Responses of fish communities to sustained removals of perch (Perca fluviatilis)

Transcription

1 Responses of fish communities to sustined removls of perch (Perc fluvitilis) SCIENCE FOR CONSERVATION Benjmin G. Ludgte nd Gerrd P. Closs Pulished y Deprtment of Conservtion PO Box -4 Wellington, New Zelnd

2 Science for Conservtion is scientific monogrph series presenting reserch funded y New Zelnd Deprtment of Conservtion (DOC). Mnuscripts re internlly nd externlly peer-reviewed; resulting pulictions re considered prt of the forml interntionl scientific literture. Titles re listed in the DOC Science Pulishing ctlogue on the deprtmentl wesite nd printed copies cn e purchsed from science.pulictions@doc.govt.nz Copyright Jnury 3, New Zelnd Deprtment of Conservtion ISSN ISBN This report ws prepred for puliction y DOC Science Pulishing, Science & Reserch Unit; editing y In Mckenzie nd lyout y Ruth Munro. Puliction ws pproved y the Mnger, Science & Reserch Unit, Science Technology nd Informtion Services, Deprtment of Conservtion, Wellington.

3 CONTENTS Astrct 5. Introduction 6. Rtionle for study 6. Ecology of perch 7.3 Perch in New Zelnd 7. Aims nd predictions 8 3. Methods 9 3. Study site 9 3. Experimentl design 3.3 Preliminry survey 3.4 Physico-chemicl nlysis Perch removl Fish popultion monitoring 999/ Perch removl Fish popultion monitoring / Finl survey 5 3. Dt nlysis 6 4. Results 7 4. Preliminry survey 7 4. Physico-chemicl nlysis Ctch selectivity of gill nd fyke nets in reltion to perch size Fish community 999/ Fish community / 4.6 Other fish Finl survey Common ully size distriution Summry of fish community dynmics (999 ) 8 5. Discussion 3 5. Physico-chemicl prmeters 3 5. Ctch selectivity of gill nd fyke nets Impct of removls on perch popultion structure nd dynmics Impct of perch removl on common ullies Presence of other fish Use of gill nd fyke netting s method of perch reduction nd erdiction Conclusions Acknowledgements References 36

4 Responses of fish communities to sustined removls of perch (Perc fluvitilis) Benjmin G. Ludgte nd Gerrd P. Closs Deprtment of Zoology, University of Otgo, PO Box 56, Dunedin, New Zelnd ABSTRACT Over two-yer period, dult perch (Perc fluvitilis) were regulrly cught nd removed from three costl wetlnd ponds (removl ponds) in Otgo, South Islnd, New Zelnd. Resulting chnges in the fish community in the ponds from which perch were removed were compred with fish communities in three similr ponds (control ponds) from which perch were not removed. At the strt of the experiment, fish communities in oth the removl nd control ponds were similr, eing dominted y reltively smll numer of lrge dult perch nd low numers of common ullies (Goiomorphus cotidinus). Few young of yer (YOY) perch were present in the ponds suggesting tht cnnilism ws significnt fctor regulting perch popultion dynmics. Periodic removl of dult perch from the removl ponds using gill nd fyke nets significntly reduced dult perch undnce. In response, the undnce of common ully incresed significntly in the two smllest removl ponds. However, in the lrgest pond perch successfully spwned either just prior to or during the initil removl nd reduced dult perch undnce following removl resulted in incresed YOY perch survivl. By the end of the experiment, the fish community in this pond ws dominted y YOY perch nd common ully undnce remined low. In contrst to the chnges tht occurred in the removl ponds over the course of the experiment, no significnt chnges in the fish communities of the control ponds were oserved. The results of the study suggest tht perch hve significnt negtive impct on the undnce of common ully, nd tht cnnilism plys significnt role in regulting the popultion dynmics of perch in smll lentic hitts. Physicl removl of perch using gill nd fyke nets my offer potentil control option in smll lentic wter odies. However, incomplete removl of dults my still llow spwning. Due to reduction in cnnilism y dult perch, incresed survivl of YOY perch resulting in high numericl undnce of YOY perch my susequently occur. Keywords: Freshwter fish, introduced species, exotic species, common ullies, Goiomorphus cotidinus, lentic, Perc fluvitilis, perch, fish removl, netting Jnury 3, Deprtment of Conservtion. This pper my e cited s: Ludgte, B.G.; Closs, G.P. 3: Responses of fish communities to sustined removls of perch (Perc fluvitilis). Science for Conservtion. 38 p. Science for Conservtion 5

5 . Introduction. RATIONALE FOR STUDY At lest species of freshwter fish hve een introduced into New Zelnd. Of these, rown nd rinow trout, quinnt slmon, koi crp, rudd, tench, goldfish, Gmusi nd perch hve estlished self-sustining popultions cross lrge res of either the North or South Islnd, or oth (McDowll 99). In mny wterodies introduced species such s rown or rinow trout, perch, rudd, or koi crp now dominte the locl ichthyofun, oth in terms of solute numers nd iomss (McDowll 99). Severl of these introduced fish re recognised y lw s sportsfish, including the vrious species of slmonid, perch, tench nd rudd (in the Aucklnd Wikto region). Little is known of the impct of introduced fish species on New Zelnd freshwter ecosystems, lthough some evidence suggests it cn e significnt. Brown trout re known to eliminte smll, non-didromous glxiid fish in smll strems (Townsend & Crowl 99; Crowl et l. 99), result tht hs consequences for periphyton, invertertes nd strem productivity (Flecker & Townsend 994; McIntosh & Townsend 995; Huryn 996). However, other thn trout, the impct of vrious introduced fish species on ntive freshwter ecosystems is not well known. Perch re freshwter fish species tht ws introduced into New Zelnd in 868 s n ngling species (McDowll 99). They re reltively lrge predtory fish (commonly to 45 mm long) nd now occur in wide vriety of lowlnd freshwter systems cross oth the North nd South Islnds of New Zelnd (McDowll 99). Bsed on Europen studies, perch hve the potentil to hve significnt impct on the ecology of the systems into which they hve een introduced, directly influencing zooplnkton, mcroinverterte, nd fish popultions (Persson & Greenerg 99; Tonn et l. 99; Persson & Eklöv 995). At present, only limited studies of the perch diet, spwning nd hitt use hve een reported from New Zelnd systems (Duncn 967; Griffiths 976; Jellymn 98). No informtion exists s to their impct on ntive fish, invertertes, or other components of ntive freshwter ecosystems. There is growing recognition nd concern regrding the spred of introduced fish species cross New Zelnd. Informtion on their distriution, impct, nd potentil control methods is required in order to prioritise control strtegies for vrious species nd regions. In the cse of widely distriuted nd undnt species such s perch, erdiction throughout New Zelnd is not relistic option. However, some iologiclly significnt wetlnd res remin free of ny introduced fish species including perch. Understnding the potentil impcts of vrious exotic freshwter species should they enter res of high conservtion significnce, nd refining potentil methods for loclized erdiction is essentil if control strtegies re to e developed. The need to understnd the likely impcts of perch nd develop strtegies for loclised control or erdiction provides the impetus for this study. 6 Ludgte & Closs Responses of fish communities to removls of perch

6 . ECOLOGY OF PERCH Perch elong to the Fmily Percide, group consisting of round 6 freshwter fish species tht occur nturlly in still or slow-flowing temperte wters throughout the Northern Hemisphere (McDowll 99). The generl iology of perch is well known in its ntive Europen environment, with reserch hving een conducted into perch diet (e.g. Crig 978; Mehner 993), hitt use (e.g. Persson & Greenerg 99; Ben & Winfield 995), predtion effects (e.g. Tonn et l. 99; Persson & Eklöv 995) nd competition (e.g. Persson & Greenerg 99). Coupled with this is extensive reserch into perch popultion size-structure, nd popultion dynmics (e.g. Tresurer et l. 99; Tresurer 993; Persson et l. ). Perch spwn in spring, where the fish egin life cycle potentilly involving three ontogenetic diet nd hitt shifts (Persson 988; Persson et l. ). Initilly, perch re primrily plnktivorous, feeding mostly on pelgic zooplnkton t sizes up to 8 mm. They then switch to feeding primrily on mcroinvertertes nd increse their use of littorl hitts. Intense intrspecific competition for zooplnkton my force juvenile perch to switch from feeding on zooplnkton to mcroinvertertes premturely, resulting in reduced rtes of growth (Persson 988). A finl shift to piscivory occurs for those perch tht grow to sufficient size, usully t round 5 mm long (Persson 988). As result of the piscivorous nture of lrger perch, cnnilistic dult perch hve the potentil to ply n importnt role in structuring perch popultions (Clessen et l. ). Cnnilism in perch ffects minly the young-of-the-yer (YOY) clss (Persson et l. ). Cnnilism of lrvl nd juvenile perch hs een reported from mny perch popultions nd cn ply significnt role in reducing the intensity of intr-specific competition mongst juvenile perch (Persson 988; Tresurer 993; Persson et l. ; Whlström et l. ). The effects of cnnilism re most commonly seen in systems lcking other top piscivores (Tresurer 993; Whlström et l. ). However, cnnilism cn still ply mjor role in structuring perch popultions when other top predtors such s pike re present (Tresurer et l. 99)..3 PERCH IN NEW ZEALAND Perch re now distriuted widely throughout New Zelnd, with centres of undnce in Northlnd, Aucklnd, Hwke s By, Trnki, Wellington, Hokitik, centrl Cnterury, Otgo, nd Southlnd (Fig. A; McDowll ). They hve recently een recorded from Motuek (Shw & Studholme ). In New Zelnd perch up to 4 45 mm nd kg re regulrly encountered, however mny popultions re dominted y smller fish (McDowll 99). They hve een recorded to ttin mximum size of 675 mm nd pproximtely 4 kg in New Zelnd (McDowll 99). They my grow to lrger size with fish in excess of 4 kg hving een recorded elsewhere (McDowll 99). Reserch on perch in New Zelnd is limited, with only three mjor studies investigting their iology. Jellymn (98) exmined perch life history nd Science for Conservtion 7

7 found tht perch spwn in spring (Septemer Novemer) with high lrvl nd juvenile growth rtes during the first six months followed y slower growth rtes, especilly during winter months. Duncn (967) nd Griffiths (976) primrily exmined perch diet, with only minor interest in popultion structure. However, oth studies noted the presence of common ullies (Goiomorphus cotidinus) in the diet of perch. Common ully re widely distriuted through the lowlnd hitts in which perch typiclly occur (McDowll 99). High rtes of predtion on common ully could hve mjor impcts on the undnce nd ehviour of this smll ntive fish. Cnnilism y dult perch hs een reported in the Northern Hemisphere on mny occsions (Persson 988; Tresurer 993; Persson et l. ; Whlström et l. ). In New Zelnd, Duncn (967) lso noted young perch in the stomchs of lrge perch. In the sence of other piscivorous fish, it is likely tht cnnilism hs significnt influence on perch popultion dynmics in New Zelnd. Cnnilism could potentilly mintin low undnce nd stilise the structure of perch popultions s is seen in the Northern Hemisphere (Tresurer 993).. Aims nd predictions This study represents the first experimentl study exmining the impct of perch on ntive fish in New Zelnd. The ims of this study were to determine whether erdiction of perch from smll wetlnd ponds could e chieved using netting nd trpping. If dult perch could e either erdicted or significntly reduced, would this influence survivl nd recruitment of juvenile perch, nd the undnce nd popultion structure of common ully. Prior to removl it ws predicted tht, in response to the removl, numers of dult perch in the remining popultion would decline. In response to reduced numers of dult perch, numers of YOY perch following the removl would either e low ecuse of n sence of spwning or increse ecuse of reduction in cnnilism hence survivl of juvenile fish. It ws lso predicted tht the numers of common ullies would increse in the removl ponds reltive to the control ponds ecuse of reduced predtion pressure from dult perch on this smll ntive fish species. 8 Ludgte & Closs Responses of fish communities to removls of perch

8 3. Methods 3. STUDY SITE The Wihol/Wipori Wetlnd Complex is locted 33 km south-west of Dunedin in the South Islnd of New Zelnd (45 59 S, 7 6 E) nd covers pproximtely hectres of the Tieri plins (Fig. ). The wetlnd is ordered y Lkes Wihol nd Wipori nd ll re drined y the Wipori River nd smller strems. Ares of the wetlnd receive regulr intrusions of sline wter from the tidlly influenced lower Wipori River, which joins the lower Tieri River on its wy to Tieri Mouth, pproximtely km downstrem. The wetlnds once formed prt of much lrger wetlnd complex tht extended throughout much of the lower Tieri River plin. The Wihol/Wipori wetlnd hs een significntly reduced in re since Europen colonistion y dringe for griculturl use. Although erlier unsuccessful ttempts were mde to drin nd develop the re, the present wetlnd complex hs not een ltered since 956, (Q.E.II Ntionl Trust 997). Regulr inundtion of the re y high river levels during floods is ongoing, with the lst mjor inundtion prior to this study occurring in 994. The wetlnd comprises A B 35º North Islnd Lke Wipori 4ºS See Fig. South Islnd Lke Wihol Tieri Mouth km 45º Lke Wihol nd Lke Wipori Stewrt Islnd 7ºE 75º 3 km 3 km Figure. (A) Distriution of perch in New Zelnd (dpted from New Zelnd Freshwter Fish Dtse). (B) Mp of the Wihol/Wipori wetlnds. Science for Conservtion 9

9 numerous tidl chnnels nd ponds (rnging from. 8.9 hectres). Mny isolted ponds do not receive wter directly from the tidl flows. Levels in these re presumly mintined y groundwter intrusion nd rinfll. Communities in these ponds re only connected to other wterodies during mjor floods. The ponds within the wetlnds were possily formed following the collpse of pet ogs (Q.E.II Ntionl Trust 997), nd re structurlly simple, lcking sloping shllow littorl res nd exhiiting only limited vrition in depth. The vegettion of the re is reltively uniform, with the mjority of the wetlnd re eing covered y ntive plnts such s flx (Phormium tenx), sedges (Crex spp.), rupo (Typh orientlis), interspersed with coprosm shrus (Coprosm propinqu). Invsion y exotic plnts hs een feture in recent yers, with crck willow (Slix frgilis) nd sweetgrss (Glyceri mxim) spreding throughout the wetlnd nd incresingly crowding the wterwys nd wterodies (Q.E.II Ntionl Trust 997). The qutic vegettion is minly comprised of pondweeds (Potmogeton spp.), uttercups (Rnunculus spp.), Myriophyllum spp., free-floting ferns (Azoll spp.), nd duckweed (Lemn spp.). The composition nd density of vegettion etween nd within ponds vries considerly etween yers. The fish community of the re is diverse y New Zelnd stndrds. Perch re widespred throughout the wetlnds, eing found in oth Lkes Wihol nd Wipori, s well s in numerous smller ponds nd chnnels (Q.E.II Ntionl Trust 997; Kttel 999; Dvid ). They re undnt in the Wipori nd Tieri Rivers where they re regulrly sought y nglers. The common ully is undnt throughout the wetlnd complex nd is found in the connected nd isolted ponds, s well s the lkes nd chnnels (Kttel 999; Dvid ). Both longfin (Anguill dieffenchii) nd shortfin eels (A. ustrlis) re found within the wetlnds in resonle numers (Dvid ). Glxiids re present in the re with inng (Glxis mcultus) eing the most undnt, while gint kokopu (G. rgenteus) re present in some res in low numers (Dvid ). Due to the tidl nture of the Wipori nd Tieri rivers, the wetlnds re n importnt reeding ground for inng (Sutherlnd & Closs ). Inng re found predominntly in the connected ponds nd chnnels, with low numers present in some of the isolted ponds (Dvid ). Popultions of inng in isolted ponds re presumly relict popultions surviving since the ponds were lst connected during floods (Dvid ). Introduced rown trout re present in the wetlnd chnnels nd connected lkes (Dvid ). Other fish species occsionlly oserved in the tidl chnnels include smelt (Retropinn retropinn), lmprey (Geotri ustrlis) nd lck flounder (Rhomosole retiri; Q.E.II Ntionl Trust 997). 3. EXPERIMENTAL DESIGN Six ponds were selected for this study from pproximtely 7 ponds contined within the wetlnds (Fig. ). Selection of the ponds ws primrily sed on four fctors tht were: the presence of perch nd common ullies, pond size, loction, nd the sence of ny connection with the min chnnels or the Wipori River. Fish composition ws determined y exmining dt collected during previous fish survey of the re (Dvid ), nd dt collected in Ludgte & Closs Responses of fish communities to removls of perch

10 Figure. Loction of the study ponds in the Wihol/Wipori wetlnds. Removl pond, lrge Removl pond, medium Control pond, smll Removl pond, smll Control pond, medium Control pond, lrge preliminry survey of the ponds prior to this study. Reltively smll ponds were selected to llow intensive netting of ech pond. The ponds lso hd to e resonly ccessile through the rough oggy terrin of the wetlnd. Isolted ponds were required to prevent the movement of fish from other res oscuring experimentl effects. At the strt of this study these ponds hd een isolted since lrge flood in 994. Prt wy through the study in August, flooding of the wetlnd re occurred nd resulted in vrying degrees of reconnection of the ponds to the wider floodplin. The six ponds selected for this study were grouped into three pired size clsses: smll, medium, nd lrge (Fig. ). One pond from ech size clss ws llocted to one of two tretments: perch removl (smll, medium nd lrge pond res of.7,.46 nd.79 hectres respectively) nd control (smll, medium nd lrge pond res of.33,.54 nd.53 hectres respectively). The perch removl tretment consisted of three ponds where progressive removl of one-yer nd older perch ws undertken. Juveniles were not removed ecuse of limittions of the smpling equipment. The control tretment consisted of the three ponds in which no perch removl ws undertken. Science for Conservtion

11 The smpling undertken in this study involved preliminry survey of the study ponds in Octoer 999; perch removls in Novemer/Decemer 999 nd Decemer ; monitoring of the fish community monthly from Novemer 999 to April. Smpling in the first seson indicted rpid chnges in fish community structure were unlikely hence smpling intensity ws reduced in the second field seson. Smpling ws undertken in Novemer, Jnury, nd Mrch. A finl gill nd fyke net survey of ll the ponds completed the study in April (Tle ). 3.3 PRELIMINARY SURVEY A preliminry survey of the fish communities ws undertken cross ll six study ponds in erly Octoer (Tle ). Fyke nets (wing length 4.5 m, stretched mesh mm) were set for five nights in ech pond. Higher numers of nets were used in lrger ponds to mintin smpling effort with respect to pond size. Four fyke nets were used in the smll control pond, six fyke nets in the medium control pond nd seven fyke nets in the lrge control pond. In the smll nd medium perch removl ponds four fyke nets were used, while five fyke nets were used in the lrge perch removl pond. Fyke nets were set y ttching the floting top line of the wing to the nk (y tying to nery vegettion, e.g. flx) then extending the net t n ngle of 45 to the nk nd nchoring the end using hevy chin. All fish cught were identified to species, mesured to the nerest millimeter, nd then relesed. Eels were identified to species level nd immeditely relesed without mesurement (due to difficulties ssocited with hndling nd ccurte mesurement). TABLE. DATES OF SAMPLING UNDERTAKEN IN THIS STUDY. Note tht the perch removls were only undertken in the perch removl ponds, while the surveys nd monitoring were undertken in ll six study ponds. SAMPLING START DATE END DATE 999/ Preliminry survey 7/9/99 6//99 Novemer monitoring 6//99 //99 Perch removl (Smll nd medium) 9//99 //99 Perch removl (Lrge) //99 7//99 Decemer monitoring //99 8//99 Jnury monitoring 9// 3// Ferury monitoring /3/ 6/3/ Mrch monitoring /3/ 6/3/ April monitoring 5/4/ 9/4/ / Novemer monitoring 8// // Perch removl (All three ponds) 4// 7// Jnury monitoring 8// 3// Mrch monitoring 8/3/ 3/3/ Finl survey /4/ 5/4/ Ludgte & Closs Responses of fish communities to removls of perch

12 3.4 PHYSICO-CHEMICAL ANALYSIS Mesurement of the physico-chemicl chrcteristics of the six study ponds ws undertken on eight occsions during the course of this study (monthly from Novemer 999 to April, Jnury nd Mrch ). Wter chemistry nlysis ws not conducted in Novemer ecuse of equipment rekdown. One set of mesurements ws tken per pond on ech of these occsions. The nlysis ws undertken prior to fish smpling in ech pond to reduce the influence of sediment disturnce on wter qulity mesurements. Vriles were mesured in the center nd edge of the ponds, with ph, conductivity, slinity, dissolved oxygen nd temperture eing mesured from the top nd ottom cm of the wter column. Mesurements tken were: depth (centimetres), wter trnsprency (centimetres), ph, conductivity (micro-siemens), slinity (prts per thousnd), dissolved oxygen (milligrms per litre), nd wter temperture (degrees Celsius). Wter trnsprency ws mesured using Secchi disc. The secchi mesurement ws tken from the verge of two mesurements: one tken s the disc disppered from view s it ws lowered into the wter column, the other tken s it reppered s it ws rised up the wter column (McMhon et l. 996). The ph mesurement ws tken using phep3 portle field ph meter. Conductivity, slinity, dissolved oxygen nd wter temperture were mesured using Yellow Springs Instruments (YSI) Model 85 meter. Air temperture nd rinfll dt over the study period were otined from the Ntionl Institute of Wter nd Atmospheric Reserch (NIWA) wether sttion locted t Dunedin Airport, pproximtely 5 km wy. 3.5 PERCH REMOVAL 999 The initil removl of perch from the three removl ponds ws conducted in mid Novemer nd Decemer 999 (Tle ). The removls in the smll nd medium ponds were conducted simultneously. The removl in the lrge pond ws crried out when ll nets were ville to deploy in the one pond. The removls were initilly plnned to occur in lte Septemer/Octoer, which would llow dult perch to e removed efore spwning occurred or newly recruited YOY fish were present. These new recruits would e smller thn the minimum size possile for cpture using the smpling methods utilised during the removl period, nd would lso e potentilly present in very high numers. It ws resoned tht pre-spwning removl would mximise the chnces of chieving successful removl. However, results from the preliminry survey suggested tht cpture rtes of perch were reduced in wter less thn C (see lso Hoknson 977). Susequent delys due to wether resulted in the perch removl from the lrge removl pond eing conducted in Decemer 999, proly fter spwning hd occurred. The removl of perch ws conducted using overnight sets of gill nd fyke nets over period of three to six nights for ech removl pond. A totl of nine 7 m multi-mesh gill nets (pnel height 3 m, pnel length 4.5 m, stretched mesh sizes: 5, 45, 55, 7, 85 nd 5 mm) nd nine fyke nets (wing length 4.5 m, stretched mesh mm) were ville to deploy cross the ponds. In the smll Science for Conservtion 3

13 pond five fyke nets nd three gill nets were used over three night period. In the medium pond four fyke nets nd six gill nets were used over three-night period. In the lrge pond eight fyke nets nd eight gill nets were used over three-night period. Six fyke nets were lso used continuously over further three-night period in this pond. Gill nets were set y ttching one end of the floted top line to the nk nd extending the net perpendiculrly into the centre of the pond. Fyke nets were set s per the method descried in the preliminry survey. All equipment ws set within 4 hrs prior to sunset with some vrition etween ponds due to trvel times etween sites. Equipment ws retrieved within 4 hrs fter sunrise, gin with some vrition etween ponds due to vrile trvel times etween sites. All perch cptured were killed immeditely using either the nesthetic n-phenoxyethnol dministered t lethl dose, or y delivering shrp low to the hed. All fish (excluding eels) were weighed to the nerest tenth of grm nd mesured to the nerest millimetre. Eels were identified to species level. All fish other thn perch were then immeditely relesed. 3.6 FISH POPULATION MONITORING 999/ Smpling of the fish popultion in ech pond ws conducted monthly from Novemer 999 through to April (six smpling occsions). All ponds were smpled using identicl methods, nd monitoring ws completed in ll six ponds over mximum of seven dys ech month. Times of equipment set nd retrievl were similr to times used in the preliminry survey. Vrition in set times ws ccommodted y reporting ll ctches s ctch per unit effort (CPUE). Collpsile, rectngulr Gnsell it fish minnow trps (stretched nylon mesh 5 mm, cm with 7 cm opening) nd fyke nets (wing length 4.5 m, stretched mesh mm) were used to monitor the fish popultions. Minnow trps were used to trget ullies nd YOY perch. Fyke nets were primrily used to cpture lrger perch nd eels. Gill nets could not e used during the monitoring given the need to return perch cptured in the control ponds live nd in good condition, nd the need to mintin consistent smpling effort cross ll ponds. Overnight sets of minnow trps nd fyke nets were used for one night per pond on ech smpling occsion. On one occsion, in April, nets were left for two nights ecuse wether nd ssistnt injury prevented ccess to the ponds (CPUE djusted ccordingly). On ech night, 4 minnow trps were set in ech of four primry microhitts within ech pond (6 trps per pond): center surfce (greter thn m from nk nd floting in top cm of wter column); center ottom (greter thn two metres from nk nd resting on sustrte); edge (djcent to nk, resting on sustrte); m from edge ( m from nk, resting on sustrte). Two fyke nets were used per pond for monitoring trips in Novemer nd Decemer. Three fyke nets were set in ech pond from Jnury onwrds s dditionl equipment ecme ville. Trps nd fyke nets were set rndomly with no selection for res with or without mcrophytes. 4 Ludgte & Closs Responses of fish communities to removls of perch

14 3.7 PERCH REMOVAL A second perch removl in the perch removl ponds ws completed in Decemer (Tle ). Agin removls were initilly plnned for Octoer, prior to perch spwning. However, delys due to wether (i.e. wter tempertures < C) resulted in the removls most proly eing completed fter perch spwning. This removl used similr methods to the 999 removl, with the ddition of m long fine monofilment multi-mesh gill nets tht hd ecome ville (pnel height.7 m, pnel length 3 m, stretched mesh sizes: 3, 5, 65, 85 mm). Two of these nets per pond were used in conjunction with the gill nd fyke nets descried previously. Removls cross ll three ponds were conducted simultneously. Netting ws conducted over three-night period for ech pond. In the smll pond four fyke nets, one 7 m gill net nd two m gill nets were used over three-night period. In the medium pond four fyke nets, three 7 m gill nets nd two m gill nets were used over three-night period. In the lrge pond three fyke nets, five 7 m gill nets nd two m gill nets were used over three-night period. The setting nd retrievl of the nets ws conducted s per the methods used in the 999 removl period. 3.8 FISH POPULATION MONITORING / Smpling of the fish popultions ws conducted in Novemer, Jnury, nd Mrch (Tle ). Methods used on ech occsion were identicl to those in the 999/ seson. 3.9 FINAL SURVEY A finl survey using overnight sets of gill nd fyke nets ws conducted cross ll six ponds in erly April. Gill nd fyke nets were used in ll ponds to provide directly comprle estimtes of undnce cross the full size rnge of perch in ll of the study ponds. Two 5 m multi-mesh gill nets (pnel height.7 m, pnel length 3 m, stretched mesh sizes:, 3, 5, 65, 85 mm) were used in ech pond. One 7 m gill net ws lso used in ech pond. In the smll pond three fyke nets were used, nd in the medium nd lrge removl ponds four fyke nets were used. Three fyke nets were used in the smll control pond, one fyke net in the medium, nd four fyke nets were used in the lrge control pond. A misunderstnding etween survey tems resulted in fewer nets eing ville to survey the medium control pond otherwise vrition in numers of fyke nets per ponds llowed for some mintennce of smpling effort cross the different sized ponds. The setting nd retrievl of the nets ws identicl to methods used in the 999 nd removl periods. All fish collected in the control ponds nd ll fish other thn perch in the removl ponds were immeditely relesed following recording. Science for Conservtion 5

15 3. DATA ANALYSIS Sttisticl nlysis of the physico-chemicl dt ws conducted using one-wy ANOVAs using Systt Version 9. to determine if ny differences in physicochemicl prmeters existed etween ponds. The ctch selectivity of gill nd fyke netting for different size clsses of perch ws exmined using size-frequency histogrms. Ctch rtes of perch using the two smpling methods were compred y pooling ll fish cught during the first nd second removls nd the finl gill netting/fyke netting. These represent the smpling times during which gill nd fyke nets were used simultneously, hence ctches t these times re directly comprle. For nlysis of perch popultion structure cross the control nd removl ponds, perch popultions were divided into two functionl groups sed on length, i.e. YOY nd lrge. The YOY group comprised perch less thn 8 mm in totl length. Eighty millimetres ws the mximum size oserved in perch in their first yer of life in this study, nd it ws ssumed tht t this size perch were not significnt piscivores. Perch lrger thn 8 mm in length were ssumed to e piscivorous (Mittlech & Persson 998). Ctch rtes t ech removl nd in the finl netting re presented grphiclly. For the fish community monitoring, independence of smples ws ssumed nd equl repliction of nets nd trps per pond occurred, hence sttisticl nlysis ws completed. Ctch dt ws converted to ctch per unit effort (CPUE: No. fish hr net or No. fish hr trp ) to ccommodte vrition in the time tht ech net ws set. Becuse of the non-norml distriution of the dt, rndomistion tests (Mnly 99) were used to compre ctch rtes cross ponds t ech time. Rndomistion tests were completed using Microsoft Excel. The size distriution of the common ully popultion in the control nd removl ponds ws compred using two-smple Kolmogorov-Smirnov test (Sokl & Rohlf 995), performed using Systt Version 9.. Length dt ws pooled cross the three ponds in ech tretment to differentite etween erlyspwning nd lte-spwning periods for the common ully. As common ullies re known to spwn from lte spring to utumn, (Stephens 98; McDowll 99) the dt ws pooled into the following groups: Novemer 999 Jnury (erly spwning); Ferury April (lte spwning); Novemer nd Jnury (erly spwning); nd Mrch (lte spwning). Dt collected on the other fish popultions present in the study ponds (long nd shortfin eels, nd inng) were grouped y yer nd vrition in CPUE y pond ws tested using the rndomistion tests. The pooling of yerly dt ws performed to exclude sesonl chnges from the nlysis, nd to llow simple comprison of the undnce of ech species in ech yer cross ponds. 6 Ludgte & Closs Responses of fish communities to removls of perch

16 4. Results 4. PRELIMINARY SURVEY The preliminry survey of the six study ponds in Octoer 999 estlished tht perch were present in ll six ponds (Tle ). Perch were reltively undnt in three ponds, with 5 perch eing cught nd relesed in the lrge control pond, 38 perch in the smll removl pond, nd 8 in the medium removl pond. Few common ullies were cught in ny ponds, in the medium removl pond eing the highest numer oserved. Eels were undnt with 7 eing cught from the medium control pond. Inng were only oserved in the smll control pond. As ll fish were cught nd relesed, fish my hve een cught more thn once over successive nights. TABLE. TOTAL NUMBER OF FISH CAUGHT AND RELEASED OVER A FIVE NIGHT PERIOD FOR EACH POND USING FYKE AND MINNOW TRAPS DURING THE PRELIMINARY SURVEY OF ALL SIX STUDY PONDS IN OCTOBER 999. PERCH COMMON LONGFIN SHORTFIN BULLY EELS EELS INANGA Control Smll 5 54 Medium Lrge 5 Perch removl Smll 38 3 Medium Lrge PHYSICO-CHEMICAL ANALYSIS Air tempertures over the study period were similr etween yers. However, the winter of 999 ws oserved s eing very dry, nd the summer of 999/ eing very wet (Tle 3). Rinfll ws more evenly distriuted through winter nd summer /, with summer eing mrginlly wetter thn winter. Depth, wter trnsprency, slinity nd conductivity levels differed significntly etween ponds (Tle 4). The lrge removl pond ws deeper nd clerer thn the other ponds. The smll control pond ws more sline nd hd higher conductivity thn the other ponds. The levels of ph, wter temperture, nd dissolved oxygen were similr cross ll six study ponds throughout the study period (Tle 4). Science for Conservtion 7

17 TABLE 3. DAILY AIR TEMPERATURES ( C) AND RAINFALL (mm) RECORDED AT DUNEDIN AIRPORT. Winter = My Septemer (i.e. etween field sesons), Summer = Octoer April (i.e. field sesons). TEMPERATURE ( C) RAINFALL (mm) AVERAGE (RANGE) AVERAGE (RANGE) TOTAL 999 Winter 7.7 ( ).3 (..) 96.8 Summer.8 (.3 3.9).4 (. 67.) 55. Winter 7.5 ( ). (. 56.) 36. Summer 3. ( ).8 (. 5.) 379. TABLE 4. MEAN AND RANGE (MINIMUM, MAXIMUM) OF PHYSICOCHEMICAL CONDITIONS OF THE SIX STUDY PONDS. The dt ws tken from eight smpling occsions (Novemer 999 to April, Jnury, Mrch ). CONTROL PERCH REMOVAL SMALL MEDIUM LARGE SMALL MEDIUM LARGE Depth (cm) * (46., 75.) (37., 96.) (6., 9.) (69., 99.) (56., 8.) (7., 95.) ph (6.7, 7.6) (6.8, 8.8) (6.8, 8.3) (6.8, 7.9) (6.8, 8.3) (6.8, 8) Wter trnsprency * (cm) (7., Su.) (4., Su.) (., Su.) (6., 46.) (., Su.) (45., Su.) Slinity (ppt) * (.,.3) (.4,.9) (.6,.) (.6,.9) (.5,.) (.3,.) Conductivity (µs) * (88, 383) (667, 73) (95, 36) (9, 398) (85, 655) (57, 947) Dissolved oxygen (mg/l) (.9,.5) (.8,.) (3., 8.3) (.3,.7) (.4,.) (5.3,.) Wter temperture ( C) (.,.8) (.6,.8) (9.8, 8.) (.4,.) (.8,.8) (.9,.) * Indictes significnt t P =.5 level. Su. indictes ottom sustrte visile. 4.3 CATCH SELECTIVITY OF GILL AND FYKE NETS IN RELATION TO PERCH SIZE Fyke nets were clerly selective for smller perch, ctching totl of 34 perch in the 5 to 49 mm size rnge over the course of the removls nd finl survey (Fig. 3). Only four fish longer thn 5 mm were cught in fyke nets during the removls nd finl survey. In contrst the gill nets mostly cught fish over 5 mm in length, the lrgest eing 4 mm perch. 8 Ludgte & Closs Responses of fish communities to removls of perch

18 Figure 3. Totl numer of perch cught y size clsses in (A) fyke nets, (B) corse monofilment gill net nd (C) fine monofilment gill net during perch removls nd finl survey A) Fyke Totl perch cught B) Corse gill C) Fine gill Perch size clsses (mm) 4.4 FISH COMMUNITY 999/ Smpling of the study ponds efore perch removl using fyke nets nd minnow trps (Novemer 999) indicted the presence of low numers of lrge perch in the smll control nd the lrge removl ponds, while 8 lrge perch were cptured in the lrge control pond (Fig. 4). Only one YOY perch ws recorded. Common ullies were collected in three of the ponds (lrge control, smll nd medium removls) in low numers t this time. The initil removl of perch using fyke nd gill nets from the three perch removl ponds in lte Novemer/erly Decemer indicted tht resonle popultions of perch hd een present in ll three removl ponds (Fig. 5). Six perch were removed from the smll removl pond, with ll perch cptured eing etween 3 6 mm in length. Similrly, in the medium removl pond two perch ( nd 37 mm in length) were removed. In the lrge removl pond 55 perch were removed rnging in size from mm. Of these perch, 47 were in the 49 mm size rnge. Few lrge perch were susequently cught during routine monitoring following the removls in the smll nd medium removl ponds (Fig. 6). Aprt from one lrge perch tht ws cught nd removed in the medium removl pond in Decemer 999, no lrge perch were cught in either the smll or medium removl ponds for the rest of the 999 seson. The lrge removl pond showed similr trends, with only two lrge perch cught nd removed in Science for Conservtion 9

19 Figure 4. CPUE (No. fish hr net ) of fish in Novemer 999 prior to perch removl: (A) lrge perch, (B) YOY perch, nd (C) common ullies. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. Note different scles on y-xes. CPUE A) Lrge perch B) YOY perch..5 C) Common ully..5 S M L S M L Control Removl Figure 5. CPUE (No. fish hr net ) of 5 mm length clsses of perch cught during the 999 perch removl in the (A) smll, (B) medium nd (C) lrge perch removl ponds. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Note different scles on y-xes A) Smll B) Medium 6.6 Perch CPUE.4..6 C) Lrge Length (mm) Ludgte & Closs Responses of fish communities to removls of perch

20 Lrge perch CPUE A) Dec 47 S B) Jn C) Fe 3 M L S M L Control Removl S D) Mr E) Apr M L S M L Control Removl Figure 6. CPUE (No. fish hr net ) of lrge perch (>8 mm) in (A) Decemer 999, (B) Jnury, (C) Ferury, (D) Mrch nd (E) April. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. Note different scles in (B) Jnury nd (E) April. Jnury nd one in Ferury. Lrge perch were cught more frequently in the control ponds, with significntly higher numers of lrge perch eing cught in the lrge control pond in Decemer 999 compred with the other ponds. Lrge perch were cught in resonle numers in Jnury in the smll nd lrge control ponds, while in Ferury ten lrge perch were cught in the smll control pond. The numers of lrge perch cught nd relesed susequently decresed, with no lrge perch cptured in ny ponds in Mrch, nd only one lrge perch cught in the smll control pond in April. Following the pre-spwning perch removls in the smll nd medium removl ponds in lte Novemer no YOY perch were oserved t ny time in these ponds during the 999 seson (Fig. 7). However, in Decemer 999 ten YOY perch were cught in the lrge removl pond, indicting tht the perch removl hd either een only prtly successful or hd occurred fter spwning. Cpture rtes of YOY perch were lso high in the lrge control pond t this time, ut decresed drmticlly y Jnury, when only one YOY perch ws cught in this pond. Similrly, the smll control pond hd low numers of YOY perch present in Jnury, nd these lso hd decresed in undnce y the following smpling dte. YOY perch were not cught in ny of the control ponds from the Mrch smpling onwrds, while the lrge removl pond supported low numers of YOY perch in oth Mrch nd April. The low undnce of common ullies in Decemer 999 ws similr to the ptterns oserved prior to the perch removls in Novemer 999 (Fig. 4C nd 8A). However, no common ullies were oserved in the lrge control pond t Science for Conservtion

21 YoY perch CPUE A) Dec c c B) Jn 5 c c D) Mr E) Apr C) Fe S M L S M L Control Removl.4. S M L S M L Control Removl Figure 7. CPUE (No. fish hr net ) of YOY perch (<8 mm) in (A) Decemer 999, (B) Jnury, (C) Ferury, (D) Mrch nd (E) April. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. Note different scle in (A) Decemer 999 these times. By Jnury the CPUE of ullies in the smll nd medium removl ponds hd incresed, s did the CPUE of common ullies in the medium control pond (Fig. 8). Ferury sw significntly higher numers of ullies cught in the smll nd medium removl ponds, with low numers oserved in ll the other ponds nd none in the lrge control pond. By the min ully spwning period of Mrch, ully undnce ws significntly higher in the smll nd medium removl ponds, s well s in the medium control pond compred with ll other ponds. This pttern ws lso seen in April where the highest numers of ullies in the 999 seson were oserved in the medium control, nd the smll nd medium removl ponds. No significnt differences in ully CPUE occurred etween these three ponds t this time. 4.5 FISH COMMUNITY / During the monitoring (Novemer ) prior to the second round of perch removl, eight lrge perch were cught in the lrge control pond, with only one nd two perch cught in the smll nd lrge removl ponds respectively (Fig. 9). Two lrge perch were cught in the smll control pond. Lrge numers of YOY perch were cught in the medium control pond indicting successful spwning event in this pond. Prior to this ctch few perch hd een cught from this pond. One YOY perch ws lso cught in the lrge removl pond. Common ullies were cught in reltively high numers in the smll nd medium removl ponds in Novemer, s well s in the medium control pond. Ludgte & Closs Responses of fish communities to removls of perch

22 Common ully CPUE S A) Dec B) Jn 4 C) Fe M L S M L Control Removl S D) Mr E) Apr 6 3 M L S M L Control Removl Figure 8. CPUE (No. fish hr net ) of common ullies in (A) Decemer 999, (B) Jnury, (C) Ferury, (D) Mrch nd (E) April. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. Figure 9. CPUE (No. fish hr net ) of fish in Novemer : (A) lrge perch, (B) YOY perch nd (C) common ullies. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. Note different scle in (B) YOY perch A) Lrge perch B) YOY perch CPUE C) Common ully S M Control L S M Removl L Science for Conservtion 3

23 The perch removl conducted in erly Decemer recorded low undnces of perch in the smll nd medium removl pond, with higher numers of perch in the lrge perch removl pond (Fig. ). When compred to the first perch removl in 999 (Fig. 5), the ctch rte of perch from the medium removl pond ws higher in the second removl. Ctch rtes during the second removl were clerly improved y the use of the six m long fine monofilment multi-mesh gill nets, which ccounted for 7% of the totl ctch of perch from gill nets t this time. In the smll removl pond only one perch ws cptured, with length of mm, nd in the medium removl pond six perch were cught, rnging in length from 4 33 mm. However, in the lrge removl pond 6 perch were cught with eing less thn 5 mm, indicting survivl of significnt numers of YOY perch from the previous yer. Figure. CPUE (No. fish hr net ) of 5 mm length clsses of perch cught during the perch removl in the (A) smll, (B) medium nd (C) lrge removl ponds. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Note different scles on y-xes A) Smll B) Medium 3 Perch CPUE C) Lrge Length (mm) In Jnury nd Mrch, no lrge perch were cptured in ny of the removl ponds (Fig. ). In Jnury no lrge perch were cptured in the control ponds, ut in Mrch low numers were cught in the smll nd medium control ponds. YOY perch were cught in low numers in the smll nd medium control ponds nd the lrge removl pond in Jnury (Fig. ) indicting tht spwning hd occurred in these ponds t some time. However, y Mrch no YOY perch were cught in the control ponds, while in the lrge removl pond YOY perch were still present. 4 Ludgte & Closs Responses of fish communities to removls of perch

24 Figure. CPUE (No. fish hr net ) of lrge perch in (A) Jnury nd (B) Mrch. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. Lrge perch CPUE A) Jn B) Mr.5..5 S M Control L S M Removl L Figure. CPUE (No. fish hr net ) of YOY perch in (A) Jnury nd (B) Mrch. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. YOY perch CPUE A) Jn B) Mr S M Control L S M Removl L Science for Conservtion 5

25 Similr to Novemer (Fig. 9C), the undnce of common ullies ws high in Jnury in the smll nd medium removl ponds, nd in the medium control pond (Fig. 3). However, y Mrch the ctch rtes of ullies hd decresed in the medium control pond, ut hd incresed drmticlly in the smll nd medium removl ponds. The CPUE of ullies in the lrge removl pond remined low. 4.6 OTHER FISH In 999/ significntly more longfin eels were found in the medium control pond compred with the other ponds, wheres the undnce of shortfin eels ws significntly greter in the smll nd medium control ponds (Fig. 4). In the / period significntly more shortfin eels were cught in the medium control pond, while the lrge control pond contined considerly higher numers of inng reltive to the other ponds. Figure 3. CPUE (No. fish hr net ) of common ullies in (A) Jnury nd (B) Mrch. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Letters indicte significnt differences t 5% level. Common ully CPUE A) Jn 35 B) Mr c S M c L S M c L Control Removl 6 Ludgte & Closs Responses of fish communities to removls of perch

26 Figure 4. CPUE (No. fish hr net ) per pond for (A) 999 nd (B). Fish species shown re long-finned eels (digonls), short-finned eels (solid) nd inng (spots). Eel dt from fyke nets only; inng dt from minnow nd fyke nets. Error rs re + SE. Numers ove rs represent the totl numer of fish cught. Asterisk ove r represents significnt difference etween ponds t 5% level. Note different scles on y-xes. CPUE * 94 * 46 * 79 * 9 54 * 5 A) B) S 7 M Control 9 L S M L Removl 4.7 FINAL SURVEY The ptterns in perch popultion structure oserved through Jnury nd Mrch were supported y the results of the finl gill nd fyke net survey cross ll of the ponds (Fig. 5). Perch rnging from 355 mm were oserved in ll three of the control ponds, with no YOY perch eing oserved t this time. No perch were cptured in the smll removl pond, nd only one 73 mm perch ws removed from the medium removl pond. In contrst, lrge numers of YOY perch were collected from the lrge removl pond, with only one 4 mm perch eing cptured t this time. 4.8 COMMON BULLY SIZE DISTRIBUTION In the erly spwning period of 999/ ullies were found in low numers in ponds cross oth tretments (Fig. 6). The length distriution of ullies did not differ significntly etween the tretments (Kolmogorov-Smirnov test: D =.393, P =.46). However, in the lte spwning period ully size distriutions did differ (K.-S. test: D =.346, P <.) with lrger ullies nd greter size rnge present in the removl ponds. The distriutions of ully lengths differed etween the control nd perch removl ponds in the erly spwning / period (K.-S. test: D =.6, P Science for Conservtion 7

27 Control Removl Perch CPUE A) Smll B) Medium D) Smll E) Medium C) Lrge F) Lrge Length (mm) Length (mm) Figure 5. CPUE (No. fish hr net ) of perch from the finl survey in April in the control ponds: (A) smll, (B) medium nd (C) lrge; nd the perch removl ponds: (D) smll, (E) medium nd (F) lrge. Error rs + SE. Numers ove rs represent the totl numer of fish cught. Note different scle on y-xis of (F) lrge perch removl pond. =.5; Fig. 6). A chnge occurred in the lte spwning period in / when the numer of ullies dropped in the control ponds, ut the smll nd medium removl ponds contined lrge numers of ullies cross wide size rnge. However, t this time there ws no significnt difference in the size distriution despite the considerle difference in undnce (K.-S. test: D =.63, P =.4). 4.9 SUMMARY OF FISH COMMUNITY DYNAMICS (999 ) Lrge perch were cught in ll six of the study ponds in the preliminry survey nd were cught in ll three of the control ponds t some stge during the fish community monitoring. However, lrge perch were rrely cught in the three perch removl ponds fter the initil removl, with no lrge perch eing cught following the second perch removl in these ponds in Decemer. The results of the finl survey lso indicted tht lrge perch were still present in resonle numers in the control ponds, ut few were present in the removl ponds. YOY perch were oserved for short periods in Decemer nd Jnury in the control ponds, ut were cught over the entire study in the lrge perch removl pond. YOY perch were not cught t ny time in the smll nd medium 8 Ludgte & Closs Responses of fish communities to removls of perch

28 Figure 6. Lengths of common ullies in the control ponds: (A) erly spwning 999/, (B) lte spwning, (C) erly spwning / nd (D) lte spwning ; nd the perch removl ponds: (E) erly spwning 999/, (F) lte spwning, (G) erly spwning / nd (H) lte spwning. Erly spwning times re Novemer to Jnury. Lte spwning times re Ferury to April. Frequency Control A) Erly 999/ E) Erly 999/ B) Lte F) Lte C) Erly / D) Lte Removl G) Erly / H) Lte Length (mm) Length (mm) removl ponds. Common ullies were undnt in the smll nd medium removl ponds nd the medium control pond. In the / monitoring seson the undnce of ullies in the smll nd medium removl ponds incresed drmticlly, while the popultion in the medium control pond decresed. The size structure of the ullies chnged in the control nd removl ponds throughout the study, with greter numer of smll nd lrge ullies oserved in the smll nd medium removl ponds. Longfin eels were present in greter numers in the medium control pond in 999/ compred with the other ponds, while shortfin eels were more undnt in ll three of the control ponds in 999/. Inng were more undnt in the lrge control pond thn the other ponds in the / monitoring seson. Science for Conservtion 9

29 5. Discussion 5. PHYSICO-CHEMICAL PARAMETERS Depth, wter trnsprency, slinity nd conductivity levels differed significntly etween ponds during this study. Slinity nd conductivity levels oserved remined within the tolernce limits of the fish species present (Thorpe 977; McDowll 99). The differences in the depth nd wter trnsprency oserved etween the ponds in this study were unlikely to hve influenced the dynmics of the fish communities. Wter temperture, ph nd dissolved oxygen did not differ significntly etween ponds. Dissolved oxygen levels s low s.3 mg L- were oserved t vrious loctions within certin ponds. Levels of less thn mg L- is lethl oxygen level for mny fish species (Mtthews 998), however these redings were only oserved round the edges nd the ottom res of the ponds. All mesurements tken from the pond surfce were greter thn mg L-. The ph nd wter tempertures oserved in the ponds remined within tolernce limits of the fish species in the ponds (Thorpe 977; Richrdson et l. 994). 5. CATCH SELECTIVITY OF GILL AND FYKE NETS Selectivity for prticulr sizes of perch ws evident in the ctch y either fyke or gill nets, with fyke nets tending to ctch smller fish reltive to the gill nets. A degree of selectivity is n inevitle feture of ny pssive smpling device (Huert 996), nd the ptterns of perch cpture over the course of this study must e interpreted in light of likely smpling is t ny prticulr time. With the exception of the finl survey, gill nets were only used to cpture perch in the removl ponds. Typiclly, the efficiency of gill nets is reltively high ut is lso often ssocited with some fish injury nd mortlity (Huert 996). Consequently, gill nets could not e used in the control ponds, where the intention ws to mintin nturl perch popultions, nd so required the return of ll cptured fish live nd in good condition. Hence, we could use only fyke nets during the regulr monitoring of fish popultions. As result, perch ctch rtes during the regulr monitoring from the medium nd smll control ponds were low, given tht perch popultions in these ponds were dominted y smll numer of lrge perch. In the lrge control pond, mostly smll perch were cptured during the regulr monitoring in the summers of oth 999/ nd /. However, it is importnt tht lrge perch (>3 mm long) were cptured from ll three control ponds when gill nets were used long with fyke nets during the finl survey in April. In contrst, no lrge perch were cptured from ny of the removl ponds during the finl fyke nd gill net survey in April. Given tht perch over 3 mm long re likely to e over 4 yers old (Jellymn 98), the finl gill nd fyke net survey confirmed tht lrge dult perch were present in ll of the control ponds over the course of the study. 3 Ludgte & Closs Responses of fish communities to removls of perch