The Effect of Predation by Wintering Cormorants Phalacrocorax Carbo on Grayling Thymallus Thymallus and Trout (Salmonidae) Popul

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The Effect of Predation by Wintering Cormorants Phalacrocorax carbo on Grayling Thymallus thymallus and Trout (Salmonidae) Populations: Two Case Studies from Swiss Rivers Author(s): W. Suter Source: Journal of Applied Ecology, Vol. 32, No. 1 (Feb., 1995), pp. 29-46 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/2404413 Accessed: 30/07/2010 16:50

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http://www.jstor.org Journal of Applied Ecology The effectof predationby winteringcormorants 1995, 32, 29-46 Phalacrocoraxcarbo on graylingThymallus thymallus and trout(Salmonidae) populations:two case studies fromSwiss rivers

W. SUTER* SchweizerischeVogelwarte, CH-6204 Sempach, Switzerland

Summary 1. The effectof cormorantpredation on one troutand two graylingpopulations was examined in two riversin north-easternSwitzerland, the only sites in the countryat presentwhere both birdand fishdata are sufficientfor such an analysis. Cormorantdensities were among the highestrecorded on Swiss rivers.Fishery yieldwas used as a means of detectingfish population changes, because population size could not be measureddirectly. Two hypotheseswere tested:strong cormorant predation resultseither in lower yields, withoutaffecting long-term population dynamics (compensatorymortality), or destabilizes the prey population, with effectsobserved in over-fishedpopulations (additive mortality). 2. No evidence was found to support predictionsof a negative effecton fish populationdynamics by cormorants.Predation intensity on graylingwas positively correlatedwith yield in the largestgrayling population of Switzerland.In the other river,grayling yield was unusuallylow aftera cormorantinflux in one winter.This had been taken widelyas evidence of populationdepredation, but yieldvariation was mainlya functionof anglingeffort and age structurein the graylingpopulation. However, some compensatoryeffect between angling success and birdpredation is possible. At the same time,a sudden decrease in troutyield was caused primarily by alteredregulations for trout fishing. 3. Variations in growthrate, age structureand age at firstmaturity in the large graylingpopulation were not relatedto cormorantpredation, but to the strengthof the recruitingcohort (age class 2). The biomass of younggrayling also explained the intensityof cormorantpredation pressure. Among the survivinggrayling of any one winter,10-16% bore marks of a recentcormorant attack, but to date no evidence existsfor negative implications of such injuryrates on graylingpopulation dynamics. Key-words:cormorant, density dependence, grayling,predation, rod fisheries. Journalof Applied Ecology (1995) 32, 29-46

(e.g. Bayer 1989). Existingliterature on the poten- Introduction tial of piscivorous birds to depredate fish stocks Blamingfish-eating birds for negative effects on wild in naturalenvironments seems equivocal. Draulans freshwaterfish populations and yields has a long (1988) pointedout thatmost earlier studies (particu- history.Increased demand on fishstocks both by larly before 1950), includingthe well-knownbird humansand birdshas recentlyaccentuated conflicts removal experiment by Elson (1962) reporting again, particularlyin Europe and North America. 'damage', lacked sufficientdata or rested on in- Many conflictshave become political issues, even appropriate methods. Both Draulans (1988) and thoughthere was no factualevidence to justifythem Suter (1991) concluded from a review of recent papers based on sound methodology,that there was * Presentaddress: Swiss Federal Instituteof Technology no indicationof piscivorousbirds causing long-term Zurich,Institute of Forest,Snow and Landscape Research, decreases in fishpopulations in naturalfreshwater 29 ZUrcherstrasse111, 8903 Birmensdorf,Switzerland. habitats,while at most scant evidence existed for 30 reduced yields because of compensatorybalance was unknown, because no tagging experiments Cormorant effects between human- and bird-inducedmortality, but were carried out, and cohort analysis or related on graying that predationby some larger divingbirds on fish proceduresassume constant mortality or recruitment ponds and similar artificialhigh-density situations (Youngs & Robson 1978; Pitcher & Hart 1982), could resultin substantiallosses. which is invalid when bird predation is involved. On the other hand, fisheries'ecologists have Instead, variation and possible trends in angling been aware for some time that cannibalism and and commercialyield were analysed for possible interspecificpredation by piscivorous fish may effectsof birdpredation. The impactof cormorants exerta strongeffect on the structureand dynamics was assessed by testing two mutually exclusive of fish populations (Pitcher & Hart 1982; He & hypotheses. Kitchell1990; Magnuson1991; Tonn, Paszkowski& 1. Cormorant predation reduces yield, but the Holopainen 1992), whichin turncan triggera series removal of fish by cormorantand humans works of effects'cascading' down throughthe food web of withinthe range of compensatorymortality. Thus, a lake (Shapiro & Wright1984; Northcote 1987; no signsof over-exploitationare apparent. Persson et al. 1988; Vanni et al. 1990). Evidence 2. Cormorantpredation is additiveto fishingmor- for the controllingcapacity of highervertebrates tality and leads to over-exploitationwith altered in terrestrialpredator-prey systems is also accumu- population dynamics,where yields decrease and lating (e.g. Erlinge et al. 1983; Newsome 1990; population parameterschange accordingto known Hanski, Hansson & Henttonen 1991). It seems, patterns of overfishing.For example, average therefore,possible that piscivorousbirds, at least fishsize decreases when older age classes become some of the larger diving species, should also be increasinglyrare, growth in youngercohorts (chiefly able to prey on fishbeyond the thresholdof com- age class 2 in grayling)accelerates as competitive pensatorymortality. pressurefrom older age classes relaxes, and age at One such species is the cormorantPhalacrocorax firstmaturity may also decrease (Backiel & Le carbo (L.). Its breeding population has increased Cren 1978; Burrough& Kennedy1979; Healy 1980; in north-centralEurope about 15-foldsince 1970, Kirchhofer& Tschumi 1986; Mills & Chalanchuk and at a similar rate in Switzerland,where the 1988; Wootton 1990; Mann 1991). species is a passage migrantand winter visitor. and riversnow may Densities on some Swiss lakes Studyareas and methods reach up to 500 birds km-2 in late autumn (Suter 1994). Concern has been expressed repeatedly, STUDY SITES, FISHERY PRACTICES, AND particularlyabout the cormorant'seffect on grayling CORMORANT PRESENCE Thymallusthymallus (L.) populations(Arbeitsgruppe 'Kormoran und Fischerei' 1987), which are the RiverRhine main prey in some free-runningrivers (W. Suter, unpublisheddata). This salmoniformfish is classified The studyarea compriseda 21-kmlong section of as 'threatened'in Switzerland(Kirchhofer, Zaugg & the River Rhine, fromits outlet at Lake Untersee Pedroli 1990), but neverthelesssubject to intense (westernpart of Lake Constance) to the barrageat angling, with yields of up to 30-50kgha-1 (see Schaffhausen(8'40'E, 47'40'N). The mean widthis Table 2), as it is still one of the more common 150m and water depth in winteris mostlyless than species (Pedroli, Zaugg & Kirchhofer1991). How- 4 m (maximum 12 m in pools). Lake Constance ever, when low or decliningcatches happened to is uncontrolledand there is free water current coincidewith the presenceof cormorants,predation (1-0-1-5 m second-1) in the upper two-thirdsof the was generally regarded responsible (Staub et al. 350-ha river area, but in the lower part the water 1992). In doingso, catchstatistics were tacitlytaken regimeis influencedby the barrage.The area holds as a directindex of fishpopulation size. Conclusions the largest grayling population of Switzerland, also neglectedthe fact that a quantitativeassessment which is spatially, and probably also genetically, of cormorantactivity and diet only existed fortwo isolated from neighbouringpopulations by the areas. hydroelectricalplant and by the lake, although In this paper, two cases with a relativelysatis- some exchange across Lake Untersee is possible. factorydata set are examined. One deals withthe Within the area fish can move freely. Grayling effectof cormorantson the largest Swiss grayling dominate the fisheryyield (80-85%) by weight population in a semi-naturalstretch of the upper and number. About half of the yield is landed by River Rhine, the otheron graylingand brown/lake anglingfrom boat and fromthe shore, mainly in trout(Salmo truttaf. fariollacustrisL.) stocksin a October-January,andt the otherhalf by a gill-net smaller, canalized river (Linth Canal). Cormorant fishery,partly during the open season (May-January) densities in these areas were among the highest and partly as spawning catch in March-April. recorded for river habitats in Switzerland(Suter Fishingrights belong to local societies. The number 1994). However, the size of the fish populations of licences issued is limitedand thereforeremains 31 relativelyconstant from year to year. Grayling in the River Rhine studyarea. Yield data have been W. Suter stockingis intensive(on average 1 millionalevins available since 1970; however,data forsome types and fry,equivalent to 1850 'fingerlingunits', ha-1), of rod fisherythat accounted for about 11% of the althoughit has been criticizedas unnecessarygiven total yield were only collected since 1987-88. the excellentnatural reproduction (Gerster 1990). These data were not used in calculationswith data Cormorantsfish on Lake Untersee in autumnand sets for longer periods. On the other hand, data early winter,where they also roost, but since the were available separatelyfor several fishing grounds beginningof the 1980sflocks of 100-300 birdshave withinthe area. The upper ('Steiner Wasser') and increasinglyused the River Rhine for foraging central ('Diessenhofener Wasser') sectors are between (November) late December and March, withinthe free-flowingstretch, whereas the water i.e. mainlyin the monthsbetween the periods of regimeof thelower sector (comprising the 'B1isinger graylingangling and spawningcatches. Numbersof Wasser' and three smaller grounds) is influenced otherlarge fish-eating birds (e.g. goosanderMergus by the barrage. merganserL.) are insignificant. Fish weightswere estimatedby the anglers,but reportedonly as monthlyor annual sums along with fishnumbers. Average fishweight in thisanalysis is Linth Canal theannual yield (kg) dividedby catch (fish numbers); The Linth Canal is a canalized, 17-kmlong river its distributionis skewed to the right(see Fig. 12). (59 ha) thatlinks oligotrophic Lake Walenstadtwith The officialdata of average trout weights from mesotrophicupper Lake Zurich (9OO'E, 47010'N). the LinthCanal were, however,inaccurate: missing Shorelinesare straightened,the profile is trapezoidal weightdata had been replaced by excessivelyhigh with a mean depth of 3m, and width is 30-35m estimates,which resulted in over-estimationsfor the throughout. Despite its structuralpoverty, the average value, especiallyin lake trout.The average riversupports dense graylingand brown/laketrout weights of brown/lake and rainbow trout were populations,because of its good waterquality, flow recalculated from the original statistic forms of (0-8-1*Om second-) and gravelbottom. The river 1983-91 (n = 4124). In 1991 and 1992, anglershad is also the main reproductionsite forthe lake trout to record the lengthof every fishon a new form. populationof the two lakes. More than 90% of the Weightestimations were stillmonthly sums, but the anglingyield consists of graylingand trout,the main values formonths with only one fishmade it possible graylingseason being autumnand earlywinter until to determine average estimated weights in each the end of December. Licences can be obtained lengthclass (in cm) forgrayling (n = 277) and brown directlyfrom the state, and their annual number (n = 196), lake (n = 246) and rainbow trout (n = 84). fluctuatesstrongly. Catches of spawning grayling Estimates for smaller grayling(<38 cm) compared are carried out in early spring,but stocking(600 well to real weights,but with increasinggrayling fingerlingunits ha-') is on average about three size, anglers tended to over-estimatethe weight, times less dense than in the River Rhine. Lake while in trout they judged the smaller individuals Walenstadtis almost free of cormorants,but Lake too heavy. The relationshipbetween length and Zurich has supported300-700 cormorantsin mid- estimatedweight was then used to calculate per- wintersince 1980. Small numbersstarted foraging centagesof differentsize classes in the catch. Again trips into the Linth Canal in the early 1980s. In onlydata formonths with single fish could be used. January1985, an influxof 300 cormorants,which This resultedin a bias towardslarger fish, because coincidedwith an extremelycold period, lasted for smallerindividuals were more likelyto be caughtin 3 weeks. Since 1985/86,a scaringprogramme has highernumbers per month. been carried out by game and fisherywardens throughoutthe winter, and no major influxhas Assessmentof cormorantpredation occurred again. However, small numbers(usually <10 cormorants) visit the canal daily between Cormorant predation (estimated biomass of fish December and March. Few goosandersare present. species i removed,in kg) was calculated as:

yi = n X c X pi, Fisherydata wheren is the numberof cormorantdays, c the daily Fisheriesyield (kg fishcaught) and catch (number food consumption,and pi the proportionof species i of fishcaught) data were collected by the cantonal in the diet. Cormorantdays on the River Rhine authorities.For the LinthCanal, the data are com- were based on data gatheredby two anglers who pletefor the period considered (since 1960),although had counted incoming cormorants almost daily all trout,i.e. brown/lakeand rainbowtrout Oncor- throughoutthe wintersince 1985/86. One to five hynchusmykiss (Walbaum), were combinedbefore censuses were carried out monthly,and usually 1974 (see the Results for identificationproblems). recorded10-40% more cormorantsthan the anglers There is a complicated systemof fisheries'rights because theytended to under-estimatelarge flocks, 32 sometimes missed smaller flocks that arrived by GRAYLING BIOMETRY AND AGE STRUCTURE Cormorant effects flyingover land, and did not includein theirfigures on graying the cormorantsfishing in the uppermostkilometre Data on length,weight, sex, sexual maturityand of the river. The data were thereforecorrected rate of injuriesinflicted by cormorantattacks were accordingly.Cormorant days at the Linth Canal collected from samples of the spawning catch, were assessed similarly,from daily countsby game 1986-192 (freshinjuries since 1985), whilehandling and fisherywardens, who were provided with a the fish,whereas age was determinedfrom scales questionnaire between 1986/87 and 1990/91, later in the laboratory.Annual sample size varied and from control counts mainly in two winters. between 195 and 758 grayling(maximum 1105 in Cormorant days of earlier years (including the 1985), but as each parameter(except injuryrate) 1984/85 influx) were based on protocols of the was notrecorded for every fish, the sample size used fisherywarden and roostcensuses at Lake Zurichby in calculationsvaries. Several anglersalso measured ornithologists. total lengthof the graylingthey caught from 1985/ Daily food consumption per cormorant was 86 to 1991/92(annual sample size 808-5844). The estimatedto be 500 g throughoutthe winter.Back- weight-length(g, mm) relationshipfor grayling was calculatedfish weights in 157 pelletsfrom the upper calculatedfor the lengthrange of 85-480mm as: River Rhine, 1986/87, were 448 g pellet-' (95% ln(weight)= 3-057 x ln(length)- 11-918, confidenceinterval for the mean, 369-508g; W. Suter,unpublished data). The value of 500 g allows (n = 691, r = 0-981, P < 0*0001). Togetherwith the for under-estimationassociated with otolith wear informationon age-length relations,it was possible and correspondsto 23% of the mean body weightof to calculatethe age-specificbiomass in the spawning a cormorant.It is twicethe percentagerecently used catchesand in the anglingyield, assuming there was for P. (carbo) lucidus in a tropical environment no growthin winter.For these calculations,only the (Linn & Campbell 1992). However, previous esti- data on age compositionfrom the spawningcatches mates in a warm climate were c. 16% (DuPlessis in the two main fishinggrounds were used, where 1957; Junor1972), and proportionsaround 20% are catches were carried out every year by the same generally accepted for larger fish-eatingbirds of equipmentwith constant mesh-size. Age is referred temperatelatitudes (Dunn 1975; Barrettet al. 1990; to as the year of life,i.e. age class 2 (= 1') is 1-5- Johnstoneet al. 1990). Although23% may be at the 1-7 yearsold in the anglingyield and nearly2 years upper end of the range (W. Suter, unpublished in the spawningcatch. Age classes 5-7 were pooled data), thisvalue does accountfor the relativelylow because of the smallsamples in age class 6 and 7 and energycontent of the freshwaterfish that form the the difficultyinvolved in correctlydetermining the bulk of the diet (Scherz & Senser 1989), and for age of older fish (Mann & Steinmetz1985). Age increasedmetabolism at low wintertemperatures. class 1 was not representedin the samples and age Estimatesof the percentage by numberand weight class 2 not fully.The legal size (30 cm) for the rod of graylingand troutin the diet were determined fisheryand mesh-size(38 mm) of the drag-netsused from the analysis of regurgitatedpellets (main in the spawningcatches mean that graylingfrom method at the upper River Rhine) and of stomach 30-32 cm onwardsare represented,but they prevent contentsof shot cormorants(primarily at the Linth the catch of smallergrayling of age 2. Canal). Fish lengthand weightwere back-calculated Injurieswere classifiedas 'fresh'when therewas fromuneroded otolithsin pellets and frompartly eitheran open wound or a less deep markwithout digested fishin stomachs(Appendix). There were regeneratedscales present,typically near the gills, no signsof a major bias introducedby the different showing a deeper imprintfrom the hook of the sampling methods. The proportionof graylingin upper mandibleon one side and a smallermark of samplesfrom the RiverRhine appeared to varylittle the lower mandible on the other side (Takashima between years, and direct observations at feed- & Niima 1957; Carss 1993). 'Healed' injuries(regen- ing places confirmedthat graylingwere always erated scales) stillhad this patternwhen therehad the primaryfood. Thus, a value of 66% was used been a deep mark,but were more difficultto assess throughout.At the LinthCanal, thehigh percentage in less severe cases; marksof doubtfulorigin were of graylingin the diet duringthe main influxin 1985 omitted. A few graylingthat had both fresh and was also confirmedby anglers who reportedthat healed injurieswere countedfor the freshrate only. cormorants,which they had scared away,had mainly regurgitatedgrayling. However, the dominance of STATISTICS graylingdecreased in later seasons, while trout and cyprinidspecies became more important.Pro- Multiple regressionanalysis was used to explain portions were thereforecalculated separately for variationin yield (normal linear model) and injury 1985 (also applied to 1983 and 1984), 1987 (also rates (generalized linear model) of grayling. A applied to 1986 and 1988), 1990 (also applied to non-lineartrend in the Linth Canal example was 1989) and 1991 (Appendix). removedby smoothing (Cleveland's LOWESS method; 33 Wilkinsonet al. 1992). Nevertheless,yields were not Althoughthese results do not supportthe hypoth- W. Suter strictlyindependent samples, since theydrew partly esis (1) of compensatoryyield reduction, the variation on the same population in successive years. The in yield (1970/71-1991/92, n = 22) was analysed dependent variables were thereforechecked for by multiplelinear regressionto examine whether autocorrelations,which were found to be weak at cormorantpredation could have neverthelesslow- most(see textfor details). Tests were two-tailedand ered fisherylandings. Dependent variableswere (a) significancelevels were 5%. total, (b) angling and (c) spawning yield for the entirearea, and (d) anglingand (e) spawningyield separately for the three main fishing grounds. Results Untransformed,ln-transformed and (because of the autocorrelations)differenced values at lag 1 were GRAYLING, RIVER RHINE used. Independentvariables tested in various com- binationswere (f) cormorantpredation either in the Yield same or in the preceding year, (g) average fish There was no trendin total graylingyield between weightin the respectivecatches, (h) fingerlingunits 1970/71and 1991/92(r=0-005, n=22, P=0.98). stocked 1 or 2 years before, and (i) variables rela- Yield was highat thebeginning and at theend of the ting to water discharge,mainly mean dischargein period, with smaller peaks in between (Fig. 1). A October-December (the principalangling season), weak tendencyfor cyclicfluctuations was apparent whichmight influence angling success, and discharge (autocorrelation: r = 0-50, n = 21, P < 0-05; no in excessof 150% of thelong-term average in April- significantpartial autocorrelations at lags >1). May of the precedingyear, which were suspectedof Increasingpredation by cormorantsfrom 1980/81 affectingsurvival of eggs, larvae and fry. Results onwardswas not followedby reduced yield (mean fromuntransformed and ln-transformedvalues were totalyield 1970/71-1980/81, 10 239 + 2819 kg, 1981/ the same, so only those fromtransformed data will 82-1991/92 10450?+-2754kg; ANOVA: F=0*032, be reported.None of the 18 finalmodels produced a P= 0-861), but tended to increase the estimated negative correlationwith cormorantpredation, as total utilizationof the population by fisheryand would be expectedif cormorants had reducedyields. cormorant (10 292 + 2791 kg vs. 13424 + 4819 kg; Stockingfry or fingerlingshad also no significant ANOVA: F= 3-481, P= 0-077). Estimated gray- positiveeffects on later yields, nor was highwater ling biomass removed by cormorantsfrom 1980/ in springfound to have a negativeinfluence. Only 81 onwards was positively correlated with total three, probably causal, relationships emerged: (= angling plus spawning) yield (r = 0-78, n = 12, anglingyields both in the centraland lower fishing P = 0-003) and with angling yield (r = 0-792, n = 12, grounds were negativelyrelated to average fish P = 0.002), but not significantlywith spawning yield weight (P < 0-001, adjusted R2 = 29% and 33%), (r= 0-51, n = 12, P = 0.094). Spawning catches indicatingthat yield was high in years with strong aim to provide a constant amount of eggs, and cohorts of graylingof age 2. The subjective im- the biomass of graylingcaught was thereforeless pressionof the fisherywarden that anglingcatches variable than in the anglingyield. were higherin those autumnswith low waterlevels

25 000 3 Cormorantpredation M Anglingyield M Spawningyield 20 000-

O 15 000 E 0

(D10 000 E

LU 5000

0 70/71 72/73 74/75 76/77 78/79 80/81 82/83 84/85 86/87 88/89 90/91 Year

Fig. 1. Estimatedgrayling biomass removedby fishery(angling and spawningyield) and cormorantsin the River Rhine studyarea, 1970/71-1991/92.Fishery data representc. 89% of the true minimumyield. Anglingincludes some gill-net fishery. 34 (K. Egloff,personal communication), was supported Table 1. Estimated biomass percentages of grayling Cormorant effects by the differencedvalues of the lowerfishing ground age classes withsample size in the anglingyield (a) and on graying (P < 0-05,adjusted R2 = 18%). No data representing spawningcatch (s), 1985/86-1991/92,of the River Rhine studyarea actual fishingeffort were available, but the local systemof fishingrights restricted the number of Winter Age 2 Age 3 Age 4 Age 5-7 n licencesissued, and thereforeprobably kept angling effortrather constant. 1985-86 a 22 6 20 8 43-8 12 8 1026 s 40 253 586 120 441 1986-87 a 59 1 12 7 17 5 10 7 5318 Populationparameters s 237 274 360 129 300 1987-88 a 17 8 49 0 8 0 25 2 2180 If predationby cormorantshad led to over-exploi- s 87 563 9.1 25 9 150 tation of the graylingpopulation (hypothesis2), 1988-89 a 43 6 25 5 19.1 117 1771 mean species size should have decreased as the s 283 328 288 10.1 313 1989-90 a 52 2 21 5 14 8 11 4 2444 older age classes had become increasinglyrare. s 464 27 9 15 8 10.0 413 However, the average weight of graylingin the 1990-91 a 32 4 50 3 13 9 3 4 1602 spawning catch of the central section (where a s 36.8 494 12.2 1-7 259 long data set exists) increased from 320 ? 42 g in 1991-92 a 183 434 365 1 8 749 the period 1951-70 to 531 ? lllg in the period s 282 618 10.0 05 195 1971-92 (ANOVA: F=64-1, P<0-001; Fig. 2). Values were similarin the anglingcatch here and both in anglingyield and spawningcatch. The weak in the lower sector and did not decrease aftercor- cohortof age 2 in 1988 produced a low biomass of morant predation set in from 1980/81 onwards. age classes 5-7 in 1991, as expected, and only in There were large annual fluctuations,and high 1992 did the oldest age class appear less numerous estimated cormorant predation was correlated thanexpected from the strengthof thiscohort in the with low average graylingweight (central sector, precedingyears. spawning catch: r = -0-582, n = 12, P = 0-047, Age distributionsin anglingyield and spawning anglingcatch: r = -0-543, n = 12, NS; lower sector, catcheswere broadlysimilar in mostyears, with the mainly angling catch: r = -0-685, n = 12, P = 0-014). exception of age class 2, which was much more Because the two cohortsborn in 1988 and 1989 stronglyrepresented in the angling catches until were verystrong, the relativeproportions of older 1989/90 (Table 1). Because cormorantpredation age classes in the catches were lower afterwards. largelyfalls between the anglingand spawningcatch But the absolute biomass of older graylingdid not periods, differencesbetween the two samples have decrease between 1985/86 and 1991/92 (Fig. 3): been interpretedas reflectinga cormoranteffect thebiomass of age class 2 showedno trendin angling (Staub et al. 1992), such as the peak of age class 4 yield, and even increased in the spawning catch present in the 1991/92angling yield and missing (r = 0-81, n = 7, P < 0-05); age class 3 increased in the 1992 spawning catch (Fig. 3). However, both in the angling yield (r = 0-79, n = 7, P < 0-05) 1991/92 was a season with moderately strong and in the spawning catch (r = 0-87, n = 7, P = 0-01); cormorantpredation, whereas after 1990/91with and age classes 4 and 5-7 showed no significant exceptionallyhigh predation (Fig. 1) no such dif- trend. Strong and weak cohorts could be traced ference emerged. Besides, cormorants mostly fromage class 2 throughto age classes 5-7, where take graylingof age classes 2 and 3 (Fig. 4), and an the correspondingpeaks and troughswere apparent effectshould be more likelyto be visible in these

800 - Diessenhofen spawning catch 303 0000 - Diessenhofen angling catch Zf --Lower sector, mainlyangling catch I,, 5Coo

E CL~~~~~~~~~~~~~~~~~~~~~~~~

5000 E

51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 Year

Fig. 2. Average graylingweight in the catchesat Diessenhofenand Busingen-Schaffhausen,the lowersector of the River Rhine studyarea. 1951-1992. and estimatedcormorant predation in the precedingseason (as in Fig. 1). 35 Angling Spawningcatches W. Suter 4500 - 1800 -

3000 - 1200- mm i ~~~~~~~~~~~~~~~2years 1500 _ 6000 -

85 86 87 8889 90 91 86 87 88 8990 91 92

4500 1800

- c,, 3000 | 1200 3 years

C: 1500 - 600

E n 85 86 87 88 89 90 91 86 87 88 8990 91 92 3000- 1200

1500 600 4 years

0 O' 85 86 87 88 89 90 91 86 87 88 8990 91 92 1500 600 5 - 7 years

85 86 87 88 89 90 91 86 87 88 89 90 91 92 Year

Fig. 3. Age-specificgrayling biomass in the anglingyields and spawningcatches of the River Rhine studyarea, 1985/86- 1991/92.Age classes: 2 years = end of second year, etc. Equivalent cohortsare alignedvertically.

40 - Age-class 2 0 Stomach a Otolith Age-class 3 30

Age-class I Age-class 4 20

13-15 16-18 19-21 22-24 25-27 28-30 31-33 34-36 37-39 40-42 43-45 46-48 49-51 Length-class(cm)

Fig. 4. Length distributionof graylingeaten by cormorantsat the upper River Rhine. Back-calculationsfrom partly digestedfish (n = 16) in stomachsof shotbirds and fromuneroded otoliths (n = 125) in pellets.Approximate spans forage classes 2 (upper end), 3 and 4 accordingto Fig. 5.

classes,but they were similarly or even morestrongly (r = 0*042,P = 0.426) or older grayling(Fig. 5). The representedin the spawning catches than in the lengthof individualsin a given cohortwas strongly angling yield after intense cormorant predation correlated between age 2 and 3 (r = 0O980, n = 6, (Table 1; e.g. 1990/91). P = 0.0006), but betweenage 3 and 4 growthdiffer- ences levelled off (r = 0*771, n = 6, P = 0.072). Decreasing body lengthwas indeed a functionof Age-specificlength increasinggrayling density (expressed as the age- The over-fishinghypothesis predicts faster growth, specificbiomass presentin the spawningcatches), because of relaxedcompetition at decreasinggrayling both for age 2 (r = -0*944, n =7, P = 0001) and density,and thereforeincreasing mean age-specific age 3 (r= -0*811, n=7, P=0.027). If the slower length.In reality,this value decreased between1986 growth rate was also due to earlier maturityin and 1992both in age class 2 (r = -0*441, P < 0.0001, females (see below), namely as compensationfor forthe n = 581 originalmeasurements) and age class increasinginvestment in egg productionat age 2 3 (r= -0*466, P<0*0001), but not in age class 4 (Staub et al. 1992; but see Koch & Wieser 1983; 36 100 -1500 336 2 years Cormorant effects 1 on graying 33 80 -|200

n581= C\J 30 60 d-900 86 87 88 89 90 91 92 E o E 40 |600 m

40- 220 -300 ,, T 3 years

37 -I 0 0 1988 1989 1990 1991 1992 n571I 34 Year 86 87 88 89 90 91 92 Fig. 6. Percentage(with 95% confidencelimits) of sexually mature,2-year-old female grayling, and graylingbiomass of age 2, in theannual spawning catch, 1988-1992. . t t *t t ; 4 years

n =368 38 86 87 88 89 90 91 92 small sample sizes rangingfrom 17 to 55). There Year was, however,a close relationshipwith the absolute biomass of age class 2: when the cohortwas strong, Fig.5. Age-specifictotal length of grayling inthe spawning mostfemales became maturetowards the end of the catchesof theRiver Rhine study area, 1986-1992(mean, 95% confidencelimits, and overallsample size). Age second year, but theyremained immature when the classes:2 years= end of secondyear, etc. Equivalent cohortwas weak. Gonadal weightin mature2-year- cohortsare alignedvertically. old femalescontributed little to the biomass of the cohort,as the maturefemales were on average only 7% heavier than the immaturefemales of similar Pauly 1986 for general reservationsagainst this body length (n = 203). concept), sex-specificdifferences in growth rate should have emerged.Two-year old males were on Injuriesfrom cormorant attacks average 0*8cm longer than females (33.0 + 14 cm vs. 32-2 +-1 cm; ANOVA:F= 34-3, P< 0-001), but In the spawningcatches from1985/86 to 1992, the bodylength in males (thathave to investlittle energy mean proportion of graylingwith injuries from in gonadal production) changed between years cormorantattacks was 98% forfresh and 13*1% for similarlyto that in females (correlationof mean healed injuries(n = 3868). The rate of freshinjuries lengthat age 2 in the sexes for 1986-92: r = 0-91, in 1985, which also appeared as healed injuriesin n = 7, P = 0-004). However,there was a slight,albeit 1986, was much higherthan in all subsequentyears not significant,tendency for the sex-specificdiffer- in spite of only 'average' pressure by cormorants ence to become largerwith decreasing female length in the winter 1984/85. Fresh injuries fluctuated (r = -0-73, n = 7, P = 0-065). between4% and 14% thereafter,and healed injuries decreased fromc. 15% to 7% between 1987 and 1992 (Fig. 7a). If 1985/86is excluded, therewas no Age at firstmaturity in females relationshipbetween the rate of freshinjuries and The proportionof mature 2-year-oldfemales fluc- that of healed injuries in the followingyear. A tuated markedlybetween 12% and 76% from1988 generalized linear model was used to explain the to 1992 (Fig. 6: large confidenceintervals because of variation in fresh injury rate for the population

'30 (a) 40 ( b) 30 ~ ~ ~ ~ ~ ~ ~ ~ 3

U 2010

85 86 87 88 89 90 91 92 2 3 4 5-7 Year Age- class

Fig. 7. Proportionof graylingwith injuries from cormorant attacks in the spawningcatch, 1985/86-1992. Rates of fresh (filledsymbols) and healed injuries(open symbols)by year (a) and age class (b); triangles,1986 only; circles,1987-1992. 37 representedby the spawning catches. One could thehealed injuryrate at age 3 would resultin a value W. Suter have expected the injuryrate to increase with the of c. 9% for both rates (c. 13% when 1985/86is intensityof cormorantpredation, and to decrease included).Therefore, about 10-13% of the grayling when a strong cohort of 2-year-oldgrayling was of age class 2 and 13-16% of older individualsthat present, assuming that handling of smaller fish survivedcormorant predation in one winter,had was easier and thereforefewer injured fishwould neverthelessbeen attacked by a cormorantand escape. However, the intensityof cormorantpre- escaped withan injuryin the same period. dationwas not foundto have a significantinfluence, while the presence of age class 2 in the spawning GRAYLING AND TROUT, LINTH CANAL catches was positivelycorrelated with the rate of fresh injuries, q: Grayling logit q = -2*848 + 0*014x = 2X81, P< 0 05). (tx The coincidence of high cormorantpredation in Here x is the percentageof the 2-year-oldcohort in 1984-85 and minimumangling yield in the four the catches,but a similarresult was obtainedwith x subsequent winters (Fig. 8a) has been taken as as the absolute biomass (P < 0-05). evidencethat cormorants are able to depredateriver Injuryrates did not differby sex (chi-square = fish populations (Arbeitsgruppe 'Kormoran und 2-89, df=2, P=0-236), but there were age- and Fischerei'1987; Staub etal. 1992). However,angling size-relateddifferences. Fewer 2-year-oldgrayling yield had been subject to pronouncedyearly fluc- carriedfresh beak marks(5-9%) than older cohorts tuationsthroughout the period of 1960/61-1991/92, (9.0%; chi-square = 4.59, df= 1, P = 0-032, with regardlessof cormorantpresence (Fig. 8a). Fishing Yates' correction),but age classes 3, 4 and 5-7 did effortwas not constant,as the numberof licences not differbetween each other(Fig. 7b). By contrast, increasedfrom 400 to 1000in 1977and thendeclined rates of healed injuriesincreased strongly with age to 400-600 (Fig. 8a). No other informationon after 1986. This apparently reflectedthe length effort,such as the numberof rod days, is available. of exposure to cormorantattacks. In 1985-86, There was no linear trendin total yield (r = -0-24, aftergrayling had been subject to major cormorant n = 32, P = 0-185), but yield per licence (r = -048, pressurefor the firsttime in the precedingwinter, n = 32, P = 0-006) and catch per licence (r = -0-44, rateswere high (>30%) but thesame in all older age n = 32, P = 0-012; Fig. 8b) decreased from1960 to classes. If healed injurieswere in factat least 1 year 1991. However, a non-linearcurve seems to fitthe old, thenthe mean rate of freshmarks in 2-year-old pattern better (Fig. 8b, smoothed curve), as the and the rate of healed injuriesin 3-year-oldgrayling decline was relativelysudden and occurred in the should be the same, giventhat sampling probability firsthalf of the period. Using yield per licence does not differand that cormorantscan catch and also reduces the apparent 4-year long (1985-88) handle small 1-year-oldgrayling (mean total body 'population low' to a single year (1985) with un- length20cm) withoutlosses. Instead, the two rates usuallylow anglingsuccess just afterthe cormorant were c. 6% for freshmarks at age 2 and 12% for influx(Fig. 8b). Average graylingweight in the healed injuries at age 3, respectively(1985-86 catch showed no trend for the entire time-span excluded, because age was not assessed in 1985). (r=0-10, n=32, P=0*602; Fig. 9a) and did not Staub et al. (1992) concluded that freshlyinjured differin the years before and aftercormorant pre- graylingwere not adequately representedin the dation started in 1982 (544 + 27 g vs. 551 + 45 g; spawningcatches, as theywould seek refugeplaces, ANOVA: F= 0.31, P = 0*584). Data forother relevant and thatfresh injury rate had to be estimatedfrom parametersare not available before 1970, so only healed injuries at age 3. However, in the river the period since then will be consideredin further sections where spawning catches are done, drag- analysis. nettingsamples the full water body that can be As for the River Rhine data, multipleregression utilizedby grayling.There is also no evidence that analysis was used to relate variation in the gray- injuredgrayling would congregatein otherareas not ling catch to several factors.The mean numberof sampledat all. It seems morelikely that a proportion graylingcaught per licence was detrendedby using of healed injuriesregarded as 'old' was nevertheless the residualsfrom the smoothedcurve as the depen- inflictedin the same season, perhapsin late autumn dent variable (Fig. 8b). Because predationby cor- or early winter. A span of 4-5 months should morantswas strongin only one winter(Fig. 8a), it be sufficientfor healing of the wound and some could not be expected to emerge as an explaining regenerationof scales (T. Wahli, personal com- variable in the long run (validated by exploratory munication). Most of the 3*2% of graylingwith models), even if it had reallyinfluenced the angling healed injuriesin age class 2 (Fig. 7b) may have catch of subsequent years. To avoid undetectable sufferedthe attackin the same winterrather than in cormoranteffects possibly influencingthe results, the previouswinter at age 1. Adding this value to only the period 1960/61-1983/84,i.e. before the the freshinjury rate at age 2 and subtractingit from large cormorantinflux, was used in the final re- 38 6000 --100 Cormorantpredation (a) 1200 Cormorant effects M Anglingyield on grayling 900 4000

(n ~~~~~~~~~~~~~~~~600 E m2000

0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~0

6 6264 66 68 70 72 74 76 78 80 82 84 8'6 88 90 20- E Number/licence (b) - Smoothed curve Model 1616 - - Z 8a 4

0 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 Year

Fig. 8. Estimatedgrayling biomass takenby the rod fisheryand by cormorantsat the LinthCanal, the numberof licences sold (a), and the numberof graylingtaken per licence (b), 1960/61-1991/92(60= 1960-61). Smoothedcurve and model, see the text.

( a) -Grayling 0*7

060 _

Lake trout O 0 (b)( )- Browntrout 0E 7 -A-Rainbow trout

0.4 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 Year

Fig. 9. Average weightsof grayling(a) and trout(b) in the rod and line catch at the LinthCanal, 1974-1991. The arrow indicateswhen legal size was increased.The troutdata pointsfor 1974-82 are the 'official'values correctedaccording to the differencefound for 1983-87, when the raw data of 1983-91 were re-analysed.

gressionmodel. Independentvariables were average [residual= -59-522 x averageweight (kg) + 31-713; graylingweight in the catch and stockingof fry/ adjusted R2 =O0261, t= -3-077, P= 0.005]. fingerlings1 and 2 years before,respectively. Only In the second step, thismodel was extrapolatedto weightwas found to explain some variation:catch 1984/85-1991/92,the period beginningwith the increased with decreasing average graylingweight cormorantinflux. If cormorantpredation in 1984/85 39 had stronglyreduced the graylingpopulation or example, out of 308 anglers who caught at least W. Suter even affectedpopulation dynamics adversely, gray- 10 trouts of either fario or lacustris in any 1 year ling catches per licence predictedfrom the model between 1983 and 1991, 167 (54.2%) assigned all should be much higherthan the observed values. troutsto only one form.Because yields of brown, However, the model fits the actual values well lake and rainbow trout were stronglycorrelated (Fig. 8b), and does so even in the season following anyway(r = 0-939-0-958, df= 15, P < 0.0001), they the strong cormorantpredation (prediction 1-34, will thereforebe combinedfor most considerations real 1-57grayling licence' h-'). This indicatedthat to follow. Trout yield increased stronglyfrom a the low catch was 'normal' given the high average stable level between1960 and 1967to a peak in 1976 graylingweight (Fig. 9a), and was not directly and decreased thereafterto reach another stable related to the estimatedbiomass removed by the level between 1985 and 1990 (Fig. 10a). Catch per cormorants.By contrast,the model failedto predict licence (Fig. 10b) followeda patternroughly similar the low catch in 1991/92,which occurred after a to total yield, indicatingthat not only anglingacti- series of years with quite insignificantcormorant vity,but also trout population size, had changed. predation.It cannot,however, be excluded thatthe The reasonsfor the decline after1976 are unknown, cormorantsinfluenced the mean graylingweight but cormorantpredation was not involvedbecause it recordedin the 1985 catch, and thereforeindirectly commencedonly after the decline and was strongest lowered the theoreticalcatch expected from the in the years of stable yields(Fig. 10a). model (see the Discussion). Afterthe main decline, troutyield in the season followingthe cormorantinflux of 1984/85dropped sharplyagain and remainedlow thereafter(Fig. 10a). Trout Cormorantpredation was claimedto be responsible Angling statistics distinguished between brown (Arbeitsgruppe 'Kormoran und Fischerei' 1987; trout,lake troutand rainbowtrout only after1973, Staub et al. 1992), despite the fact that troutcon- butit is not knownhow manyanglers could correctly sumptionby the birds had been only about 16% of separatethem, particularly the two truttaforms. For the anglingyield in 1984/85.If the varyingnumber

8000 - Numberof licences 1200 * Cormorantpredation a B Anglingyield 6000 - 900

E U) o 4000 600

E Ui 2000 300

0 0 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90

t16 - Number/licence (b) s Estimate - Forecast

E 8

0 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 Year

Fig. 10. Estimatedtrout biomass taken by the rod fisheryand by cormorantsat the LinthCanal, the numberof licences sold (a). and the numberof trouttaken per licence(b), 1960/61-1991/92(60 = 1960-61). Estimateis the hyperboliccurve fittedto the 1976-84 values ( 1.2 + 31-2/x,where x = year 1974; correctedr2 = 0-939), forecastis the extrapolationto 1985-91 (see the Results). 40 of licences is taken into account, numbersof trout Most of the 64% decrease in troutcatch per licence Cormorant effects per licence after 1984 are more in line with the between 1984 and 1985, and of the differenceto on graying previoustrend, although an unusuallylow figurestill higher expectations in later catches, which were remainsfor 1985. A simplehyperbolic function that regarded as cormoranteffects, can thus simplybe fittedthe 1976-84 values well (R 2= 0-939) was attributedto the change in legal trout size. The extrapolatedto 1985-91 to predictexpected values expected, slightincrease in the catch fromthe first of a troutcatch not influencedby a new factorsuch to the second year afterthe implementationis also as cormorantpredation (Fig. lOb). The actual catch clearlyvisible from the data (Fig. 10b). in 1985 (1.45 troutlicence-) was 64% lower than the expected (4-04 trout licence-'), while in the subsequent years the catch was between 9% and Discussion 44% below prediction. However,in 1985 a new rod fisheryregulation for CORMORANT EFFECTS ON GRAYLING the protectionof immaturelake troutincreased the POPULATION STRUCTURE IN THE RIVER legal size of all troutforms from 30 cm to 35 cm. Two RHINE? effectscould thusbe expected: (i) troutyield should become lower, particularlyin the firstyear after The data on yield variation at the River Rhine implementation,while in the second year a small do not support the hypothesisthat cormorants increase could be expected, because a part of the increased mortality of the grayling population sparedtrout (depending on therelationship between beyond the limitwhere compensatorymechanisms natural and fishingmortality) population now be- could work. No substitutionalshifts between fishing comes available to anglersas heavier fish;and (ii) and bird predation were found. On the contrary, average troutweight should increase. In all three highyield coincided with high cormorant predation. troutforms, the average weightincreased sharply Because fishingeffort may be regardedas constant from 1984 to 1985 (Fig. 9b) and remained signifi- in thelonger term, the higher yields probably reflected cantlyhigher in the period 1985-90 than it was in largergrayling populations. The second hypothesis, 1974-84 (brown trout 538?+ 36g vs. 464?+ 30g, thatgrayling population parameters showed signsof ANOVA: F = 20-8, P = 0-001; lake trout660 ? 43 g vs. over-fishing,must also be dismissed.Mean grayling 509 38 g, F = 56-7, P < 0-001; rainbow trout size in the recruitedpopulation did not decrease 617 ?44 g vs. 483 ? 27 g, F = 59-7, P < 0.001). For after cormorantpredation began, but fluctuated 1991, the legal size was lowered to 32 cm, and with the strengthof the age 2 cohort. Older age average weights decreased accordingly,although classes were also not missing,as has been asserted catch per licence did not increase. (Staub et al. 1992), but had either increased in To estimatethe percentageof the troutsize class biomass (3-yearclass) or fluctuatedwithout trend (4 30-34cm in the catchbefore the legal size increase, years and older). Very large grayling(age classes the medians of the weight estimatesfor trout at 5-7) were rare in 1991 when a weak cohort had 34 cm and 35 cm total length were determined become 5 yearsold, but also in 1992when a stronger with the 1991-92 data (see Methods). They were cohort was expected. However, the calculation of 400/450g for brown trout,420/470 g for lake trout biomass proportionsin 1992 was based on a smaller and 400/500g for rainbow trout. The arithmetic sample taken fromthe spawningcatches (n = 195) mean gave very similarresults. The upper weight than in most previousyears (n = between 250 and limitfor 34-cm long browntrout was thereforeset at 550 except in 1988) and may have been biased by a 400 g, forlake and rainbowtrout at 450 g. Assuming samplingeffect during fish processing. Nevertheless, thatmisplacements above and below the limitwould ifthis scarcity in 1 year is to be attributedto (over-) cancel out each otherwas justified,despite the fact utilizationof the graylingpopulation, angling must that31-32cm long troutwere available in 1983-84 also be considered.Anglers removed almost 3600 kg but banned in 1991-92 (misplacementsin brown of 4-year-oldgrayling in the winter1991/92 (Fig. 3), troutbelow the limit27/above 31; lake trout29/34; whereas cormorantstook a total of 3700 kg of all rainbowtrout 10/10). The mean percentagesof the age classes, of whichprobably less than a third(c. trout size class 30-34cm could now be obtained 1200kg) belongedto age class 4, because cormorants fromthe weightdistribution in the 1983 and 1984 do not select the largest grayling(Fig. 4). In any catch. They were 58% in the browntrout, 41% in case, equally low proportions(<5%) of 5-7 year the lake troutand 39% in the rainbow trout.The old individualshave been found in many grayling combined value for all trout, weighted by their populations not subject to significantbird pre- frequencyin the catch,was 46%. This is a minimum dation,both in Switzerland(R. Ensmenger,unpub- estimate,since it is derived froma frequencydis- lished observations)and elsewhere(Peterson 1968; tributionbiased towardslarge fish (see theMethods). Hellawell 1969; Lusk 1975; Woolland & Jones1975; Therefore,at least 46-50% of the troutcatchable in Witkowski& Kowalewski 1988). 1983-84 became unavailable to anglers in 1985. Age-specificgrowth and age at firstmaturity in 41 female graylingdecreased withthe strengthof the 10000o W. Suter age class 2. An inverserelationship between growth E_ 8000 rate and populationdensity has been foundin many o m fishes (Backiel & Le Cren 1978), and was also 0c 6000

suggestedfor grayling(Woolland & Jones 1975). ()Q 4000- Mechanismsmay be varied,but competitive pressure - * inhibitingoptimal feeding is ofteninvolved (Woot- (I, 2000 ton 1990), and also seems likelyfor 2-year-old gray- 0 * ling whichhave a similardiet to the older cohorts 0 300 600 900 1200 1500 Biomassage 2 (kg) (Mtiller 1961; Peterson 1968; Hellawell 1971). Density-dependentgrowth was indeed predicted Fig. 11. Total estimatedcormorant predation on grayling fromthe over-fishinghypothesis, but as havingthe as a functionof biomass of age 2 individualsin thespawning oppositeeffect. Growth rates should have increased catch of the River Rhine studyarea, 1985/86-1991/92. under relaxed competition,if cormorantpredation had reduced population size and especially the GRAYLING AND TROUT POPULATIONS proportionof older age classes. Normallya decrease DEPREDATED AT THE LINTH CANAL? in the age of firstmaturity is linkedto an increasein growthrate (Wootton 1990). Social stimulationas The Linth Canal example offers less biological well as inhibitionof maturityis knownfrom several insightbecause (i) cormorantpredation was import- fishspecies (Wootton 1990) and mayalso be important in only one winter,and (ii) few fishpopulation ant in grayling(see Carl, Walty& Rimmer1992 for data were available. Instead,it demonstrated various similarfindings in Thymallusarcticus). It may be problemswhen conclusionson fishstocks are drawn hypothesizedthat individualsof age 2 formgroups fromrod fisherystatistics. A populationcollapse in composed only of their own age class when their graylingand trout,related to the cormorantinflux, cohortis strong,but mixwith older individualswhen may be postulated only if total yield or catch are theircohort is weak. The onset of sexual maturity considered. Such a claim does, however,pose the would be enhancedin uniformage groupsbut inhi- question of whythe collapse should have occurred, bited by the presence of larger individuals. The when the total estimatedbiomass taken by human situationapparently conforms to the 'community- and bird was not higherthan the biomass removed dependentrecruitment' model proposed by Danielson by anglers alone in several of the previous years & Stenseth (1992), where both floaterand adult (Fig. 8a for grayling; e.g. 1969/70, 1973/74, densityinfluence recruitment. It predictsstronger 1978/79).By using catch per unit effort,where the population densityvariations than for populations unit effortis simplythe number of licences sold, with recruitmentonly dependent on the density unusuallylow catches were still obvious for 1985, of adult individuals. In general, however, sexual but not for the subsequent years. Therefore,no developmentis linked to growth,sexual size di- signs of a population collapse lasting for several morphismand survivalby complicatedinteractions yearswere apparent.Similar discrepancies between that determinelife history tactics (Mann 1991). totalcatch and catchcorrected for effort were found The abundance of age class 2 was not only a in British Atlantic salmon (Salmo salar L.) rod key factorfor grayling population structure, growth fisheries(Shearer 1992). and maturity,but also influencedyield and popu- Catch per licence sold does not account for vari- lation size. Low average fishweight was found to ation in effortinvested per licence, or for possible increase yield both at the River Rhine and at the changesin the returnrate of statisticforms. In 1985, Linth Canal. Cormorantpredation pressure could 89% of theforms were returned, which is exactlythe thereforealso be expected to be a functionof age mean for1983-91. An effectof the returnrate can class2 abundance.This was indeedthe case, although thereforebe dismissed.There are a few indications based on a small sample (Fig. 11): estimatedgray- that the effortinvested per licence in 1985 was ling biomass removed by the cormorant (which slightlylower than normal, when the impression is equivalentto influxstrength) was positivelycor- prevailedamong anglersthat the canal was 'empty'. related with the biomass of 2-year-oldgrayling in However, the mean graylingcatch of the threemost the spawning catch (r = 0-73, P < 0-05; the same successfulanglers in 1983-91 is stronglycorrelated tendencyexisted with the anglingyield: r = 0-58, with the mean catch per licence (r = 0-93, n = 8, P> 0-05). Even the overall rate of freshinjuries P < 0.001), and the 1985 value is no outlier. This increasedwith increasing abundance of younggray- suggeststhat neitherchanges in effortper licence ling. On average, 10-16% of all graylingof age 2 nor a possibletendency to under-reportcatches (see and older that escaped capture by fishermenand Shearer1992 for general findings) were major factors cormorantsnevertheless bore marks of cormorant loweringthe 1985 catch. attacks sufferedin the same winter,but no impli- Effectsof changesin anglingregulations also need cations on survivalrate were apparent. be considered: in the case of trout, most of the 42 differenceto the expected catch could be explained 30 - 1991 Cormorant effects by the increased legal size. Bearing in mind the 20 - on grayling small proportionof trout removed by cormorants 20 compared to the fishery,it can be safelyconcluded that cormorantpredation of trout in 1984/85did 30 1990 not significantlyaffect later catches, nor the popu- 20 ~* lation size itself. 10 l In the case of grayling,angling practice cannot explainthe low catchjust afterthe cormorantinflux. 30 - 1989 It probablyrepresented a relativelylow, thoughnot 20 9 collapsed, populationlevel. Annual catch variation 10F in the LinthCanal was explainedby averagegrayling weight better than in the River Rhine. The low 30 - I1988 catch of 1985 was almost exactlypredicted by the model fromthe highaverage graylingweight, which suggestedthat the cohort of age 2 fishwas weak. This was confirmedwhen the weightclass distribu- ,, ,,1030 -1987 1_ tionsin the catchfrom 1983 to 1991 were compared 20 - (Fig. 12): althoughthe 1985 distributiondoes not stand out as markedlydiverging, smaller grayling withan estimatedweight of up to 500 g formedthe 30 - 1986 lowest, and relativelylarge grayling(>800g) the 20 L highest,percentage of the period. Even thoughthe average weightin 1985 was not higherthan in 1982 (Fig. 9a), whenno cormorantswere present(and no 30 985 data on weightclass distributionwere available), it 20 F could be argued thatthe cormorantspreyed mainly upon smallergrayling and had thereforelifted the average weightin the catch, whichin turnhad led 30 to a low expected catch. If the assumptionthat 1984 20 cormorantshad indeed altered the age class com- position was correct,and that the average weight had otherwisebeen 570g, i.e. the average of the 30 - period 1980-84, the expected catch per licence 1983 20 would have amountedto 4-5 graylinginstead of 1-6. i The differenceis equivalent to a yield of 770kg, representing38% of the estimatedbiomass removed 200 400 600 800 1000 1200 1400 by cormorants.On theoreticalgrounds, Houston Weight-class( g) (1992) showed that the fish biomass consumed Fig. 12. Distributionsof estimatedgrayling weight classes need loss by seabirds not necessarilyequal a to at the Linth Canal, 19831-91 (rounded to 10g). The the fishery. distributionsarc biased against lower weightclasses (see the Methods), but comparisonsare not affected.

GRAYLING BlOMASS AND PREDATION RATES

Cormorant predation increased the exploitation of graylingat the River Rhine by 25% on average Table 2. Area-specificgrayling yield (kg ha- l) and by 50% maximum, and at the Linth Canal by 80% in the 1 year (Table 2). As there was no Fishery Cormorant Total informationon population size or production, itis notknown what proportion was takenby humans River Rhine and otherpredators, or whetherpredation accounted 1970-82 Mean 320 (c. 40)* 01 321 Maximum 53 3 53 3 for most of the mortality.However, published 1983-91 Mean 35.0 85 435 data fromsimilar habitats can be used to allow a Maximum 51 3 25 3 76 6 roughestimate of predationpressure. Area-specific yieldat LinthCanal was 20-50% higherthan at the Linth Canal River Rhine, but both areas are consideredexcel- 1960-84 Mean 48 5 - 48 5 Maximum c. 96 c. lent graylinghabitat, and the Rhine population is 96 1984-85 465 377 842 the largestin Switzerland.Growth (Fig. 5) is similar in bothareas and fasterthan in mostother European F Primegraying habitat only. 43 populations(reviewed by Woolland & Jones 1975), prevailedamong fisherymanagers that exploitation W. Suter includingstocks in French (Persat & Pattee 1981) ratesby humanswere near themaximum sustainable and Central European rivers recentlyexamined yield, and that additional predation pressure by (southernBavaria: Schmid1991; Austria:Jungwirth, cormorantswould affectthe populations(J. Walter, Schmutz & Waidbacher 1989; Wiesbauer et al. personalcommunication). 1991; Czechoslovakia: Lusk 1975; Witkowski & In both study sites, the area-specificpredation Kowalewski 1988). If fast growthin taken as an pressure by cormorants (1-6 individuals ha-') indicationof good habitat quality, it is likelythat was among the highestrecorded for riverhabitats. population densities in the River Rhine and the Despite this,and contraryto the prevailingbelief of LinthCanal comparewith those in the best Austrian fishermenand fisherymanagers, no evidence was habitats,where graylingbiomasses of up to 150- found that cormorantpredation had depredated 450kg ha-' were recorded (Jungwirthet al. 1989; graylingand troutpopulations at the two studysites. Wiesbauer et al. 1991). If the average graylingbio- At the River Rhine, predationpressure was itself mass presentin the River Rhine is supposed to be rathera functionof fishbiomass available. For the around 150kg ha-1, thefishery would remove 22-5% largegrayling population at the RiverRhine and the and cormorants5-7% of the standingcrop annually troutfishery in the LinthCanal, therewere also no (Table 2). Utilizationof the LinthCanal fishpopu- indicationsthat cormorantpredation resulted in a lation may be somewhat stronger.Ruhle (1985) compensatoryreduction of fisheryyield. The effects assumed thatanglers take 25-30% of the recruited on graylingyield in the Linth Canal by strongpre- stockannually. Based on the averageyield of 1970- dation pressurein 1 year are less clear, and a sub- 84 (47 kgha- 1), an average standing stock (and stitutionalshift to some extent between angling similar production) of 160-190kg ha-1 would be catch and bird predationcannot be excluded. The calculated, which puts the exploitation rate by results are in line with earlier conclusions that cormorantsin 1984/85at 18-22%. Linn & Campbell piscivorousbirds do not normallydestabilize fresh- (1992) considered a consumptionof 17% of prey waterfish populations in naturalhabitats (Draulans biomass by white-breastedcormorants P. carbo 1988; Suter 1991). However, predation-mediated lucidusin Lake Malawi to be indefinitelysustainable processes in the regulationof freshwaterfish popu- by the fishpopulation, while in termsof production lations are poorly understood (Wootton 1990), 30-50% should be safelyharvestable (Henderson, despite the recent progress made in modelling Ryder& Kudhongania1973). For adultbrown trout, the effectsof larval survivalon the abundance of annual mortalityrates of 70% were found in a adult fish(e.g. Jensen1993; Rice et al. 1993), or in sustained population that was not supported by unravellingmechanisms for densitydependence in stocking(Davies, Sloane & Andrew 1988). These the survivalof juvenile fish (e.g. Elliott 1985, 1989a, comparisons support the earlier conclusion that b). The question remains whetherbird predation cormorantpredation had not put an unusuallyhigh could triggeran irreversibledecline in small, frag- strainon the graylingpopulation of the LinthCanal. mented fish populations broughtto the brink of extinctionby humans,and calls for a conservation- minded approach in the investigationof freshwater CONCLUSIONS fishpopulation dynamics. Fisherypractice and regulationsin Switzerlandare guided by research only for a few commercially Acknowledgements importantspecies on a minorityof lakes (Muller 1990), but not usuallyfor cyprinid fishes or forriver This paper draws on the help of many people. fishcommunities exploited mainlyby angling.No Cormorantcensus data were providedmainly by the biomonitoring,or at least an assessmentof stocks late E. Bbhni, H. Seiler, H. Riget and M. Zanoli, and stocking operations, was done in the study whileP. Morel and S. Imfeldanalysed the cormorant areas. Intensive stocking of graylingand other pellets. Data on biometryand age structureof the salmoniformspecies is still being continuedin dis- graylingpopulation in the River Rhine are due to regard of warningsby fisherybiologists (Peter & severalmembers of the local fishingsocieties, and to Meier 1989; Gerster 1990), that such operations J. Walterand to K. Egloff,who also kindlyprovided may not only be a waste of resourcesbut thatthey the age determinations.Fishery data were made could also affect populations of other fishes as available by A. Kramer, M. Straub and J. Walter, well as the genetic diversityof the targetspecies who, togetherwith R. Muller and E. Staub, par- (for graylingsee Surre, Persat & Gaillard 1986; ticipatedin (sometimesheated) debates thathelped Bouvet, Soewardi & Pattee 1990; Bouvet, Pattee & to shape some of the concepts presented in this Maslin 1992). Regulations such as the increase of paper. Several points were also clarifiedin dis- legal size for troutat Linth Canal were based on cussions withB. Buhrer,J. Guthrufand A. Peter. ad hoc decisions, when a negative trend in yield L.G. Underhillhelped withstatistical analysis, and apparentlyindicated declining stocks. The impression L. Schifferliprovided much encouragement through- 44 out the study.He, A. Kramer,R. Muller,B. Naef- Elliott, J.M. (1989b) The critical-periodconcept for Cormorant effects Daenzer, E. Staub, N.R. Webb and two anonymous juvenile survivaland its relevance for populationregu- lation in youngsea trout,Salmo trutta.Journal of Fish on graying refereescommented on an earlierdraft of the paper. Biology, 35 (SupplementA), 91-98. I am gratefulfor theirhelp and I thank them all, Elson, P.F. (1962) Predator-prey relationshipsbetween includingthe manymore contributors that could not fish-eatingbirds and Atlantic salmon. Bulletin of the be mentioned personally. Financial support was FisheriesResearch Board Canada, 133, 1-87. providedby the SANDOZ AG Rhine Foundation Erlinge, S., Goransson, G., Hansson, L., Hogstedt, G., and the BUWAL (Federal Officeof Environment, Liberg, O., Nilsson, I.N., Nilsson, T., von Schantz,T. & Sylv6n,M. (1983) Predationas a regulatingfactor on Forest and Landscape). small rodent populations in southernSweden. Oikos, 40, 36-52. References Gerster, S. (1990) Verdnderungender Fischbestdndeam Hochrheinund deren Ursachen.Report to the Federal Arbeitsgruppe'Kormoran und Fischerei'(1987) Kormoran Officeof Environment,Forest and Landscape, Bern. und Fischerei.Schriftenreihe Fischerei, 47, 1-56. Hanski, I., Hansson, L. & Henttonen,H. (1991) Specialist Backiel, T. & Le Cren, E.D. (1978) Some densityrelation- predators,generalist predators, and themicrotine rodent ships forfish population parameters. Ecology of Fresh- cycle. Journalof Animal Ecology, 60, 353-367. waterFish Production(ed. S.D. Gerking),pp. 279-302. He, X. & Kitchell,J.F. (1990) Direct and indirecteffects Blackwell ScientificPublications, Oxford. of predationon a fishcommunity: a whole-lakeexper- Barrett,R.T., R0v, N., Loen, J. & Montevecchi,W.A. iment. Transactionsof the American FisheriesSociety, (1990) Diets of shags Phalacrocorax aristotelisand 119, 825-835. cormorantsP. carbo in Norway and possible impli- Healy, M.C. (1980) Growthand recruitmentin experiment- cations for gadoid stock recruitment.Marine Ecology ally exploited lake whitefish(Coregonus clupeaformis) ProgressSeries, 66, 205-218. populations.Canadian Journalof Fisheriesand Aquatic Bayer, R.D. (1989) The cormorant/fishermanconflict Sciences,37, 255-267. in Tillamook County, Oregon. Studies in Oregon Hellawell, J.M. (1969) Age determinationand growthof Ornithology,6, 1-99. thegrayling Thymallus thymallus (L.) of the RiverLugg, Bouvet, Y., Pattee,E. & Maslin, J.L. (1992) Comparaison Herefordshire.Journal of Fish Biology, 1, 373-382. de la variabilityg6n6tique de deux especes de poissons, Hellawell, J.M. (1971) The food of the graylingThymallus l'ombrecommun et le gardon,dans un fleuveam6nage. thymallus(L.) of the RiverLugg, Herefordshire.Journal BulletinFranpais de Peche et Pisciculture,324, 26-35. of Fish Biology, 3, 187-197. Bouvet, Y., Soewardi, K. & Pattee, E. (1990) Genetic Henderson, H.F., Ryder, R.A. & Kudhongania, A.W. divergence within natural populations of grayling (1973) Assessingfishery potentials of lakes and reservoirs. (Thymallusthymallus) from two French riversystems. Journal of the FisheriesResearch Board Canada, 30, Archivfur Hydrobiologie,119, 89-101. 2000-2009. Burrough,R.J. & Kennedy, C.R. (1979) The occurrence Houston, A.I. (1992) The cost imposed on a fisheryby a and natural alleviation of stuntingin a population of seabirdpopulation. Journal of TheoreticalBiology, 158, roach, Rutilusrutilus (L.). Journalof Fish Biology, 15, 313-327. 93-109. Jensen, A.L. (1993) Dynamics of fish populations with Carl, L.M., Walty,D. & Rimmer,D.M. (1992) Demogra- different compensatory processes when subjected phy of spawninggrayling (Thymallus arcticus) in the to random survival of eggs and larvae. Ecological Beaverlodge River, Alberta. Hydrobiologia, 243, Modelling,68, 249-256. 237-247. Johnstone,I.G., Harris, M.P., Wanless, S. & Graves, Carss, D.N. (1993) CormorantsPhalacrocorax carbo at J.A. (1990) The usefulnessof pellets for assessing the cage fishfarms in Argyll,western Scotland. Seabird, 15, diet of adult shags Phalacrocoraxaristotelis. Bird Study, 38-44. 37, 5-11. Danielson, B.J. & Stenseth,N.C. (1992) The ecological Jungwirth,M., Schmutz, S. & Waidbacher, H. (1989) and evolutionaryimplications of recruitmentfor com- FischoikologischeFallstudie Inn. Fischerei-Revierausschuss petitivelystructured communities. Oikos, 65, 34-44. InnsbruckStadt und Land, Innsbruck. Davies, P.E., Sloane, R.D. & Andrew,J. (1988) Effectsof Junor,F.J.R. (1972) Estimationof the dailyfood intakeof hydrologicalchange and the cessation of stockingon a piscivorousbirds. The Ostrich,43, 193-205. streampopulation of Salmo truttaL. AustralianJournal Kirchhofer,A. & Tschumi, P.-A. (1986) Age structure of Marine and FreshwaterResearch, 39, 337-354. and growthof coregonidfish populations in Lake Thun. Draulans, D. (1988) Effectsof fish-eatingbirds on fresh- Archivfur Hydrobiologie,Beiheft, 22, 303-318. waterfish stocks: an evaluation.Biological Conservation, Kirchhofer,A., Zaugg, B. & Pedroli, J.-C. (1990) Rote 44, 251-263. Liste der Fische und Rundmdulerder Schweiz. Docu- Dunn, E. (1975) Caloric intakeof nestlingdouble-crested menta Faunistica Helvetiae 9, Centre Suisse de Carto- cormorants.The Auk, 92, 553-565. graphiede la Faune, Neuchatel. DuPlessis, S.S. (1957) Growthand dailyfood intakeof the Koch, F. & Wieser, W. (1983) Partitioningof energyin white-breastedcormorant in captivity.The Ostrich,28, fish:can reductionof swimmingactivity compensate for 197-201. thecost of production?Journal of ExperimentalBiology, Elliott, J.M. (1985) Population regulationfor different 107, 141-146. life-stagesof migratorytrout Salmo truttain a Lake Linn, I.J. & Campbell, K.L.I. (1992) Interactionsbetween Districtstream, 1966-83. Journalof Animal Ecology, white-breastedcormorants Phalacrocorax carbo (Aves: 54, 617-638. Phalacrocoracidae) and the fisheriesof Lake Malawi. Elliott, J.M. (1989a) Mechanisms responsiblefor popu- Journalof Applied Ecology, 29, 619-634. lation regulationin youngmigratory trout, Salmo trutta. Lusk, S. (1975) Distributionand growthrate of grayling I. The critical time for survival. Journal of Animal ( Thymallusthymallus) in thedrainage area of theSvratka Ecology, 58, 987-1001. River. Zoologicke Listy,24, 385-399. 45 Magnuson, J.J. (1991) Fish and fisheriesecology. Ecol- Ruhl, C. (1985) Der Einfluss der Kormorane auf die W. Suter ogical Applications,1, 13-26. Fischbestande im Linthkanal. SchweizerischeFischer- Mann, R.H.K. (1991) Growthand production.Cyprinid eiwissenschaft,2, 9-10. Fishes. Systematics,Biology and Exploitation(ed. I.J. Scherz, H. & Senser, F. (1989) Food Compositionand Winfield& J.S. Nelson), pp. 456-482. Chapman & NutritionTables 1989190. WissenschaftlicheVerlags- Hall, London. gesellschaftmbH, Stuttgart. Mann, R.H.K. & Steinmetz,B. (1985) On the accuracyof Schmid, J. (1991) Einiges uber die Asche. Fischer & age-determinationusing scales from rudd, Scardinius Teichwirt,42, 357-358. erythrophthalmus(L.), of known age. Journalof Fish Shapiro, J. & Wright,D.I. (1984) Lake restorationby Biology, 26, 621-628. biomanipulation:Round Lake, Minnesota,the firsttwo Mills, K.H. & Chalanchuk, S.M. (1988) Population years. FreshwaterBiology, 14, 371-383. dynamics of unexploited lake whitefish(Coregonus Shearer,W.M. (1992) The AtlanticSalmon. FishingNews clupeaformis)in one experimentallyfertilized lake and Books, London. three exploited lakes. Finnish Fisheries Research, 9, Staub, E., Kramer,A., Muller,R., Ruhl6, C. & Walter,J. 145- 153. (1992) Einflussdes Kormorans (Phalacrocorax carbo) MUller, K. (1961) Die Biologie der Asche (Thymallus auf Fischbestande und Fangertrage in der Schweiz. thymallusL.) im Lule Alv (Schwedisch Lappland). SchriftenreiheFischerei, 50, 1-138. Zeitschriftfur Fischerei,10, 173-201. Surre,C., Persat,H. & Gaillard, J.M. (1986) A biometric Muller,R. (1990) Managementpractices for lake fisheries study of three populations of the European grayling, in Switzerland. Management of FreshwaterFisheries Thymallusthymallus (L.), fromthe FrenchJura Moun- (eds W.L.T. van Densen, B. Steinmetz& R.H. Hughes), tains. Canadian Journalof Zoology, 64, 2430-2438. pp. 477-492. Proceedingsof a symposiumorganized by Suter, W. (1991) Der Einflussfischfressender Vogelarten the European Inland Fisheries Advisory Commission, auf Siisswasserfisch-Bestande- eine Ubersicht.Journal Giteborg, Sweden, 31 May-3 June 1988; Pudoc, fur Ornithologie,132, 29-45. Wageningen. Suter, W. (1994) Are CormorantsPhalacrocorax carbo Newsome, A. (1990) The controlof vertebratepests by winteringin Switzerlandapproaching carrying capacity? vertebratepredators. Trends in Ecology and Evolution, An analysis of increase patternsand habitat choice. 5, 187-191. Ardea, 82, (in press). Northcote,T.G. (1987) Fish in the structureand function Takashima, H. & Niima, Z. (1957) How the cormorant of freshwaterecosystems: a 'top-down'view. Canadian findsand catches fishand the cuts on the fishmarked Journalof Fisheriesand Aquatic Sciences,45, 361-379. withits beak. MiscellaneousReports of the Yamashina's Pauly, D. (1986) A simplemethod for estimating the food Institutefor Ornithologyand Zoology, 10, 13- 17. consumptionof fishpopulations from growth data and Tonn, W.M., Paszkowski,C.A. & Holopainen, I.J. (1992) food conversion experiments. Fishery Bulletin, 84, Piscivoryand recruitment:mechanisms structuring prey 827-839. populationsin small lakes. Ecology, 73, 951-958. Pedroli, J.-C., Zaugg, B. & Kirchhofer,A. (1991) Ver- Vanni, M.J., Luecke, C., Kitchell,J.F., Allen, Y., Temte, breitungsatlasder Fische und Rundmdulerder Schweiz. J. & Magnuson, J.J. (1990) Effectson lower trophic Documenta Faunistica Helvetiae 11, Centre Suisse de levels of massivefish mortality. Nature, 344, 333-335. Cartographiede la Faune, Neuchatel. Wiesbauer, H., Bauer, T., Jagsch, A., Jungwirth,M. Persat, H. & Pattee, E. (1989) The growthrate of young & Uiblein, F. (1991) Fischo5kologischeStudie Mittlere graylingin some Frenchrivers. Verhandlungen der Inter- Salzach. TauernkraftwerkeAG, Wien. nationalenVereinigung fur Limnologie,21, 1270-1275. Wilkinson,L., Hill, M., Welna, J.P. & Birkenbeuel,G.K. Persson, L., Andersson,G., Hamrin, S.F. & Johansson, (1992) SYSTATfor Windows:Statistics, Version 5 Edition. L. (1988) Predator regulationand primaryproduction SYSTAT Inc., Evanston. along the productivitygradient of temperatelake eco- Witkowski,A. & Kowalewski, M. (1988) Migrationand systems. Complex Interactionsin Lake Communities structureof spawningpopulation of European grayling (ed. S.R. Carpenter), pp. 45-65. Springer-Verlag, Thymallusthymallus (L.) in the Dunajec basin. Archiv New York. fur Hydrobiologie,112, 279-297. Peter, A. & Meier, W. (1989) Fischbesatzheute - eine Woolland, J.V. & Jones,J.W. (1975) Studieson grayling, okologischeBetrachtung. Schweizerische Fischereizeitung Thymallusthymallus L., in Llyn Tegid and the upper Petri-Heil,40(4), 1-2. River Dee, NorthWales. I. Age and growth.Journal of Peterson,H.H. (1968) The grayling,Thymallus thymallus Fish Biology, 7, 749-773. (L.), of the SundsvallBay area. Reportsof theInstitute Wootton,R.J. (1990) Ecology of TeleostFishes. Chapman of FreshwaterResearch Drottningholm, 48, 36-56. and Hall, London. Pitcher, T.J. & Hart, P.J. (1982) Fisheries Ecology. Youngs, W.D. & Robson, D.S. (1978) Estimation of Chapman and Hall, London. population number and mortalityrates. Methods for Rice, J.A., Miller, T.J., Rose, K.A., Crowder, L.B., Assessmentof Fish Productionin Fresh Waters(ed. T. Marschall, E.A., Trebitz, A.S. & Deangelis, D.L. Bagenal), pp. 137-164. IBP Handbook No. 3. Blackwell (1993) Growth rate variation and larval survival - ScientificPublications, Oxford. inferences from an individual-based size-dependent predation model. Canadian Journal of Fisheries and Aquatic Sciences,50, 133-142. Received1 July1993; revisionreceived 18 January1994 46 Appendix Cormorant effects on graying Diet compositionof cormorantsthe upper River Rhine and the Linth Canal.

RiverRhine. (a) Analysisof 336 pellets,1985/86-1990/91; (b) analysisof 21 stomachcontents of shotcormorants, 1986/87 and 1989/90-1992/93;(c) fishregurgitated by disturbedcormorants

Grayling Barbel Barbus Chub Leuciscus Burbot Other & T. thymallus(L.) barbus (L.) cephalus (L.) Lota iota (L.) unidentified Total

(a) n 343 9 81 33 51 517 % 663 17 157 64 99 1000 (b) n 21 2 1 - 9 33 % 64 6 3 - 27 100 (c) n 17 2 4 - - 23 % 74 9 17 - - 100

Linth Canal. (a) Analysisof 30 pellets (January1985); (b) analysisof 208 stomachcontents of shot cormorants,1985- 1990/91;contents with >10 small fishare excluded

Grayling Trout Cyprinids Burbot Other Total

(a) n 24 - - - 2 26 (b) n 41 71 42 15 31 200 % 21 35 21 8 15 100

Annual proportionsby weight,and sample size (backcalculatedweight)

% grayling % trout % otherspecies Weight(kg)

1984-85 82 12 6 15 6 1986-87 53 29 18 1-8 1989-90 19 42 38 60 1990-91 22 39 39 9 0