Diel variation in trawl catchability: is it as clear as day and night?

Diel variation in the catchability of over 50 species was examined using research vessel surveys conducted off the coast of Newfoundland and Labrador from 1972 to 1995. Catchability during the day and night was estimated for several seasons and geographic areas in the Northwest Atlantic using two generalized linear models. In general, species exhibiting diel vertical migrations, such as redfish (Sebastes sp.), northern sand lance (Ammodytes dubius), and haddock (Melanogrammus aeglefinus), were caught in higher proportions during the day. Nonmigrating species, such as flatfish, skate, and scuplin, which rely on visibility of the trawl as a means of escapement, were caught in higher proportions during the night. Analysis of the effect of depth indicated that catchability during the day, relative to the night, increased significantly with depth for 21 species. We demonstrate that the accepted methods of estimating standard errors for generalized linear models are not valid for survey trawl data and suggest alternative methods.