Utilization of discards and offal from commercial fisheries by seabirds in the Baltic Sea

The distribution and abundance of scavenging seabirds and their utilization of discards and offal between June and December 1998 were studied in the western Baltic Sea. Herring gulls were clearly the most numerous scavenging species in all areas and all seasons, followed by great black-backed gulls, lesser black-backed gulls and mew gulls. High percentages of discarded gadoids (cod, whiting), clupeids (herring, sprat), scad, rockling and offal were consumed by seabirds during experimental discarding on fishing boats, whereas the percentages of flatfish consumed were extremely low. There was a clear effect of cod length on total and species-specific consumption by birds but this pattern was hardly evident for clupeids or dab. By combining official discard and offal statistics and our experimental discarding, we estimate that 6500 t of fish discards and 16 000 t of offal were consumed annually by seabirds in the Baltic Sea. Bivalves, especially blue mussels Mytilus edulis, were the most frequently represented food item in herring gull pellets. Fish identified in the pellets consisted mainly of gadoids, in particular cod. The proportion of discards in herring gull pellets was on average 1.6% (range: 0–4.5%) at Laboe and 17.5% at Warnemünde (range: 9.4–25.5%), but pellets bias diet assessment as offal and other soft prey (including clupeids) will be under-represented. Scavenging on discards and offal is a widespread phenomenon in the Baltic Sea as it is in other shelf areas of Europe, but the number of bird species involved is generally lower and strongly biased towards gulls, especially herring gulls.