3 0.1 900saMer Ilm!!s 1l!l'j!llser CO 0.0651 5.' 2.7 0.' 1.5 0.7 0.1 mblUSteIlll!1'qaDSU "m semtgr RIIII o.om 1.1 0.3 0.0 D.O 0.7 0.0 SII!Y !m ajrellllj S O.OOBC 1.0 D" D.l D.O CGnllM Pba!.lcr!pm a,-tg c:l 0.2918 ·221.7 1.7 1.4 3.l 0.1 0.1 7.3

fisWy 23.2 U 1.1 3.9 1.0 3.0 3.5 1.0 TttlL 1501.4 1322.2 3.7 12.6 12.1 100.2 21.2 7.2 U 1.1 3.1 '" 3.7 1.0