Seabird ecology in relation to fisheries
Previous research has hinted at changes in the migratory patterns of seabirds nesting in
Scotland, including a decreasing number of Northern Gannets Morus bassanus wintering in
the North Sea, and an increase in numbers of Northern Gannets and Great Skuas Stercorarius
skua spending the winter off north-western Africa (NWA). Both species show increasing
numbers of colonies in northern areas, including Norway and Russia. These seabird species
move through the North Sea during autumn migration, and from there search for favourable
wintering grounds mainly around Iberia, including the Atlantic coast of Portugal, the Bay of
Biscay 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 increasing
in recent years from NWA coasts, despite having to attend their nests in the colonies and, as a
result, having limited time to migrate south. Differences were observed in ring recovery
locations between years and months. The number of ring recoveries by month coincides with
records from observation points along the coast of Western Europe. However, ring recovery
data are limited and potentially biased. Using data loggers, it was possible to establish that
both species are diurnal in habits during the entire winter period, showing noticeable
differences in the times spent flying during the migration months (September-October) and
during the wintering and breeding months (January and March respectively), and to confirm
the increasing tendency to winter off NWA in recent years.
Analyses of fishing landings, discard rates, and sea surface temperature data, show that
food available to Northern Gannets and Great Skuas is increasing in NWA coasts where
oceanographic conditions are stable; in contrast in the North Sea fisheries are decreasing and
the sea surface is warming. Both species are apparently changing their migratory behaviour in
order to face the constant changes in the abundance of food. Given the long life-span of
Northern Gannets and Great Skuas, genetic changes can be ruled out of an explanation for the
changes in migration behaviour, and the fact that the changes in winter distribution appear to
be occurring within one generation of the birds. The winter distribution of Northern Gannets
and Great Skuas may be due to an ideal free distribution over a wide range, in response to
changes in the distribution of fish and the availability of discards.