On the influence of towing speed and gear size on the selective properties of bottom trawls

Abstract

Among the multitude of factors possibly affecting the cod-end selectivity of bottom trawls, in the commercial fishery gear speed and gear size are thought to be of high importance. A binational experiment, funded by the EU, carried out with a German and a Norwegian ship in the period between 1994 and 1997 tested this opinion. Of four data sets collected only two demonstrated a relationship between speed through water and cod-end selectivity. A decrease of haddock selectivity by increased towing speed in one data set was equalled by an increase of cod selectivity in another. Hence, the results of this experiments do not support the hypothesis of an essential effect of trawl speed on the selectivity of the cod-end. Likewise, no significant difference could be detected between the selectivity of the same cod-end attached to one trawl on one hand and to a scaled down version of it on the other. An increase of cod-end selectivity related to catch size was observed in three of six series of hauls. Though being in contradiction to results of other authors a possible explanation is the magnitude of the catches made during the experiments. At this range it is rather probable that with the building up of the catch in the cod-end, the meshes open increasingly, thus improving the selectivity, until a point is reached where it either levels out or begins to decrease.