Spatial and temporal changes in the southern species component of North Sea Bottom fish assemblages

Abstract

A survey series was started in 1987 to monitor changes in North Sea fish assemblages and to investigate the meso-scale natural variability in catch rates of individual species. The survey began in four areas (boxes) of 10×10 nm, but has been extended to 10 boxes. The boxes are distributed throughout the North Sea.

Randomly located trawl tows were conducted in each box during summer, using standard trawls and standard procedures. Data were available from nearly 2500 hauls. In total 85 species occurred in the catches, of which 37 were southern, 34 northern and 14 intermediate species.

The results indicate southern species migrate into the North Sea by two paths: through the Straits of Dover along the coast into the German Bight and by the Fair Isle current between Orkney and Shetland Isles into the north-western North Sea. The faunal exchange between both seems to be very low.

The highest percentages of southern species were found in the boxes south of the line, which runs north of the Dogger Bank from 54°30′N at the English coast to 56N at the Danish coast. The location of this line approximates to the northern boundary of the transition area between thermal stratified (Northern and Central North Sea) and thermal homogenous water masses in the Southern North Sea and the German Bight. The lowest proportions of southern fish species were observed in the centre of the North Sea north of the Dogger Bank, rather than in the northern areas.

Only in the German Bight a significant temporal increase in the frequency of southern fish species was noted. This appears to be determined by the winter conditions, for they are related to the comparatively high number of mild winters during the recent decade combined with a dominance of south-westerly winds, which increase the inflow of water through the Straits of Dover. The water temperatures in summer seem to be not relevant for the increasing abundance of southern species.