In the southern North Sea a gradual increase in depth is found from south to north, accompanied by gradually dropping tidal current velocities (CREUTZBERG, 1985). At a certain latitude (ca. 50 km north of Texel) a critical velocity is reached, inducing stratification of the water in summer and deposition of mud and detritus particles. This boundary , or front, is characterized by a highly enriched benthic fauna, higher chlorophyll a and zooplankton concentrations, and also relatively high fish-stocks, as compared to the sea on both sides. I have started to study whether certain fish-eating seabirds, as the top-carnivores in the system, are able to benefit from the relatively high biomass at lower trophic levels. If this would be so, higher densities and probably higher "qualities" of the birds involved would be expected in the area of the front, as compared to the areas on either side. Seabirds were counted from the R. V. " Aurelia" in the period February-November 1983 in an area extending on both sides of the frontal zone. Individual counts lasted for 10 minutes at ship speeds ranging from 7.5 to 9.5 knots. All birds seen in each count were identified, and if possible their age and state of moult was noted. Among the non ship-followers, enhanced numbers were found for the Guillemot Uria aalge in the periods February-April, and October-November. Guillemots from the frontal zone and to the north of this area were of a better quality ( older or in better condition) after the winter than birds occurring south of the front. A higher percentage of birds still in winter plumage was found in the south, whereas from the frontal zone northward relatively many summer-plumage birds were found in the early spring. Guillemots that had regained their summer plumage appeared to leave the area before conspecifics that were still in winter plumage, so wintering south of the front is linked to a late departure to the breeding colonies. Razorbills Alca torda were also found in relatively high numbers in the frontal zone at the end of the winter. In summer, Razorbills were not found within the area studied, but Guillemots, being always the more common species of the two, were seen in every month of the counting period. Trawler-scavenging seabirds are all highly mobile and their distribution at sea is for a great part determined by the fishing activities of man, bringing otherwise inexplorable food sources within reach of the birds. Fishery seems particularly intense in the frontal area, making this enriched area even more profitable for scavengers. Several species were found to occur in relatively high numbers in the frontal zone for at least part of the year: Fulmar Fulmaris glacialis in July (mainly moulting birds); Lesser Black-backed Gull Larus fuscus in summer; Greater Black-backed Gull L. marinus in autumn and Kittiwake Rirsa tridactyla in February and from September to November (NB: no counts are available for mid-winter). The general conclusion that can be drawn from these first results is that pursuit divers as well as scavenging seabirds profit from the locally enriched zone of the tidal front in the southern North Sea. (This summary has been published elsewhere: LEOPOLD et al. , 1986). |
