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PREDATION on JUVENILE FLATFISH by CORMORANTS PHALACROCORAX CARBO in the DUTCH WADDEN SEA Nederlands Instituut Voor Onderzoek

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PREDATION on JUVENILE FLATFISH by CORMORANTS PHALACROCORAX CARBO in the DUTCH WADDEN SEA Nederlands Instituut Voor Onderzoek NIOZ-RAPPORT 1995 - 10 75 PREDATION ON JUVENILE FLATFISH BY CORMORANTS PHALACROCORAX CARBO IN THE DUTCH WADDEN SEA Cindy J.G van Damme I Nederlands Instituut voor Onderzoek der Zee Mariene Ecologie This report is not to be cited without acknowledgement of the source. Netherlands Institute for Sea Research (NIOZ) P.O. Box 59, 1790 AB Den Burg, Texel The Netherlands ISSN 0923 - 3210 Cover design: H. Hobbelink Photo on front cover reproduced from “Mens en Vogel” with permission of D. Hubant/L. Vogelenzang PREDATION ON JUVENILE FLATFISH BY CORMORANTS PHALACROCORAX CARBO IN THE DUTCH WADDEN SEA Cindy J.G van Damme NETHERLANDS INSTITUTE FOR SEA RESEARCH Marine Ecology SAMENVATTING SUMMARY Sterfte van jonge platvis in het Nederlandse Predation by great cormorants Phalacrocorax deel van de Waddenzee als gevolg van predatie carbo on juvenile flatfish in the Dutch Wadden door aalscholvers Phalacrocorax carbo werd Sea was studied. Predation pressure was bestudeerd door middel van analyse van braak- estimated based on the analysis of regurgitated ballen en de reconstructie van vissoort en pellets and the reconstruction of fish species grootte aan de hand van de gevonden otolieten. and size based on otoliths found. Pellets were Braakballen werden in de herfst verzameld op collected in one main colony and in the main de slaapplaatsen van de belangrijkste kolonies. night roosts in autumn. Cormorants arrived in Aalscholvers arriveerden in mei en de aantallen May and total numbers in the Dutch Wadden in de Nederlandse Waddenzee namen ver­ Sea increased from about 1600 in May to 5100 volgens toe van 1600 in mei tot 5100 in in August. Hereafter numbers decreased to augustus. Vervolgens namen de aantallen af tot about 1400 in October. From November 1400 in oktober. Vanaf november waren onwards no significant numbers were found in aalscholvers niet meer in groten getale aan­ the area. On average about 3400 cormorants wezig. Gemiddeld waren er ongeveer 3400 were present between May and October. In total aalscholvers aanwezig van mei tot november. 24 different fish species were found in 198 Totaal werden er 24 verschillende soorten vis pellets. In numbers flatfish amounted on gevonden in 198 geanalyseerde braakballen. In average 73% of the fish found. Among the aantallen gemeten droeg platvis voor 73% bij. flatfish species, plaice was found most Schol werd het meest frequent gevonden (in frequently (17.5%), followed by dab (12.1%), 17.5% van de gevallen), gevolgd door schar flounder (8.5) and sole (1.1%). Total predation (12.1%), bot (8.5%) en tong (1.1%). De totale pressure by cormorants was estimated by predatie door aalscholvers werd berekend door multiplying the mean number of cormorants with vermenigvuldiging van het gemiddeld aantal the time the cormorants were present in the aanwezige aalscholvers met het aantal dagen Wadden Sea (180 days) and with the mean dat zij in de Waddenzee aanwezig zijn (180) en number of flatfish found in the pellets, under the met het gemiddeld aantal platvissen in de assumption that each pellet contains the braakballen, onder de aanname dat per dag één remains of the fish eaten during the previous 24 braakbal per aalscholver gevormd wordt en dat hours. In total about 28.5 millions of juvenile iedere gegeten platvis in de braakbal wordt flatfish were consumed, of which about 60% teruggevonden. In totaal werden ongeveer 28.5 could be identified to species level. Predation miljoen jonge platvissen gegeten, waarbij van appeared to be size-selective in all flatfish ongeveer 60% de soort achterhaald kon species. Juvenile plaice was most heavily worden. Predatie door aalscholvers bleek preyed upon, more than 7.3 millions were grootte-selektief te zijn in alle vier de consumed. Also 5.3 million juvenile dab, 3.1 platvissoorten. Schol werd het meest gegeten million flounder and about 0.4 million sole were (7.3 miljoen), gevolgd door schar (5.3. miljoen), consumed. A comparison with data from a bot (3.1 miljoen) en tong (0.4 miljoen). In ver­ flatfish survey in the area revealed that gelijking tot schattingen van de totale hoeveel­ instantaneous mortality rates M (d'1) were low heid platvis aanwezig in de Nederlandse and ranged from respectively 0.0013 for dab Waddenzee, leverde het lage dagelijkse morta- and 0.00003 for sole. This leads to the liteiten M (d'1) op, variërend van respectievelijk conclusion that cormorant predation is not an 0.0013 voor schar tot 0.00003 voor tong. Dit important source of mortality in flatfish in the leidt tot de konklusie dat predatie door Dutch Wadden Sea during the summer period. aalscholvers niet de belangrijkste mortaliteits- factor is voor jonge platvis in de zomer in de Nederlandse Waddenzee. 1. INTRODUCTION predators present in their area of study, Trallee bay, Scotland. Assuming a similar situation for The Wadden Sea is an important nursery area the Wadden Sea, this means that predation by for several North Sea flatfish species, such as seals and/or birds must be significant during the plaice Pleuronectes platessa, flounder P/a- summer period. No quantitative information is tichthys flesus and sole Solea solea (Z ijlstr a, present about these sources of predation. This 1972; Danker s et al., 1978; van Beek et al., study aims to quantify one of these sources, i.e. 1989). All three flatfish species spawn in winter the predation by an avian predator, the in the open North Sea or in the coastal zone. cormorant Phalacrocorax carbo, on juvenile The developing pelagic larvae settle in the flatfish in the Dutch Wadden Sea. Wadden Sea in spring and the demersal Two main types of cormorant predation on juveniles stay and grow in the area during their juvenile flatfish can be distinguished: the first years of life. In autumn they migrate to feeding of chicks by the parents with small 0- deeper waters and in spring they return in the group flatfish and the catch of larger sized area. After a few years they become mature juveniles by adults and fledged juveniles for and leave the area to join the adult stock. their own consumption. Previous research in Several studies have been carried out in the Cornwall and in the Wadden Sea has shown Wadden Sea on the mortality of juvenile flatfish that more than 40% of cormorant prey consisted and their predators. Larval flatfish are preyed of juvenile flatfish. (St e v e n , 1933; van den upon by coelenterates (van der V e er , 1985), Berg 1993, van Dam m e 1994a). Cormorants and just settled juveniles suffer from predation arrive in the Dutch Wadden Sea in April-May to by crustaceans, shrimps Crangon crangon and build up energy reserves and they leave the crabs Carcinus meanas (van der V e er , 1986; area again in September-October. Regurgitated van der V eer & Ber g m a n , 1987). With pellets were collected from a colony and roosts increasing size, other predators, such as fish, and prey species were identified from the birds and seals become dominant (Ma c e r , morphology of the otoliths that were retained 1967; Edw ards & St e e le , 1968; van der V e e r , from these pellets. For each species, 1986). This sequence in type of predation relationships between fish length and otolith appears to be a rather general pattern also size were used to reconstruct the original fish operating in other coastal flatfish nurseries size from each otolith. Based on these (P ih l, 1990; van der V eer et al„ 1990; 1996; estimates of the flatfish consumption of Ellis & G ibs o n , 1995). individual cormorants, together with information Information on both plaice and flounder on the number of cormorants present during the suggests that in these species year-class season in the area, total predation on flatfish strength is determined already in the pelagic was calculated. These figures were compared stages (van der V e e r , 1986; van der V eer et with estimates of the total juvenile flatfish stock al., 1991). Mortality by shrimps and crabs present in the area, to reveal: appeared to be significant (van der V e e r , 1986; [1] the importance of this source of predation van der V eer & Be r g m a n , 1987; Pih l, 1990; [2] to what extent this type of predation is size- v an der V eer et al., 1990; 1991) but acting only selective. in a regulating way, damping the interannual variability in year-class strength (van der V e er , 2. MATERIAL AND METHODS 1986; van der V eer et al., 1990, 1991; 2.1. AREA OF STUDY Beverto n & Ile s , 1992). Beverton & Iles (1992) reviewed all available information on The Wadden Sea is a shallow coastal sea along mortality of juvenile flatfish in European waters the northern and western coast of the and concluded that also in summer after the Netherlands, Germany and Denmark, separated period of predation by crustaceans significant from the North Sea by a row of barrier islands. It mortality among juvenile flatfish did occur. This is connected with the North Sea by deep tidal suggests that the combined effect of predation channels. In the Dutch part of the area the by fish, seals and birds might be of importance rivers IJssel and Ems discharge into the in the W adden Sea. Ellis & G ibson (1995) Wadden Sea. concluded that once flatfishes had attained a In the Dutch Wadden Sea several colonies size of 45 mm, they had outgrown the fish and important night roosts for cormorants are found. At a number of night roosts and at the Ku ip er s , 1975; Berg m an et al., 1989).
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