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ICES Journal of Marine Science: Journal du Conseil 1997 54(4):684-694; doi:10.1006/jmsc.1997.0239
© 1997 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
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Occurrence and consumption of seabirds scavenging on shrimp trawler discards in the Wadden Sea

U. Walter and P. H. Becker

Institut für Vogelforschung ‘Vogelwarte Helgoland’ An der Vogelwarte 21 D 26386, Wilhelmshaven, Germany

Correspondence to U. Walter: tel: +494421365440.

The fishery for the brown shrimp (Crangon crangon) on the coast of Germany (Lower Saxony) produces a rich food source for ship-following seabirds. Large quantities of discarded fish and invertebrates attract flocks of gulls and terns. Between March 1993 and August 1994 the number and species composition of birds following two shrimp trawlers were determined. The main scavengers were herring gull (Larus argentatus) and black-headed gull (L. ridibundus). Common gull (L. canus), lesser black-backed gull (L. fuscus), great black-backed gull (L. marinus), and common/arctic tern (Sterna hirundo/paradisaea) were less numerous. The distribution of scavenging seabirds behind the shrimp trawlers showed a marked species-specific seasonal and spatial pattern. The scavenging behaviour of seabirds feeding on discards was studied by means of experimental discarding. Of 10 356 items offered, 8253 were picked up by the birds, and in 8072 of these the consuming bird was identified. Herring gulls were most successful. Although this species represents only 45% of all birds counted, they took 82% of the total number of items consumed. In total, ship-following seabirds consumed 41% of the offered mass of flatfish, 79% of the roundfish, and 23% of four invertebrate species. In feeding experiments birds consumed 10% of undersized shrimps. Applying these consumption rates and bioenergetic data to the total quantity of discarded biota, approximately 90 000 seabirds may be supported by this source throughout the entire fishing season (April–November 1993). Potentially, the discards of the shrimp trawler fleet of Lower Saxony were sufficient to meet the energy demand of 60 000 birds for the whole year. The results suggest that discards may have strong effects on the bird populations and the ecosystem of the Wadden Sea.

Keywords: discards, feeding experiments, seabirds, shrimp fishery, Wadden Sea


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