The role of fish stock management in the control of eutrophication in

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.
Carpenter,R.S.&Lodge,D.M.,1986.Effects ofsubmergedmacrophytesonecosystemprocesses.
AquaticBotany.3/4:341-370.
Cazemier,W.,1982.Thegrowthofbream (Abramisbrama) inrelationtohabitat and population
density.Hydrobiol. Bull.16:269-277.
Densen,W.L.T.van, 1985.Bream.In:Pike,pikeperch andbream;biology,population dynamics
and management. Report of theworkinggroupon evaluation of management methods. LH,
OVB, Rivo,S&B.Nieuwegein. pp.64-87.(In Dutch).
Densen, W.L.T. van, Dijkers, C. &Veerman, R., 1986.The fish community of the Loosdrecht
lakes and the perspectives for biomanipulation. Hydrob.Bull.20:61-75.
Donk,E.van,Gulati,R.D.&Grimm,M.P.,(inpress).Restoration bybiomanipulation inasmall
hypertrophic lake:preliminaryresults. Hydrobiologia.
Donk, E. van, Slim, F.J. & Grimm, M.P., 1988. Herstel van de recreatiepias 'Zwemlust' door
toepassing van actief biologisch beheer: eerste resultaten. H2O 13:338-343. (English summary).
Feith, A.F., 1982.Visstand, visserij en waterkwaliteit. Bijdrage t.b.v. het opstellen van rijks- en
provincialewaterkwaliteitsplannen. DirectievandeVisserijen, Den Haag.Doccrapp.nr24.
Grimm, M.P., 1981. Intraspecific prédation as a principal factor controlling the biomass of
northern pike (Esoxlucius). Fish Mgmt. 12: 77-80.
Grimm, M.P., 1983.Regulation of biomasses of small northern pike (Esox lucius), with special
reference to the contribution of individuals stocked asfingerlings (4-6cm).Fish.Mgmt.13:
115-134.
Grimm, M.P., 1985. Pike. In: Pike, pikeperch and bream; Report of the working group on
evaluationofmanagementmethods.LH,OVB,Rivo,S&B.Nieuwegein.pp.4-28.(inDutch).
Hosper,S.H.,Meijer,M.-L.&Jagtman,E.,1987.Actiefbiologischbeheer,nieuwe mogelijkheden
bijhet herstelvan meren en plassen.H2O 12:274-279(English summary).
Lammens,E.H.R.R., 1986.Interactionsbetween fishes and thestructureof fish communities in
Dutch shalloweutrophic lakes.PhD Thesis,Agricultural University, Wageningen. 100pp.
Meijer,M.-L.,Lammens,E.H.R.R.,Raat,A.J.P.,Grimm,M.P.&Hosper,S.H., (inpress).Impact
ofcyprinidsonzooplankton and algaeinten drainableponds.Preliminaryresults.Hydrobiologia.
Meijer, M.-L., Raat, A.J.P. & Doef, R.W., 1988.Actief biologisch beheer als herstelmaatregel
voor deBleiswijkse Zoom, eerste resultaten. H2O21:344-348. (English summary).
N W S , 1985.Stek tachtig.Deelnota 2.Visstandbeheer. N W S , Amersfoort. 48pp.
Richter, AF., 1985.Possibilities of biomanipulation for restoringwater quality in The Netherlands.Limnologisch Instituut. Oosterzee/Nieuwersluis.Report 1985-08.109pp.
Scheffer, M., 1989.Alternative stable states in eutophic shallow freshwater systems: a minimal
model.Hydrobiol.Bull.23:73-83.
Vierssen,W.van,Hootsmans,M.J.M.&Vermaat,J.E., 1985.Waterplanten:bondgenotenbijhet
waterkwaliteitsbeheer? H2O 18:122-126. (English summary).
Winkel, E.H. ten &Meulemans,J. T., 1984.Effects offish upon submerged vegetation. Hydrobiol.Bull. 18:157-158.
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