Variations in catch rates in relation to the spatial distribution of beam trawlers 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 65% of the fishing trips, catch per unit of effort is equalized among fishing grounds. In the remaining trips, catch rates were below average, suggesting that vessels showed exploratory fishing. These results corroborate predictions of the ideal free distribution theory. More powerful vessels were over-represented on better fishing grounds as compared with less powerful vessels. As a special case the effect of vessel density on catch rate was investigated utilizing the convenient fact that a segment of the Dutch fleet stayed in port during one week each year ('week of prayer'). The catch rate of 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 reference area where no change in vessel density was observed. Implications of this evidence of competitive interactions among vessels in relation to fisheries management are discussed. (C) 2000 International Council for the Exploration of the Sea.