The Biology of Seabirds Utilising Fishery Waste in Shetland

Hudson, Anne Valerie (1986) The Biology of Seabirds Utilising Fishery Waste in Shetland. PhD thesis, University of Glasgow.

Abstract

The aim of this study was to examine the role of fishery waste in the ecology of scavenging seabirds in Shetland. Changes in the populations of these scavenging seabirds were discussed. Fulmars, Gannets and possibly Great Black-backed Gulls are still increasing but Herring and Lesser Black-backed Gulls and probably Great Skuas are decreasing. The breeding performances of Herring Gulls, Great Skuas and Fulmars were studied on the Isle of Noss in 1983 and 1984. Great Skuas fledged fewer chicks than elsewhere but the breeding success of Herring Gulls and Fulmars was comparable to other areas. Chick condition of the three species did not differ significantly between years or from chicks of the same species measured elsewhere. Seabird diet was examined on Noss from 1983 to 1985. Whitefish occurred most commonly in Herring Gull chick regurgitates while intertidal invertebrates were most important in adult food remains. Most Great Skua chick regurgitates contained whitefish. Whitefish and seabird were the commonest food types recorded in the diet of breeding and non-breeding Great Skuas and Sandeel was recorded more often early in the season. Whitefish (including offal) occurred more frequently than Sandeel in the regurgitates of Fulmar adults and chicks. Haddock, Whiting, Norway Pout and Sandeel were the commonest fish species to occur in pellets regurgitated by gulls and Great Skuas. Otolith lengths of Whiting and Haddock regurgitated by Great Skuas were smaller than the lengths regurgitated by Great Black-backed Gulls which, in turn, were smaller than those regurgitated by Herring Gulls. Behind whitefish trawlers in Shetland, Fulmars occurred in highest numbers and Great Black-backed Gulls were next in importance. Fewer Great Skuas, Gannets, Herring and Lesser Black-backed Gulls were present. More birds attended trawlers far out to sea than close inshore. About 27% of whitefish catches was discarded and about 90% of offal and 75% of discarded fish were consumed by seabirds. Trawler waste around Shetland could support approximately 200,000 seabirds. Haddock and Whiting were the two commonest discard species. Fulmars consumed most of the offal. Great Black-backed Gulls took most of the discards, but Gannets and Great Skuas also consumed many. Herring Gulls obtained little fishery waste at sea. Flatfish and gurnards were swallowed less often and gadoids more often than expected. Gannets and Great Black-backed Gulls swallowed most fish that they handled but the other bird species had a lower success rate. Fewer Haddock and Whiting than other fish species were dropped. Great Black-backed Gulls and Great Skuas stole more fish than other bird species. Great Black-backed Gulls and Gannets stole more fish than they lost to kleptoparasites but the other bird species had more fish stolen. Haddock and whiting swallowed by Great Skuas, Herring and Lesser Black-backed Gulls were smaller than the mean discard length. The proportion of fish dropped by seabirds increased with fish length. The lengths of dropped and stolen fish were longer than the mean lengths swallowed by each bird species. Larger birds swallowed larger fish. Handling times of fish increased with increasing fish length. Great Black-backed Gulls and Gannets swallowed fish more quickly than other birds.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Ecology
• Date of Award: 1986
• Depositing User: Enlighten Team
• Unique ID: glathesis:1986-77475
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77475