Diet of cormorants and the impact of cormorant predation on juvenile flatfish in the Dutch Wadden Sea

Predation by great cormorants Phalacrocorax carbo on juvenile flatfish in the Dutch Wadden Sea was estimated from otoliths found in 182 regurgitated pellets. Pellets were collected at the main night roosts and in one colony, in late summer when cormorant numbers peak in the area. Otoliths of at least 24 different fish species were found, including both marine and freshwater species. Flatfish on average amounted to 73% of the fish found in numbers and contributed 79% by mass to the birds' diet. Among the flatfish species, 0-group fish were predominantly taken. Plaice Pleuronectes platessa was found most frequently (46% of all 0-group flatfish, n=6318), followed by dab Limanda limanda (34%), flounder Platichthys flesus (19%) and sole Solea solea (1%). Total consumption by cormorants was estimated by multiplying the number of cormorant-days over summer (184 days) by the mean number of flatfish found in the pellets per sampling location, under the assumption that each pellet contained the remains of the fish eaten during 24 hours and that the diet was similar in all summer months. In total about 28.58 million flatfish were consumed over summer, 27.46 million of which were 0-group fish, including 12.55 million 0-group plaice, 9.45 million dab, 5.17 million flounder and 0.29 million sole. These figures are underestimates, because some otoliths were clearly missing, and estimated average daily intake per cormorant on the basis of all otoliths found in the pellets (460 grams of fish) was only 70 to 90% of the theoretical daily food requirement. In 1992 and 1993 0-group flatfish abundance was very low and cormorant predation during the period July to September (92 days) accounted for 49.5% of the total stage mortality in 0-group plaice in 1992 and 27.3% in 1993. However, in normal years the impact of cormorant predation will be much lower.