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 170–240 mm total length) came closest to the proposed model of avoidance-based escapement, indicating less behavioral impairment than other species. Small eulachon (<150 mm) were not frequently encountered. Almost 80% of the large eulachon maintained an upright vertical orientation throughout their escape and exited the trawl in a forward-swimming orientation. Large eulachon maintained distance from the deflecting grid better than the other species encountered (P < 0.001) and typically showed no contact or only minimal contact with it (63%). Only about 20–30% of the large eulachon showed behaviors indicating fatigue, such as laying on or sliding along the grid. In contrast, both adult and juvenile Pacific hake (Merluccius productus) frequently showed signs of fatigue, including sliding along or laying on the grid, exiting the trawl in physical contact with the grid or failing to maintain an upright vertical orientation throughout their escape. Lingcod (Ophiodon elongatus) and juvenile rockfish (Sebastes) were intermediate in their escape behavior between Pacific hake and large eulachon. They more frequently maintained an upright vertical orientation throughout their escape than Pacific hake, but also showed signs of fatigue, such as sliding along the grid or exiting the trawl in physical contact with the grid. Our data suggest that observing escape behaviors provides additional useful information to evaluate the effectiveness of bycatch reduction devices in reducing fishery-related mortality rates.