Evaluating the behavioral impairment of escaping fish can help measure the effectiveness of bycatch reduction devices

The behavior of roundfish excluded from an ocean shrimp (Pandalus jordani) trawl with a deflecting grid was studied using underwater video. The main study objective was to evaluate the condition of escaping eulachon (Thaleichthys pacificus), a species considered “threatened” under the United States Endangered Species Act. Observed behaviors were quantified in relation to a proposed model of an ideal trawl escapement based on an actively swimming fish avoiding contact with the grid. This model of avoidance-based escapement assumed that a roundfish in excellent condition would, (1) maintain distance from the grid, (2) avoid physical contact with the grid, (3) maintain a forward swimming orientation, and (4) maintain an upright vertical orientation. Of the species and size classes of fish encountered, large eulachon (approximately 170–240mm total length) came closest to the proposed model of avoidance-based escapement, indicating less behavioral impairment than other species. Small eulachon (