Comparison of a bottom longline survey and a bottom trawl survey for 2 groundfish species in the Gulf of Maine to evaluate habitat-related availability of large fish

Understanding fishery-independent survey selectivity is fundamental to relating relative abundance indices to total population size. The selectivity of a survey in an assessment model represents a combination of gear selectivity and availability of fish to the gear. Concerns have been raised about possible bias in sampling of the bottom trawl survey (BTS) of the NOAA Northeast Fisheries Science Center (NEFSC) caused by the challenges associated with towing in rough-bottom habitat. These difficulties may affect the availability of some fish, such as large (≥100 cm in total length [TL]) Atlantic cod (Gadus morhua). To evaluate the potential presence of BTS habitat-related bias, we compared catches of Atlantic cod and white hake (Urophycis tenuis) from BTS sampling in the Gulf of Maine with catches from the NEFSC bottom longline survey, which focuses on rough-bottom habitats in the same region. Differences between survey catches were apparent for large white hake (≥90 cm TL), supporting the premise of availability differences between surveys for white hake and the assumption of dome-shaped selectivity for the BTS. In contrast, results for Atlantic cod did not support the hypothesis of habitat-related bias in sampling of the BTS, supporting continued use of asymptotic selectivity for Atlantic cod in the BTS.