Evaluating Cooperative Research Longline Vs. Bottom Trawl Survey Results: Improving Stock Assessments

The National Cooperative Research Program funded a Gulf of Maine bottom longline survey (LLS) to address industry concerns about the Northeast Fisheries Science Center’s (NEFSC) research bottom trawl survey (BTS) catchability of specific species that purportedly prefer ‘hard’ bottom, and to enhance data collection for several species specifically associated with rocky habitat. The LLS was stratified by area and depth, similar to the BTS, and by bottom type (rough or smooth) as well. The LLS results suggest that about 40% of species common to both the LLS and BTS have higher catchability in the LLS. Selectivity differs between the gears with the LLS catching fewer smaller fish than the BTS, but with maximum lengths for most species being generally similar between the two gears. Preliminary estimates of ‘area fished’ on the LLS, derived from current speed and velocity, suggest that they may be comparable to the swept area estimates from the BTS, thus providing independent biomass estimates to improve current stock assessments. The results of the four surveys to date indicate that a LLS time series will substantially improve some data-poor stock assessments e.g. cusk, and provide supplemental indices to improve several data-rich stock assessments, e.g. Atlantic cod.