MICROSCALE DISTRIBUTION OF BEAM TRAWL EFFORT IN THE SOUTHERN NORTH-SEA BETWEEN 1993 AND 1996 IN RELATION TO THE TRAWLING FREQUENCY OF THE SEA-BED AND THE IMPACT ON BENTHIC ORGANISMS

This paper analyses the spatial distribution of fishing effort in a sa mple of 25 Dutch commercial beam trawlers fishing for sole and plaice in the period 1993-1996, based on an automated recording system with a n accuracy of about 0.1 nautical mile. Intensive fishing occurred alon g the borders of the closed areas (12 mile zone and the ''plaice-box'' , a protected area in the eastern part of the North Sea) and at certai n off-shore grounds in the southern and central North Sea. Effort dist ribution was studied within 30 x 30 (ICES rectangles), 10 x 10, 3 x 3 and 1 x 1 nautical mile squares and showed a patchy distribution. The degree of patchiness decreased with resolution. Within 3 x 3 mile squa res, beam trawling was randomly distributed in some parts of the most heavily fished ICES rectangles but patchily distributed in others. Wit hin 1 x 1 mile squares, the distribution became random within more tha n 90% of the squares. The micro-distribution showed a remarkable simil arity between the 4 years with a mean coefficient of overlap of 0.66, range 0.56-0.76. The microdistribution of the sampled vessels was rais ed to the total Dutch fleet in order to estimate the frequency at whic h the sea bed was trawled. It was estimated that during the four year study period in eight of the most heavily fished rectangles of the Nor th Sea, 5% of the surface area was trawled less than once in 5 years a nd 29% less than once in a year. The surface area of the sea bed that was trawled between 1 and 2 times in a year was estimated at 30%. The surface area trawled more than five times in a year was estimated at 9 %. The relevance of the findings for the study of the impact of beam t rawling on the benthic fauna is discussed. (C) 1998 International Coun cil for the Exploration of the Sea.