Diel Variability in Trawl Catches of Juvenile and Adult Yellowtail Flounder on the Grand Banks and the Effect on Resource Assessment

Abstract

Abstract Trawl catches of yellowtail flounder Limanda ferruginea on the Grand Banks off the coast of Newfoundland were analyzed for diel variability. The numbers and weights of catches were significantly higher during the hours of darkness than during daylight. Catches of juvenile yellowtail flounder were very low during daylight hours, although use of a tickler chain attached to the footgear increased the catches of both juveniles and adults during daylight hours. Catches of both juveniles and adults were significantly higher at night than during the day. Changes in the vulnerability of various size-groups is thought to be related to active feeding behavior and increased gear avoidance during daylight and a reduction in both behaviors at night. Estimates of catch rates of the stock were significantly higher at night independent of season and trawling gear used. These diel patterns in vulnerability are a serious source of bias in estimation of stock abundance and indices of recruitment.