Seabird ecology in relation to fisheries

• Main Author: Meraz Hernando, Juan Francisco
• Format: Dissertation
• Language: English
• Published: 2011
• Subjects: GC Oceanography; QH301 Biology; QL Zoology

Previous research has hinted at changes in the migratory patterns of seabirds nesting inScotland, including a decreasing number of Northern Gannets Morus bassanus wintering inthe North Sea, and an increase in numbers of Northern Gannets and Great Skuas Stercorariusskua spending the winter off north-western Africa (NWA). Both species show increasingnumbers of colonies in northern areas, including Norway and Russia. These seabird speciesmove through the North Sea during autumn migration, and from there search for favourablewintering grounds mainly around Iberia, including the Atlantic coast of Portugal, the Bay ofBiscay and the Gulf of Cadiz.By means of historic ring recovery data, provided by the British Trust for Ornithology,it was possible to establish that the number of records of adults of both species are increasingin recent years from NWA coasts, despite having to attend their nests in the colonies and, as aresult, having limited time to migrate south. Differences were observed in ring recoverylocations between years and months. The number of ring recoveries by month coincides withrecords from observation points along the coast of Western Europe. However, ring recoverydata are limited and potentially biased. Using data loggers, it was possible to establish thatboth species are diurnal in habits during the entire winter period, showing noticeabledifferences in the times spent flying during the migration months (September-October) andduring the wintering and breeding months (January and March respectively), and to confirmthe increasing tendency to winter off NWA in recent years.Analyses of fishing landings, discard rates, and sea surface temperature data, show thatfood available to Northern Gannets and Great Skuas is increasing in NWA coasts whereoceanographic conditions are stable; in contrast in the North Sea fisheries are decreasing andthe sea surface is warming. Both species are apparently changing their migratory behaviour inorder to face the constant changes in the abundance of food. Given the long life-span ofNorthern Gannets and Great Skuas, genetic changes can be ruled out of an explanation for thechanges in migration behaviour, and the fact that the changes in winter distribution appear tobe occurring within one generation of the birds. The winter distribution of Northern Gannetsand Great Skuas may be due to an ideal free distribution over a wide range, in response tochanges in the distribution of fish and the availability of discards.