In:W.L.T. vanDensen, B.Steinmetz&RH. Hughes(Eds).1990.Managementoffreshwaterfisheries.Proceedingsofa symposiumorganizedbytheEuropeanInlandFisheriesAdvisoryCommission, Göteborg, Sweden, 31MaySJune1988. Pudoc.Wageningen,pp. 328-343. The role of fish stock management in the control of eutrophication in shallow lakes in The Netherlands E.Jagtman ,S.H. Hosper ,M.-L.Meijer &E.van Donk . Directie Flevoland, Rijkswaterstaat, Lelystad,The Netherlands Institute ofInland Water Management and Waste Water Treatment, Rijkswaterstaat, Lelystad,The Netherlands ProvincialWater Board ofUtrecht,Utrecht,The Netherlands Abstract Eutrophication of Dutch shallow lakes (1-3 m deep) has led to a dramatic change in the structure and functioningofthefoodweb.Overthelasttwenty-fiveyearslittoralvegetation hasalmostdisappearedfrom Dutch shallowlakesasaconsequenceofeutrophication.Simultaneously thehabitatoftheimportantpredatorfishpike has disappeared. As a result, bream has reached very high biomasses in most shallow lakes. In controlling eutrophication the national policy isaimingat reduction ofnutrient loadingoflakes. It seems,however, that a widervarietyofmeasuresisneeded tospeedlakerecovery.Fishstockmanagement can beofimportance insuch an integrated approach. Bream maynegatively influence the recovery processasa result of its feeding strategy. Inthepresenceofanabundantplanktivorousandbenthivorous breamstock,zooplanktongrazingofalgaeislow, and thewatermaybecomeveryturbid.Thissituation mayslowdownlakerecoveryeven iftheexternal nutrient loading has been severely reduced. Regulation of the bream population may therefore contribute to solving eutrophication problems byloweringthe internal resistance oftheecosystem against changes. Biomanipulation experiments have started recently insmall lakes in the Netherlands. The experiments are primarily directed at reducingthebreampopulationbyfisheries,andintroducingpredatoryfish.Fishstockmanagementincombination with nutrient reduction may thus help in solving the eutrophication problem byinitiating a positive feed back process in the food web that may convert ecosystem structure into a socially desirable,i.e., a stable and highly diverseecosystem. 1.Introduction Livingin the Netherlands one always haswater nearby. Ponds, canals, ditches,lakes, streams and rivers are plentiful and cover a surface area which amounts to about 3500km2. These aquatic ecosystems^are being used for fishery, recreational boating, swimming,boat traffic, industrial use,drinkingwater supply and agricultural purposes. Each ofthesefunctions makesitsownspecific demands.These demands cannot always be complementary. Industrial discharges and drinkingwater supply obviouslywon't go together. Water quality management in the Netherlands focuses on the protection of specificfunctions ofsurface watersasmentioned above,but alsodirectly onthe protectionanddevelopment ofecosystems.Thislatterobjective hasmuch incommonwith the general objectives offish stockmanagement. Water qualitymanagement and fish stock management thusshareacommoninterest.InspiteofthisthereisintheNetherlands a situation inwhichresponsibilitiesfor fish stockmanagement andwater qualitymanage328 Theroleoffish stockmanagementineutrophicationcontrolinshallowlakesinTheNetherlands ment areseparatedbylaw.Inthispaper attention ispaidtofish stockmanagement asa toolineutrophication control.Resultsofrecent biomanipulation experiments (regulating fisheries) in eutrophic lakes are given. The results show that this type of lake restoration mayspeed uprecoveryprocesses.Thisintegrated approach oflake restorationmayturn outprofitable both tomanagers offish stocksandwater quality, and thus emphasises theneed for a dialoguebetween allparties involved. Figure 1. Thestructure ofpikepopulations insituationswhere therearedecreasingamounts ofvegetation. Small pike (<45 cm) protect themselvesfrom cannibalism byhidingamongplants.Inpassingfrom situation a tosituation c,plantgrowthdecreasesbecauseofeutrophication, and thestructureof thepikepopulation becomesunbalanced (Source:Organisation for the ImprovementofInland Fisheries). 2.Consequences ofeutrophication Eutrophication isaseriousproblem inDutch shallow(1-3mdepth) lakes.Ithasled to a dramatic changeinthestructureand functioning ofthe aquaticfood chain.In general the eutrophication process can be described as follows. Eutrophication starts with an increase innutrient load.The water remains clear as the extra nutrients are processed bythe abundant littoralvegetation and thelakesediment (ecosystem resistance). Continuous nutrient loading,however, leadstoa prolific growth of filamentous algae upon the higher plants.This results inthe disappearance of macrophytes due to a lack oflightforgrowth.Thisdisappearance hasdrasticconsequencesfor theecosystem (Van Vierssen, Hootsmans & Vermaat 1985, Carpenter, Stephen & Lodge 1986, Grimm 329 Jagtman,E.,Hosper,S.H.,Meijer,M.-L.&Donk,E.van 1985).Themostcharacteristicchangeisasharpincreaseintheabundanceof planktonic algae.Persistentnutrientloadingeventuallyleadstopermanentbloomof cyanobacteria, whichresultsina highlystableecosystemofpoor quality. Overthelasttwenty-five years,littoralvegetation hasalmost disappeared from Dutch shallowlakesasaconsequenceofeutrophication.Withthedisappearanceof submerged vegetation the habitat ofthenorthern pike (Esoxlucius)wasdestroyed.The survivalof thisspecieshasbeen found tobe stronglydependent ontheavailabilityofhidingplaces, which in general are plants or plant remains (Grimm 1981,1983). Pike hatchlings are attached to plants in the first days of their lives.Pike seek shelter between submerged plantstoprotectthemselvesfrom cannibalism,ortohidethemselveswhenhunting.The relation between hiding places and the structure of a pike population is illustrated in Figure 1.In water rich in macrophytes the survival of young pike is relatively high and their large numbers can effectively regulate young bream (Abramis brama). It will be clearthatthedisappearance oflittoralvegetationisanimportant causeofthe declinein numbersofpike,andanincreaseinthoseofbreaminshallowwaters.Breamhave found exceptionally good conditions in the algae rich open waters, since these provide an abundant food supply (zooplankton and midge larvae) and greatly reduce the risk of prédation.AlmostallDutchshallowlakesarenowbream infested (Lammens 1986,van Densen 1985,van Densen, Dijkers & Veerman 1986) with dense populations having biomasses of several hundred kg/ha (Cazemier 1982). This enormous bream stock enhances the algal biomass by reducing the zooplankton density (lowered grazing pressure)andbyrenewingthesupplyofnutrientsfromthelakesediments(bioturbation). Thus, the process of eutrophication has led to a situation inwhich most Dutch shallow lakeshavealifecommunity dominatedbyalgaeand bream. 3.Current strategyofeutrophication control Eutrophication controlintheNetherlands aimsatreducingthephosphorus load from the environment. Measures are being taken to reduce the phosphorus contents of detergents,toremovephosphatefrom theeffluents ofsewagetreatment stations,and to lower industrial discharges and the release of phosphorus from agricultural areas. Loweringphosphorusloadingshouldsolvetheproblem ofalgalbloomsbyloweringlake productivity.Despite major efforts over the past fewyears,hardly anyrecovery can be seenatall.Wethinkthatbiologicalfeedback mechanismsunderliethislack ofrgcdvery. Intheprocessofeutrophication, acriticalthreshold innutrient concentrations hastobe exceeded before any biological response by the system may be seen (Figure 2). In the samewayitmightbe expected that inlakerestorationprogrammes,nutrient concentrationshavetobereduced toacertainthresholdvaluebefore aresponse canbe expected. In fact, two levelshave tobe passed before changes can take place.As a first step itis necessary to remove the nutrient store in the lake to make phosphorus or nitrogen a limiting factor for algal growth. A positive biological response of the system may then follow directly,but usuallya second step isneeded because ofthe internal resistanceof theecosystemtochanges.Largequantitiesofdetritushavebeenformed onthe bottoms of the shallow lakes from the abundant algae.Benthivorous fish, feeding on this, may then increase nutrient concentrations and turbidity, thus lowering the effectiveness of measures aimed at reducing nutrient loadings from external sources.Release of phos- 330 The roleoffish stockmanagementin eutrophicationcontrolin shallowlakes in TheNetherlands phonis from lake sediments is another factor which can frustrate lake recovery. High algalbiomasscanmaintain abigfluxofphosphorus from thelakesediment.In summer thisinternalfluxmaybeashighastheexternalphosphorusloadingofthelake(Boström, Jansson & Forsberg 1987).These feedback mechanisms oblige thewater quality manager to put an extra effort into reducing the phosphorus load of the lake before the primary objective of the restoration programme, lowering the algal biomass, can be achieved.Whenthisgoalisreachedproblemsstillremaintobesolved.Ecosystem quality staysrather poor asplant growthisimpaired bylargequantities ofbenthivorous bream whichturnupplantroots(tenWinkel&Meulemans1984).Largequantitiesofnewborn bream may decimate the zooplankton,thereby reducing the consumption of algae.As bream can live up to an age of 20years the present heavy infestation of bream in the Dutch shallow lakes may continue to influence water quality for a long time. These biological mechanisms lead to the conclusion that solving the eutrophication problem simplybyreducingthe(external)phosphorusloadwillbesuccessful inthelongterm,but averytimeconsuming method. internal ecosystem resistance Q biological control route nutrient route step 2 B'. breaking internal ecosystem resistance step 1 removal of nutrient store nutrient loading Figure2. Diagram representingecosystem quality inresponse tonutrient loading.Nutrient loadingbeyond optimal situation A leadstoecosystem decay (A' toB)after aperiodofinternal ecosystem resistance.Beyond point B, ecosystem delayislimited byother factors (e.g. light).Lake restoration bynutrient reduction isa two-stepprocess (dark upward arrow).Biologicalcontrol methods may speed uplakerecovery (lightarrow) byalteringthreshold concentration B'intoB(adaptedfrom Hosper, Meijer &Jagtman 1987). 331 Jagtman,E.,Hosper,S.H.,Meijer,M.-L,&Donk,E. van 4.Objectives from theviewpointoffish stockmanagement Intheprecedingparagraphsitwasrecognizedthateutrophicationleadstoecosystem decay. From the viewpoint of water quality management this is a problem because protection anddevelopment oftheecosystemisageneralobjective.Clearly,from the viewpoint of fish stockmanagement, eutrophication isa problem too.Fish makedemandsontheirenvironment.Theirbasicneedsaregoodwaterquality,asufficient supply ofsuitablefood,anopportunitytofindcover(vegetation),goodspawningconditionsand migrationopportunities.Eutrophicationinterfereswithmanyofthesedemands.Individualgrowth ofcyprinids,forinstance,ispoorinmosteutrophiclakes,asaconsequence midge larvae Figure3. Principal food chain relationsinaeutrophiclake.Algaeandbream dominate,resultingin turbid water (negativespiral).Zooplankton, pikeandmacrophytes arefewinnumber (from Hosperetal 1987). 332 Theroleoffish stockmanagementineutrophicationcontrolinshallowlakesinTheNetherlands ofthedensityofthestockpresent.Fisheryorganizationshave determined thisand seek remedialmeasures.Theeutrophication ofDutchinlandwatershastobe reduced (Feith 1982, N W S 1985). An overcrowded stock of cyprinids is recognized as an effect of eutrophication and although regulating fisheries are considered useful in reducing this stock,itisbelieved that thesemeasures onlytake awaythe symptoms of eutrophication ( N W S 1985). Reduction of nutrient loading has to be the main objective. Without nutrient reduction the eutrophication problem will not be solved. It is important that fisheries regulation doesmorethan'justtakeawaythesymptoms ofeutrophication'.By regulatingthecyprinidstockinaeutrophiclake,conditionsaresetwhichfavour aswitch to a more diverse ecosystem. Considering the biological feedback mechanisms in an eutrophic lake, it might even be expected that the critical threshold concentration at which lake recovery sets in might be altered in a positive way. Objectives like species diversity, good growth of the fish stock, and the presence of pike and rudd (Scardinius erythrophthalmus) ( N W S 1985)thus comewithin reach.This isa strong argument for usingbiological measures inparallelwith nutrient load reducing measures in eutrophication control. 5.Biological controlin practice How canwe get from the situation of turbid, algal-rich, bream-infested water, to one ofclearwater,arichlittoralvegetation and astrongpikepopulation?Thisquestionwas raised when we started to think about the possibilities of biological control methods in water quality management. On the basis of preliminary research (Meijer et al. 1987), literature reviews (Richter 1985) and interviews with fish scientists, a theory was developedwhichproved helpful inchoosingthekeyelementsinthefood chain atwhich to direct measures (Hosper, Meijer & Jagtman 1987).The principle food chain relations aredepicted inFigure3. Reducingthenutrient load istheprimaryobjective to decrease thebiomassofalgae.Whennutrient concentrationsstarttolimitthegrowth ofalgae the importance of biological measures grows.Artificial reduction of the bream population may lead to an increase in the number of (large) zooplankton, which causes, through their grazing, a lower algalbiomass. In turn this may lead to clearer water, chances for higher plants to grow, the return of pike, an increase inprédation of bream and more zooplankton etc. A positive spiral may thus be set in motion. Less re-suspension of sediments as well as the reduced disturbance of rooted macrophytes also brings about positivefeedback processesleadingtothe desired lake conditions. Pikeperch (Stizostedion lucioperca)is known to be able to maintain itself in turbid algal-rich waters. This makes it a suitable predatory species for reducing the large numbers ofcyprinidsineutrophiclakes.There isprobablylittlepoint inreleasingyoung pikein'bare'waters,duetothestrongintraspecific traitofcannibalism.Consideringthis, pikeperch seemed to be the most attractive species to experiment with. Following successful pond experiments in 1986(Meijer etal.1987),itwas decided to start experiments under natural conditions in 1987. The results of two of these experiments are discussed inthe following section. 333 Jagtman,E.,Hosper, S.H.,Meijer,M.-L.&Donk,E. van Table 1. Data onfishremovedfromtheGalgjecompartment in the BleiswijkseZoominApril 1987 (from Meijer, Raat &Doef 1988). Species length(cm) number weight(kg) <12 200 0.4 12-22 11 0.7 22-37 205 55.3 37-46 101 73.4 >46 45 96.9 Abramisbrama(bream) 8-16 33 521 636.5 Bliccabjoerkna(silverbream) 16-24 398 41.3 Stizostedionlucioperca (pikeperch) Subtotal: 226.7 24-29 158 45.1 29-34 219 110.8 >34 455 Subtotal: Cyprinuscarpio (carp) 353.8 1187.5 108 550.0 Esoxlucius(pike) 5 16.3 Percaßtviatilis (perch) 7 0.8 Rutihisrutilus(roach) 63 1.4 3 1.5 Carassiuscarassius(cruciancarp) Anguillaanguilla(eel) Totalweightremoved: 62 circa2 000 6.Case studies 6.1 Case 1:Bleiswijkse Zoom (Meijer, Raat&Doef 1988) TheBleiswijkseZoomsystemofthreesmallinter-connectedlakes,coveringatotalof 14.4ha,wasformed in1970forrecreationalpurposes.Initsfirstyearsofexistencethe systemwascharacterisedbyhighphosphorusconcentrations(0.4mg/1)resultinginpeak chlorophylllevelsof300g/1andatransparencydepthofabout0.2m.In1986thequestion wasraisedastowhetherfishstockmanagementcouldhelpinimprovingwaterquality. Inco-operationwiththeOrganizationfortheImprovement ofInlandFisheries,which usesthelakesfor research purposes,amanagement schemewasdevisedwhichaimed atcontrollingtheratherdensestockofcyprinids.Thelakesystemwasdividedintotwo 334 The roleoffishstockmanagementmeutrophicationcontrolinshallowlakes in TheNetherlands top Zeeltje (control area) Galgje (treated area) 0 0 O.O-i 0.1 - ts ^g 0.2- Q. 0.3- r- -o 0.4- U in Q. 0.50.6- h- 1 0.70.80.91.01 bottom •1- 1 May Jun Jul Aug 1987 1 Sep 1 Oct 1 Nov •Zeeltje (control area) Galgje (treated area) ~i 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1987 Figure4. Changesin theBleiswijkseZoom following management of the fish stockinApril 1987. a. transparency depth,b.chlorophyll concentration (from Meijer et al1988). 335 Jagtman,E.,Hosper,S.H.,Meijer,M.-L.&Donk,E. van a Zeeltje (control area) Galgje (treated area) 50 30- 20 V'* / '\ v/ \/\/ \ /* 10- V\ V May )un Jul Aug Sep Oct Nov 1987 Zeeltje (control area) Galgje (treated area) May Jun Jul Aug Sep Oct Nov 1987 Figure 5. Changes intheBleiswijkseZoomfollowingmanagementofthefish stock inApril 1987. a. inorganicsuspendedsolidconcentration,b.total phosphorusconcentration (from Meijer et aLl988). 336 The roleoffish stockmanagementin eutrophicationcontrolinshallowlakesin TheNetherlands Zeeltje (control area) Galgje (treated area) O.S-i 0.4E « 0.3 Sa c 02- o e E < 0.1 May Jun Jul Aug Sep Oct Nov 1987 d Zeeltje (control area) Galgje (treated area) 0.8 May Jun Figure5. Changesinthe BleiswijkseZoom following management ofthe fishstockinApril 1987. c.ammonium concentration,d.nitriteand nitrate concentration (from Meijeretal.1988). 337 Jagtman,E.,Hosper, S.H.,Meijer,M.-L.&Donk,E. van 500-1 - Galgje (treated area) 1i f i 1 , 400- ê K 1\ 300- î > ' Cl 200-O E 100- •f V A • ' '1' 1 1 1 t t 1 /\ /\ /ƒ/ \\ ƒ ƒ ƒ / ƒ / î 11 / \ \1 1* f / \ V f s \\ \\ \ 0 May Jun lul Aug 1 N —T= Sep ^ n Oct A Nov 1987 Figure6. Total number ofDaphniain the BleiswijkseZoomfollowingfishstockmanagement inApril 1987 (from Meijer etal 1988). compartments called Galgje (3.1ha; treated area) and Zeeltje (11.3ha; control area). Water quality in these two compartments was similar before treatment. After being separated by a dam, exchange of fish between the compartments could not occur. The treatment of the Galgje compartment consisted of removing about 2000 kg of fish (Table1), mainly (1200 kg) bream and silver bream (Bliccabjoerkna), carp (Cyprinus carpio) (500kg)andpikeperch (200kg).Bytheend ofApril19871200young pikeperch of2.5cmlengthwereintroduced toregulatenewlybornbreamfrom theresidual bream stock.IntheZeeltje compartment notreatmentwasundertaken.Within afewweeksof treatmentwaterqualityhadimprovedtremendouslyintheGalgjecompartment(Figures 4a, b,5a, b,c, d).Nutrientandsuspended solidconcentrationswerereduced, chlorophyll levels were extremely low and transparency had increased to 1.1 m (the bottom). Macrophytes, predominantly Chara vulgaris var. longibracteata,grew rapidly. Large zooplankters likeDaphnia hyalina first increased rapidly, but their numbers dropped fromJuneonwards(Figure6).However,algalbiomassremainedlow,despitethereduced grazingbyzooplankton.ThereareindicationsthattheprolificgrowthofCharaceae has somethingtodowiththis.Allelopathymayhavesuppressed thegrowth ofthealgae,but it is possible that a lack of nutrients (nitrogen) may have been a limiting factor. By contrast, in the Zeeltje compartment no improvement was seen.However, the growth and survival ofthe newlyintroduced pikeperch waspoor,presumably due tothe dense vegetation (100% cover) which would have hindered them when hunting. In fact, surprisingly, the measures taken resulted in the development of a pike habitat rather thanapikeperchhabitat(moreopenwater).Sincepikehadbeen almosttotallyremoved 338 Theroleoffish stockmanagementineutrophicationcontrolinshallowlakesinTheNetherlands from the system, and their numbers are still low, a number of young pike will be re-introducedin1988. MeasurestakenresultedinthefastrecoveryoftheGalgjecompartment,withecosystem alterationsbeinginlinewithexpectations andweanticipatethatthepresenceof macrophyteswillnowstabilizetheecosystem(Scheffer 1989).Ecosystemmonitoringwill becontinuedforthecomingthreeyears. Table 2. DataonfishremovedfromLakeZwemlust (March 1987) byseine-netting and electro-fishing (Fromvan Donk,Slim &.Grimm, 1988). Species Abramisbrama(bream) Rutilasrutilas(roach) length(cm) >20 weight(kg) 116 99.4 10-20 10 005 435.0 <10 6 208 50.6 >10 4 025 64.0 <10 2 912 Esoxlucius(pike) Leucaspiusdelineatus Scardiniuserythrophthalmus(rudd) number >20 10-22 <10 , 24.7 44 56.6 18 547 30.8 4 1.0 223 5.5 115 0.6 575 9.2 24 15.7 Tineatinea(tench) 7 6.4 Cyprinuscarpio(carp) 1 6.4 >20 9 3.2 <20 357 1.7 1 0.2 43 173 811.0 Bliccabjoerkna(silverbream) Anguillaanguilla(eel) PercafluviatiUs(perch) Stizostedionlucioperca(pikeperch) Totalnumberandweightremoved: 6.2 Case 2:Lake Zwemlust (vanDonk,Slim&Grimm 1988,vanDonk,Gulati & Grimm1987) LakeZwemlustisashallow(meandepth1.5m)hypertrophiclakeof1.5hainthecentre ofTheNetherlands.Lowtransparencydepths(0.1-0.3m)andalgalblooms (Microcystis aeruginosa) havecharacterisedthislakeforyearsatastretch.LakeZwemlustisinuse asa natural swimmingpool and water qualitywasnotveryattractive for recreational activities.Thecommonstrategyofreducingthephosphorusloadingofthelaketoinduce 339 Jagfman, E., Hosper,S.H.,Meijer,M.-L.&Donk,E. van Bottom J'F'M'A ' M ' J ' J ' A ' S ' O ' N ' O ' J ' F ' M ' A ' M ' J ' J ' A ' S ' O ' N ' D ' 1986 1987 t Biomanipulation 200 '3 150 o 100 50 J'F'M'A 'M' J ' J ' A ' S ' O ' N ' D ' J ' F ' M ' A ' M " " " " " ' s ' a ' N ' D ' 1986 f 1987 Biomanipulation Figure7. ChangesinLakeZwemlustbefore and after biomanipulation inMarch 1987.a.tranparency depth. b. chlorophyll concentration (from van Donketal1988). 340 Theroleoffish stockmanagementineutrophicationcontrolinshallowlakesinTheNetherlands 1.5- ao E 1.0 0.5- J F MA M J J A S O N 0 IJ F M A M J J t 1986 A S O N D 1987 Biomanipulation 1.5- E — 1.0H 8 0.5 J F MA M J J A S O 1986 Figure8. Changesin LakeZwemlustbefore and after biomanipulation inMarch 1987.a. Phosphorus concentration .• orthophosphate, O total phosphate, b.nitrogen concentration, • ammonium, Onitrate. 341 Jagtman,E.,Hosper, S.H.,Meijer,M.-L. &Donk, E. van lake recovery proved not to be possible, as thislake onlyreceives seepage water from the nearby nutrient richRiver Vecht.The tool offish stockmanagementwas therefore tested. Measures taken consisted of draining the lake bypumping, and eliminating the predominantlyplanktivorousandbenthivorousfishpopulation (Table2)byseine-netting and electro-fishing. The lake was subsequently restocked with 1500 northern pike fingerlings and a lowdensity of rudd asfood supplyfor thepike.Stacks ofwillowtwigs, roots of Nuphar luteum and seedlings of Chora sp.were brought in as a refuge and spawningplacefor thepike and asshelter for the zooplankton. These measures resulted inincreased transparencies of 1-2 m (Figure7a),lowchlorophylllevels,below5ftg/\(Figure7b),andadensezooplanktonpopulation, predominantly Daphnia sp. In contrast to the Bleiswijkse Zoom system, nutrient concentrations remainedextremelyhigh (Figures8aandb).Thisisprobablyduetothe dominant effect of seepage,whereasintheBleiswijkse Zoomsystem,nutrient concentrations dropped asa result of reduced bioturbation bybream and carp.Growth of rudd wasextremely good duetotheabundant food supply.Survivalofthe stocked pikewaslow,asexpected, due tocannibalismandalackoffish larvaetofeedupon.Midgelarvaeandzooplanktonwere dominant elements inthedietofthepike.Finally,growth ofmacrophyteswasgood,but was hampered by the development of large green algae like Hydrodyction sp. and Enteromorpha sp.These results are generallyinlinewith expectations.The anticipated decrease in the biomass of cyanophyceae, due to enhanced zooplankton grazing, was confirmed experimentally and led to increased transparency. Growth of macro-algae mayhowever, pose athreat tocontinued macrophyte development and may eventually affect ecosystem stability.InthiswaytheZwemlust and Bleiswijkse Zoom systems have exhibited differential development. 7.Concluding remarks Preliminaryresultsofmanaging fish stockstoreduce eutrophication inDutch shallow lakesarehopeful. Expectations ofecosystem development after regulatingfisheries are confirmed in general. Even when extremely high nutrient concentrations are present, ecosystemqualityseemstobecapableofimprovementfollowingfishstock management. Subsequent development of macrophytes isa key factor in stabilizing newly recovered ecosystems.Macrophytesmayreducenutrient concentrations,butitseemsthat, conversely,highnutrientconcentrationsdecreasethechancesofcontinuousmacrophyte development. So the basic need to reduce nutrient loadings remains and is crucial. It is however, important to recognize that fish stock management may effect nutrient concentrations aswell, either in a positive or a negative way, depending upon the density and composition of the stockpresent.Thisisa strong argument for allparties involved discussingthe possibilities ofintegrated lakemanagement, notwithstanding the current situationofresponsibilitiesbeingseparated.Effective fishstockmanagement mayspeed uplakerecoverybyreducingeutrophication problemsandbringingaboutthepossibility of a new ecosystem equilibrium. The stability of this new ecosystem equilibrium will depend upon thetrophic state ofthelake.Research willbe continued tofind out which levelofnutrientconcentrationmaybeconsidered'safe'forretainingasociallydesirable, stable and diverse ecosystem. 342 Theroleoffish stockmanagementineutrophicationcontrolinshallowlakesinTheNetherlands 8.References Boström, B.,Jansson M. & Forsberg, G, 1987.Phosphorus release from lake sediment. Arch. Hydrobiol. 18:5-59. 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