Effect of deviations from target speed and of time of day on catch rates of some abundant species under North Sea International Bottom Trawl Survey protocol conditions

Abstract

Effort in trawl surveys is standardized by using a common gear and fixing haul duration and vessel speed. Such standardization should result in a fairly constant distance trawled and area or volume swept. Protocols for the North Sea International Bottom Trawl Survey (IBTS) establish hauls of 30 min at a target speed of 4 knots over ground with a standard GOV (Grande Ouverture Verticale) trawl. Primarily to evaluate the effect of departures from the target speed and of trawl speed through water, a fishing experiment was performed under IBTS protocol conditions. The experiment consisted of 30 hauls made by RV “Walther Herwig III” in a small area in the northern North Sea during 5 days in November 1997. Speed over ground was calculated from the distance between GPS shooting and hauling positions. Current speed and direction were continuously recorded by a current metre set a few metres above the sea bottom in the centre of the area to allow trawl speed through water to be calculated. We used generalized additive and linear models to analyse variation in catch rates of Norway pout (Trisopterus esmarki), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), dab (Limanda limanda) and grey gurnard (Eutrigla gurnardus) with speed over ground and through water, and also with area and volume swept by the gear, together with time of day to account for diel fluctuations. Catch rates of small haddock and whiting, grey gurnard and dab increased significantly with speed over ground while rates of Norway pout and large whiting increased with speed through water. We propose that this difference is indicative of the vertical distribution of the fish. Most affected by speed were small haddock and whiting: catches in numbers doubled within the range of 3.9 to 5.2 knot ground speed observed during the experiment. Catches of large haddock were stable. Area swept affected small haddock and whiting and volume swept affected small haddock only. With the exception of large whiting, catch rates of all species and sizes varied with time of day typically within a factor of 2 between day and night. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.