Effects of fishing power and competitive interactions among vessels on theeffort allocation on the trip level of the Dutch beam trawl fleet

Variations in catch rates in relation to the spatial distribution of beam t rawlers were analyzed using mandatory logbook data from the Dutch fleet for 1990-1996. Catch rates by trip, corrected for differences in market value, showed a consistent seasonal pattern with the highest values found during autumn and winter. Catch rates showed a log-linear relationship with engine power, indicating differences in competitive abilities among vessels. In 6 5% of the fishing trips, catch per unit of effort is equalized among fishin g grounds. In the remaining trips, catch rates were below average, suggesti ng that vessels showed exploratory fishing. These results corroborate predi ctions of the ideal free distribution theory. More powerful vessels were ov er-represented on better fishing grounds as compared with less powerful ves sels. As a special case the effect of vessel density on catch rate was inve stigated utilizing the convenient fact that a segment of the Dutch fleet st ayed in port during one week each year ("week of prayer"). The catch rate o f vessels that continued fishing was 10% higher than in the week before or following the week of prayer. No such differences were observed in a refere nce area where no change in vessel density was observed. Implications of th is evidence of competitive interactions among vessels in relation to fisher ies management are discussed. (C) 2000 International Council for the Explor ation of the Sea.