Garthe, Stefan (1996) Distribution and abundance of North Sea seabirds and their feeding ecology in relation to fisheries and hydrography. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 156 pp.

Text Diss. 1996 Garthe, S.pdf - Published Version Restricted to Registered users only Download (2MB) | Contact

Abstract

Patterns in seabird distribution and abundance and their relationship to environmental parameters were described and analysed using data obtained by ship-based transect counts in the North Sea. Principal component analysis and other statistics showed that distance to colony, trawler abundance and hydrography strongly influenced the distribution of seabirds in the German Bight. Salinity, transparency and thermal stratification appeared to be the most important hydrographic features. Cluster analysis revealed a group of four offshore species and seven inshore species. Relationships between important abiotic parameters and seabird diet are discussed. Both fisheries and hydrography are discussed as determinants of seabird distribution with the Northern Fulmar (Fulmarns glacialis) as an example in detail. It is concluded that the occurrence of Northern Fulmars in the German Bight can be basically explained by hydrographic features, the birds preferentially frequenting Central North Sea Water Masses. The utilization of trawler discards (fish and other animals that are caught by fisheries but not landed for various reasons) and offal by scavenging seabirds is described and quantified. Based on a literature review, the total amount of fishery waste in the North Sea region is estimated at about 789,000 t per year. The numbers of seabirds potentially supported by that food is estimated to be roughly 5.9 million individuals in an average scavenger community. It is calculated on the basis of numerous discard experiments that the mass of discards and offal consumed annually by birds in the whole North Sea in 1993-1994 amounted to 55,000 t of offal, 206,000 t of roundfish, 38,000 t of flatfish, 2,000 t of elasmobranchs and 9,000 t of benthic invertebrates per year, with considerable seasonal and regional differences. There are inter-specific differences in discards types (fish species and lengths) taken by birds, which can be partly explained by bird body morphometrics. Scavenging seabirds exploit discards with different success and by different feeding techniques. Both body length and body mass of the birds can well explain species order in a kleptoparasitism index but not in a foraging success index. It is concluded that competition between species often occurs when feeding at trawlers. During specific experiments, gulls were found to exploit discards apparently equally well during the day and at night. It is shown that seabirds often exhibit clear relationships with biotic and abiotic parameters which can be successfully used to indicate conditions in the marine environment. Recommendations for further field studies and analyses are given.

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