185

FOOD OF CORMORANTS Phalacrocorax carbo sinensis WINTERING IN , SOUTHERN GERMANY

THOMAS KELLER

ABSTRACT Following the spectacular population increase in conti­ nental Cormorants, numbers of wintering birds on lakes and rivers in Bavaria, southern Germany, have risen sharply. Because of a growing .concern among commercial fishermen and anglers, a diet study has been carried out at two major roosts: one at a lake (lake ) and one at a dammed river (lower river). Main prey species were cyprinids (Rudd, Roach, Chub and Bream), but at lake Chiemsee the commercially most important Whitefish also appeared in the diet during spawning in December/January. Mean daily intake per bird increased in the course of winter, because of larger specimens taken. Possible causes and effects of this phenomenon are discussed. Assuming an average daily uptake of 400 g fresh mass, it was estimated that at lake Chiemsee 3.3% of the total annual fish production was taken by Cormorants (vs. 28% by commercial fish­ ermen). The total catch of Whitefish by Cormorants amounted to a mere 3.2% of the total commercial catch of this species. Larger proportions were scored for Eel (22.3%) and Pike (6.2%). At the lower Inn river it was esti­ mated that Cormorants took 21 % of the total annual fish production. Both in view of species composition in the Cormorants' diet and of the consumption estimates specified above, it is considered unlikely that the birds impose a serious threat to commercial fisheries. Interference with recreational angling (e.g. for Grayling) may, however, occur. Weinbergstrasse 9, D-97753 Karlstadt-Stetten, Germany.

INTRODUCTION & Hafner 1984), in The Netherlands (Marteijn & Dirksen 1991, Platteeuw et at. 1992, Dirksen et at. As a consequence of the increase of the Conno­ 1995) and in Austria (Trauttmansdorff 1992). rant Phatacrocorax carbo sinensis as a breeding Most of these have shown cyprinids to be the bird in NW-Europe since the early 1980s (Van main prey items. This paper provides a brief over­ Eerden & Gregersen 1995), the numbers of view of the food choice of the birds from two of wintering individuals in Bavaria (southern the major Bavarian roosts, based on the analysis Gennany) have increased substantially. Details of pellets. on the development of this wintering population are presented by Wtist (1981), Leibl & Vidal (1983), Bezzel & Engler (1985), Zach (1987), STUDY AREA AND METHODS Franz & Sombrutzki (1991) and Bezzel (1989, 1992). This increase has caused concern among The two roosts were selected for their suitability both commercial fishennen and anglers. Food for pellet collection. One of them was located on studies in the wintering regions of the Connorant the smallest of three islets in lake Chiemsee have mainly been carried out in other areas within (47°52' N, 12°25' E), the largest Bavarian lake Europe, i.e. Switzerland (EAWAG 1975, 1979, (79.9 km2), the other on the banks of the lower Ruhle 1985, Imfeld et at. 1986, MUller 1986, Inn river (48° 18' N, 13° 11' E), which is dammed Bundesamt fur Umweltschutz 1987, Morel & for power generation (Fig. 1). At each of the two Hausmann 1989, Suter 1991a, b, c, d), France (1m roosts pellets were collected on five consecutive

ARDEA 83: 185-192 186 ARDEA 83(1), 1995

mass should be considered as minimum values, since (partial) digestion may cause fish remains to wear or even disappear (EAWAG 1975, 1979, Muller 1986, Worthmann & Spratte 1987, Morel & Hausmann 1989, Marteijn & Dirksen 1991, Zijlstra & Van Eerden 1995). No attempt for correction of wear was made.

RESULTS

At lake Chiemsee the average monthly numbers of Cormorants were 244 in November, 255 in December, 231 in January and 165 in February. In October and March 150 birds were recorded (Lohmann pers. comm.). At the lower Inn river the numbers of Cormorants were somewhat higher; average monthly Cormorant numbers being 288 in December, 302 in January and 211 in Fig. 1. Map of Gennany with the location of the February. No counts were made in October, study areas lake Chiemsee and lower Inn river. November and March. As the counts of the winter before (1989/90) showed very similar results (Franz & Sombrutzki 1991), the numbers of days each month throughout the winter of 1990/91 October 1989 (185), November 1989 (327) and (November-February at lake Chiemsee, Decem­ March 1990 (95) were used for calculation of bird ber-February at the lower Inn river). Pellet collec­ days. tion always took place in the early morning, Identifiable fish remains were found in 62% of before scavengers could get at them, and only all pellets (n =1758),20% only contained uniden­ fresh and complete ones were taken. A total of 'tifiable remains, while about 18% were comple­ 1758 pellets from both roosts were analysed. The tely empty. A total of 2303 individual prey items numbers of Cormorants present at the roosts were was found, belonging to 22 different species of recorded in the evenings before the pellet collec­ fish (Table 1). The length spectrum of the fish tions. At lake Chiemsee additional counts were caught ranged from 3 cm cyprinid fry to 70 cm made in October and March. Eel Anguilla anguilla (corresponding to a mass All identifiable fish remains (otoliths, pharyn­ spectrum of 1-900 g). The food composition of geal bones, opercula, eye lenses, vertebrae, etc.) the Cormorants was quite similar at the two were separated and analysed (for methods see roosts, consisting mainly of cyprinids (50-76% in EAWAG 1975, 1979, Jonsson 1979, Muller 1986, numbers, 31-64% in biomass at lake Chiemsee Worthmann & Spratte 1987, Morel & Hausmann and 64-90% in numbers, 58-82% in biomass at 1989, Marteijn & Dirksen 1991, Trauttmansdorff the lower Inn river; Fig. 2). At lake Chiemsee the 1992). Otoliths and pharyngeal bones were meas­ most important cyprinid species were Rudd ured in order to estimate length and mass of each Scardinius erythrophthalmus, Roach Rutilus individual prey item, according to regression rutilus and Chub Leuciscus cephalus. At the formulae (Berg 1985, Worthmann & Spratte 1987, lower Inn river mainly Bream Abramis brama R. Muller, pers. comm., own data). Estimated and Roach were caught. The economically very numbers of fish per pellet, fish length and fish important Whitefish Coregonus spp. were only Keller: DIET OF WINTERING CORMORANTS IN BAVARIA 187

Table 1. Prey species of Cormorants in the winter of 1990/91 at lake Chiemsee and the lower Inn river, southern Bavaria.

Fish length, em Fish mass, g N

Mean Min Max Mean Min Max

Anguilla anguilla 45.6 23.2 70.4 322 32 900 57 Salmo trutta fario 24.5 12.0 34.3 300 18 700 5 Oncorhynchus mykiss 24.6 11.2 35.3 157 19 350 7 Coregonus spp. 28.6 13.0 40.8 277 16 636 130 Thymallus thymallus 27.5 13.2 42.2 221 24 646 87 Rutilus rutilus 27.3 9.3 35.0 202 6 632 133

Leuciscus leuciscus 24.7 18.2 31.2 204 55 398 5 Leuciscus cephalus 25.4 16.9 34.6 220 49 512 16 Leuciscus idus 19.3 69 1 Scardinius erythrophthalmus 24.5 10.0 33.0 276 15 517 45 Aspius aspius 37.2 32.7 41.6 623 420 825 2 Tinea tinea 27.4 25.2 31.2 286 213 416 3

Chondrostoma nasus 34.1 27.4 40.6 634 266 928 9 Barbus barbus 30.3 28.8 32.9 269 228 345 4 Albumus albumus 18.0 16.2 19.7 63 42 83 2 BUcca bjoerkna 22.7 18.6 26.8 145 59 230 2 Abramis brama 23.5 16.7 35.7 144 49 532 22 Cyprinidae, not identified 15.2 3.0 38.7 80 1 876 1348

Esox lucius 37.3 25.2 47.2 383 117 688 16 Lota Iota 29.8 17.9 41.0 204 42 490 60 Perea fiuviatilis 13.3 6.8 31.2 35 3 367 177 Stizostedion lucioperca 20.7 8.3 46.4 108 11 557 36 Gymnocephalus cemua 13.1 11.9 14.7 22 14 35 4 consumed in their spawning period at lake and up to 8% in numbers (15% in biomass) at the Chiemsee during the months of December and lower Inn river. The only other species reaching a January (up to 29% in numbers, 41 % in biomass; proportion of over 10% (either in numbers or in Fig. 2). Consumption of Eel (another economi­ biomass) in any month were Perch Perea fluviat­ cally important species) was also virtually ilis (in November 12% in numbers, 6% in restricted to lake Chiemsee, where only in biomass at lake Chiemsee and in December 23% November some large individuals (6% in in numbers, 9% in biomass at the lower Inn river; numbers and 31 % in biomass) were taken (Fig. Fig. 2) and Burbot Lota Iota (in November 12% 2). Grayling Thymallus thymallus, an important in biomass, 4% in numbers at lake Chiemsee). recreational species for anglers, were found at Two other species of recreational importance to both sites, but only in January and February at the anglers, Pike Esox lucius and Pikeperch onset of their spawning period (Fig. 2). In the Stizostedion lucioperca, were never found in latter month their proportion reached up to 23% proportions exceeding 1% of the prey items. in numbers (21 % in biomass) at lake Chiemsee Thus, except for the absence of the typically pre- 188 ARDEA 83(1), 1995

n=912 n=1318

.r:: III :;::: 20

o ~_.....d ~--=-lJL ~lJ ~c:lb.CL.._ eel whitefish grayling pike burbot perch pike-perch eel whitefish grayling cyprinids pike burbot perch pike-perch 100,------, DNov 1990 I Dec 1990 80 DJan 1991 III Feb 1991 ~ 60

Fig. 2. Food composition (number and biomass) of Cormorants at lake Chiemsee and the lower Inn river in the winter of 1990/91. alpine lake dwelling Whitefish in the diet of The estimated mean fish mass per pellet, Connorants of the lower Inn: there were only usually considered to be the individual bird's minor differences in the food choice of both mean daily food intake (cf. Voslamber 1988, roosts. Dirksen et al. 1995, Veldkamp 1995), was 273 g. Seasonal changes in food composition suggest The frequency distribution of fish mass per pellet that in lake Chiemsee cyprinids are partially was skewed towards the upper end (Fig. 3). Two substituted by Whitefish during the latter's main overall tendencies have been found: an increase spawning period in December and, to a lesser in the mean fish mass per pellet in the course of extent, January. When Whitefish disappears from the winter (values for January and February well the diet by February, the proportion of cyprinids above those for November and December) and a does not recover its original November level, decrease in the numbers of individual fish per since it is now being replaced by Grayling, which pellet. Furthennore, an increase occurred through­ start their spawning period by then. At the lower out the winter in the mass of individual fish Inn river a sharp decrease in the proportion of consumed and pellets containing one to three prey Perch between December and January was items (279 g) occurred more often than those with compensated for entirely by an increase in the more than three (161-236 g). proportion of cyprinids. Marginal increases Using the bird numbers of each month (recal­ between January and February in the proportions culated to bird days) and an individual daily food of Perch and Grayling were accompanied by a ration of 400 g/day, the total fish consumption slight decrease in cyprinids. was estimated. At lake Chiemsee the Connorants Keller: DIET OF WINTERING CORMORANTS IN BAVARIA 189

winter 1990/91 lakes and some rivers, flowing into the river Inn (e.g. Rott, Mattig, Salzach), during some weeks of the winter, the predation pressure on the river Inn will be somewhat lower in reality.

DISCUSSION

Prey species composition as well as the seasonal changes in it found in this study, agree very well with earlier findings in Swiss lakes and dammed rivers (EAWAG 1975, 1979, Imfeld et aI. 1986, 100 300 500 700 900 1100 Muller 1986, Suter 1991a, b, c, d). A similar study fish mass per pellet (g) carried out in summer in Schleswig-Holstein, however, revealed a completely different prey Fig. 3. Reconstructed fish mass per pellet in the composition (Worthmann & Spratte 1987). In this winter of 1990/91, lake Chiemsee and lower Inn river case, Eel was preyed upon with much higher combined (n = 973 pellets, mean 273 g; 0.5 and 0.75 frequency. Although sufficient knowledge about percentiles 242 g and 378 g respectively). fish densities, fish ecology and fish behaviour at lake Chiemsee and the lower Inn river is lacking, consumed about 14 522 kg of fish (36 306 bird the prey composition found suggests that serious days) during the winter (October-March). From damage to either commercial fisheries or anglers data on diet composition it was calculated, that is unlikely to occur, but there might be some they mostly took cyprinids (7150 kg). The competition between anglers and birds. At lake proportions of the commercially important Chiemsee, commercially interesting species as Whitefish, Eel and Pike were about 2300 kg, 1679 Whitefish and Eel are only caught by Cormorants kg and 270 kg respectively. If only considering at very restricted periods of time. the lake area of 7990 ha, the predation pressure In 1990 the total commercial catch was was calculated to be 1.8 kg/ha. This figure will be 122946 kg. This figure was quite low compared too high, as the birds sometimes use the adjacent to the five year average (1986-1990) of 152 159 rivers , , Tiroler Ache and Inn as addi­ kg. The Cormorants consumed 11.8% of the tional foraging areas. Especially Grayling must commercial catch. Lohmann (1991) estimated an have been taken from the rivers, as this is a very annual fish production of 55 kg/ha for the lake. rare species in the lake. The Cormorants Comparing this figure to predation pressure and consumed about 864 kg of it during the winter. At commercial catch, it is found that the Cormorants the lower Inn river the Cormorants took about consumed about 3.3% (1.8 kg/ha) of the annual 17 075 kg of fish (42 688 bird days). Again, they fish production, whereas the fishermen took 28% mostly consumed cyprinids (11 369 kg). The (15.4 kg/ha) ofit. Unfortunately, there is no infor­ proportions of the recreationally important Pike, mation avialable on angling yields. Considering Pikeperch, Eel and Grayling were 1185 kg, 806 the commercially important Whitefish, it is found kg, 879 kg and 1100 kg. During their stay, the that Cormorants took only 3.2% of the commer­ birds use a stretch of 80 km of the river cial catch (1990: 72 012 kg). Eel (not native to (Reichholf 1988), with an area of approximately the lake) and Pike are not equally important to 2730 ha. Thus, their predation pressure was fisheries. The commercial catches were 7524 kg calculated to be 6.3 kg/ha (213.4 kg/km). As the and 1682 kg in 1990 respectively, of which the birds additionally forage in some smaller gravel Cormorants consumed 22.3% and 6.2%. 190 ARDEA 83(1), 1995

The annual fish production of the lower Inn river Mattig, which is a tributary of the river Inn river was estimated to be about 30 kg/ha (Kainz, and only 12 km away from the roost of the pers. comm.). The Cormorants consumed about Cormorants. 21 % (6.3 kg/ha) of it. Also in this case only very When comparing the situations at lake few data are available on angling yields, because Chiemsee and the lower Inn river to similar Swiss a variety of German and Austrian angling associ­ waters, it is seen that much higher numbers of ations fish in the river and also some private Cormorants than those recorded at either study fishing occurs. The only data available came from site, did not have appreciable negative impacts on one fishing association that uses about 1700 ha of fish stocks present (Suter 1991b). However, for the river Inn, issuing 1035 annual licenses. As more exact predictions of impact of Cormorants there are quite some additional anglers in the on fish stocks more extensive studies on fish are same stretch of the river, which are not registered needed. Once again, it has become apparent that in that particular association, the available data Cormorants do not show any clear species prefer­ were multiplied by an estimated correction factor ence, but take whatever prey species present at its of 1.25 in order to get an idea of the total angling fishing grounds. Judging by the range of lengths yield. Thus, the anglers took about 32 646 kg of found (Table 1), the size of the fish does not seem fish in 1990, that equals 19.2 kg/ha (64% of the to impose a real problem either. Within the pisciv­ annual fish production). According to the preda­ orous waterbirds, Cormorants should thus be tion pressure found (6.3 kg/ha), the Cormorants considered generalists, able to exploit the most consumed about 33% of the angling yield. abundant stocks of any fish species. Comparing the yield to the species composi­ Daily food intake by individual Cormorants in tion in the Cormorants' diet, it was found that Bavaria in winter, as estimated by mean fish mass both anglers and birds go for pretty much the per pellet, seems to be low in comparison with the same species. The estimated angling yield of value of 500 g generally proposed (Deufel 1984, cyprinids was 22 855 kg and Cormorants Imfeld et at. 1986, Muller 1986, Worthmann & consumed about 31 % of it (values from diet Spratte 1987). A lower value (330 g), however, composition data, calculated down to the 1700 ha was also found by Voslamber (1988), while the angling association uses). But there was one Reichholf (1990) even estimated it to be a mere interesting difference. Within the cyprinid family 100-150 g. The finding that fish mass per pellet the anglers took mostly carp (12125 kg), whereas was higher in pellets containing only few indi­ the birds took virtually none of it. Considering vidual prey items suggests that birds, lucky Pike, Pikeperch and Eel, Cormorants consumed enough to catch some big fish at the beginning of about 26.3%,75.1% and 17.8% of the yield (Pike: their day's foraging, satisfy their needs earlier 2805 kg, Pikeperch: 668 kg, Eel: 3079 kg). For than less fortunate ones. Thus, it can be argued the endangered Grayling, it was found that it is that fish mass per pellet for pellets containing the taken only at the very end of the Cormorants' remains of many small fish is more likely to stay, which coincides with the onset of the represent the minimum daily needs of the indi­ Graylings' spawning. From the data of the vidual bird (cf. Van Dobben 1952). On the other angling yields, it is obvious that the Cormorants hand, the bird catching larger prey items may do not catch their Graylings in the river Inn. have the advantage of putting on body reserves. There the proportion of Grayling in the angling This could be an explanation for the higher daily catch was only 0.3% (104 kg) compared to an rations found at the end of the winter, when the average of 8% in the Cormorants' diet. Further­ birds should be preparing for the return journey more, Kainz (pers. comm.) reported 100-150 and the following breeding season. The possibil­ Cormorants feeding on Grayling for about two ities ofcatching larger prey later in winter may be weeks in February 1991, at the small Austrian greater too, since the lower water temperatures at Keller: DIET OF WINTERING CORMORANTS IN BAVARIA 191 this time are likely to reduce the maximum swim­ Kormoran-Gewollen. Contract for the Bundesamt ming speed of all fish (Wardle 1975). Thus, the fur Umweltschutz, Bern. Franz, D. & A. Sombrutzki 1991. Zur Bestands­ larger specimens (generally the faster swimmers; situation des Kormorans Phalacrocorax carbo in Wardle 1977), which earlier might have been too Bayem in den Wintem 1988/89 und 1989/90. Om. quick to be caught, only become available to the Anz.30:l-lO. Cormorants at lower water temperatures. 1m, B.H. & H. Hafner 1984. Impact des oiseaux pisci­ vores et plus particulierement du Grand Cormoran (Phalacrocorax carbo) sur les exploitations pisci­ coles en Camargue. Station BioI. de la Tour du ACKNOWLEDGEMENTS Valat, Camargue. Contrat Nr. ENV - 491 -F (Ss). Imfeld, I., L. Schifferli, P. Staub & A. Zimmermarm Collecting pellets at lake Chiemsee was only possible 1986. Der Kormoran - ein Wintergast aus dem because of the help of H. Lex, who provided transpor­ Norden. Zuger Neujahrsblatt. tation to the islet of the roost. R. Muller kindly made Jonsson, B. 1979. Skarvarna och yrkefisket i Kalmarsund. Calidris 8:171-220. available his regression formulae for calculating fish Leibl, F. & A. Vidal 1983. Zur Situation des Kormorans lengths and fish masses. W. Suter, P. Morel, M. Klein, J. (Phalacrocorax carbo) in Bayem. Ber. Dtsch. Reichholf and M. MUhlenberg also provided much Sekt. Int. Rat Vogelschutz 23 [1984]:81-89. valuable information. The Landesbund fUr Vogelschutz Lohmarm, M. 1991. Die Fische des Chiemsees. . in Bayem e.v. (a Bavarian non-profit conservation Columba Verlag. association) helped to finance this study. RM. Sellers Marteijn, E.C.L. & S. Dirksen 1991. Cormorants commented on an earlier draft of the manuscript. Phalacrocorax carbo sinensis feeding in shallow eutrophic freshwater lakes in The Netherlands in the non-breeding period: prey choice and fish REFERENCES consumption. In: M.R. Van Eerden & M. Zijlstra (eds) Proceedings workshop 1989 on Cormorants Berg, R 1985. Alterbestimmung und Wachstumsrtick­ Phalacrocorax carbo: 135-155. Rijkswaterstaat berechnung mit Hilfe markierter Aale. Arb. Directorate Flevoland, Lelystad. deutsch. Fischereiverb. 38:25-36. Morel, P. & S. Hausmarm 1989. Erste Resultate von Bezzel, E. & U. Engler 1985. Zunahme rastender Untersuchungen an Gewollen von Kormoranen Kormorane (Phalacrocorax carbo) in Sudbayem. (Phalacrocorax carbo) am Rhein bei Basel Garm. vogelkdl. Ber. 14:30-42. (Kembs). Jahresbericht der Om. Gesellschaft Bezzel, E. 1989. Rastende Kormorane (Phalacrocorax Basel 119:27-32. carbo) in Sudbayem: Entwicklung 1978/79 bis Mtiller, R 1986. Die Nahrung des Kormorans 1987/88. Garm. vogelkdl. Ber. 18:37-45. (Phalacrocorax carbo sinensis) am Bodensee. Bezzel, E. 1992. Rastende Kormorane (Phalacrocorax "Petri-Heil"-Beilage, Schweiz. Fischereiwiss. 3:1­ carbo) in Sudbayem: Entwicklung der BesHinde 2. im Winterhalbjahr bis 1991/92. Garm. vogelkdl. Platteeuw, M., J.H. Beekman, TJ. Boudewijn & E.C.L. Ber.2l:46-55. Marteijn 1992. 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Der Einfiuss der Kormorane auf die Gewollen auf Fischreste. Contract for the Fischbestande im Linthkanal. Schweiz. Fische­ Bundesamt fUr Umweltschutz, Bern. reiwiss. 2:9-10. EAWAG 1979. Systematische Untersuchung von Suter, W. 1991a. Beeintrachtigen fischfressende 192 ARDEA 83(1),1995

Vogelarten unsere Susswasserfisch-Bestande? Zijlstra, M. & M. R. van Eerden 1995. Pellet produc­ Vogelschutz in Osterreich 6:11-15. tion and the use of otoliths in determining the diet Suter, W. 1991b. Der Einfiuss fischfressender of Cormorants Phalacrocorax carbo sinensis: Wasservogel auf Susswasserfischbestande - eine trials with captive birds. Ardea 83:123-131. Ubersicht. J. Om. 132:29-45. Suter, W. 1991c. Food and feeding of Cormorants Phalacrocorax carbo wintering in Switzerland. In: M.R. Van Eerden & M. Zijlstra (eds) Proceedings SAMENVATTING workshop 1989 on Cormorants Phalacrocorax carbo: 156-165. Rijkswaterstaat Directorate Flevoland, Lelystad. Vanwege de spectaculaire toename van de continentale Suter, W. 1991d. Nahrungsokologie des Kormorans in vorm van de Aalscholver, nam ook het aantal overwin­ der Schweiz. Vogelschutz in Osterreich 6:75-80. terende vogels in Beieren (Zuid-Duitsland) sterk toe. Trauttmansdorff, J. 1992. Ethologie und Okologie des Dit zorgde voor nogal wat opschudding onder zowel Kormorans (Phalacrocorax carbo sinensis) an der beroeps- als sportvissers. In 1990/91 werd daarom een osterreichischen Donau. Umwelt-Schriftenreihe studie naar het voedselpakket - aan de hand van braak­ fUr Okologie und Ethologie, Heft 19. ballen - uitgevoerd op twee belangrijke slaapplaatsen: Van Dobben, W.H. 1952. The food of the Cormorant in de Chiemsee en de afgedamde rivier de Inn. Op beide The Netherlands. Ardea 40: 1-63. plaatsen bleken karperachtigen de belangrijkste prooi Van Eerden, M.R. & J. Gregersen 1995. Long-term te vormen (Rietvoorn, Blankvoorn en Kopvoorn). changes in the northwest European population of Cormorants Phalacrocorax carbo sinensis. Ardea Echter, op de Chiemsee kwamen tijdens de paai in 83:61-79. december/januari ook de commercieel belangrijke Veldkamp, R. 1995. Diet of Cormorants Phalacrocorax marenen (Coregonus spp.) in het dieet voor. Het gemid­ carbo sinensis at Wanneperveen, The Netherlands, deld dagrantsoen van de vogels nam toe in de loop van with special reference to Bream Abramis brama. de winter als gevolg van een toename in het formaat Ardea 83:143-155. van de gevangen vissen. Voslamber, B. 1988. Visplaatskeuze, foerageerwijze en Mogelijke oorzaken en gevolgen van dit ver­ voedselkeuze van Aalscholvers Phalacrocorax schijnsel worden besproken. Uitgaande van een gemid­ carbo in het IJsselmeergebied in 1982. Flevo­ deld dagrantsoen van 400 g verse vis en het aantal bere­ bericht nr. 286. Rijksdienst voor de IJssel­ meerpolders, Lelystad. kende vogeldagen werd geschat dat op Chiemsee 3.3% Wardle, C.S. 1975. Limit of fish swimming speed. van de totale jaarlijkse visproductie van 55 kg/ha door Nature 255:725-727. Aalscholvers werd weggevangen (tegen maar liefst Wardle, C.S. 1977. Effects of size on the swimming 28% door de beroepsvisserij). De predatiedruk van de speeds of fish. In: T.J. Pedley (ed.) Scale effects in vogels werd hier geschat op maximaal1.8 kg/ha. Op de animal locomotion: 299-313. Academic Press, rivier de Inn bedroeg de predatiedruk 6.3 kg/ha, London. waarmee naar schatting 21 % van de totale jaarlijkse Worthmann, H. & S. Spratte 1987. Nahrungsunter­ visproductie door de vogels werd geconsumeerd. suchungen am Kormoran (Phalacrocorax carbo) ­ Vooral vanwege de soortsamenstelling van de Die Auswirkungen der Kormorane auf die prooien en de bovengenoemde schattingen lijkt het schleswig-holsteinische Binnenfischerei. Landes­ fischereiamt, Kiel. onwaarschijnlijk dat Aalscholvers de beroepsvisserij in Wust, W. (ed.) 1981. Avifauna Bavariae, Bd 1:117-121. de bestaande situatie aantoonbare schade berokkenen. Munchen. WeI is het niet uit te sluiten dat sportvissers en Zach, P. 1987. Zum Auftreten des Kormorans (Phala­ Aalscholvers concurreren, met name bij de visserij op crocorax carbo) im Rotelsee Weihergebiet bei Vlagzalm. Cham. Opf. Ther. OAG Ostbayern 14:113-126.