Fleet dynamics of herring trawlers-change in gear size and implications for interpretation of catch per unit effort

Catch per unit effort (CPUE) is widely used as an index of stock abundance. It is as widely acknowledged that CPUE can be a misleading index of abundance owing to a multitude of factors including fish behavior, fishing fleet interaction, and the increase in catchability over time caused by improvement in fishing technology. Based on information concerning the size of herring trawls manufactured in Finland since the early 1980s, an increase in fishing power of the fleet was postulated. Because we lacked direct information about the size of trawls aboard, we applied a model to estimate the changes over time. In the analysis, an analogy between fish and trawls was created by adopting the concepts and algorithms from fish stock assessment into assessment of the trawl population, where both the total number of trawls and the size of individual trawls were being analyzed. The results indicate that the average gear size has nearly tripled in 20 years. Accepting the assumption that larger trawls are generally more effective than smaller ones, a substantial increase in fishing power has taken place. As a result, sequential population models calibrated with CPUE data will be severely biased as well.