Incorporating experimentally derived estimates of survey trawl catchability into the stock assessment process

During the course of the April, 1996, meeting of the Fishing Technology and Fish Behavior Working Group it became clear to many that the methodology for experimentally estimating the catchability of survey trawls might have progressed sufficiently to produce estimates useful for stock assessment. To determine whether the formation of a formal Study Group would be warranted to study the methodology and utility of experimentally derived estimates of trawl catchability, Steve Walsh, chairman of the FTFB, requested David Somerton to form an ad hoc group to consider the question and report its findings at the next FTFB meeting. The group included four members who have conducted experiments to measure trawl performance (Godø, Ramm, Somerton, Walsh,) and two members who have analyzed or fit stock assessment models to trawl survey data (Ianelli, Smith). The following report, which attempts to synthesize a rather brief e-mail discussion among the members of the ad hoc group, will consider the problem from the perspective of three questions: 1) What does the term "catchability" really mean?, 2) Are experimentally derived catchability estimates useful for stock assessment? and 3) Can experimentalists now produce good estimates of catchability