Differences in orientation and swimming of walleye pollock Theragra chalcogramma in a trawl net under light and dark conditions: concordance between field and laboratory observations

The success of modifications to fishing gear in reducing or eliminating byc atch mortality requires in many cases that escaping fish be able to use vis ion for volitional guiding out of gear. Light adequate for vision is often not present as fishing operations are commonly conducted at depths or at ni ght when light quantity is below visual capability. Previous observations o f walleye pollock behavior in the laboratory indicated that darkness inhibi ted their ability to respond and orient to trawling gear. The goal of the p resent study was to test this result in situ by comparing swimming and orie ntation capabilities of walleye pollock under light and dark conditions in commercial type trawl gear using infrared illumination and video cameras to monitor these behaviors. Under light conditions, fish swam actively and we re oriented parallel to the principle axis of the trawl. In contrast, under dark conditions there was little or no active swimming and the fish showed a significant degree of deviation from the principle axis of the trawl. Th ese results showed that vision plays a major role in trawl gear interaction s for walleye pollock and points out the importance of measuring light quan tity in field studies of bycatch processes and mortality. Published by Else vier Science B.V.