Re-analysis of comparative fishing experiments in the Gulf of St. Lawrence and other analyses to derive stock-wide bottom-trawl survey indices beginning in 1971 for 4RST Greenland halibut, Reinhardtius hippoglossoides

Abstract

Standardized bottom-trawl surveys provide fishery-independent estimates of relative abundance that are key to the assessment and management of demersal fish stocks worldwide. Over time it may be necessary or desirable to change the vessels, fishing gear or other protocols employed in the surveys to maintain or enhance survey efficiency. Such changes can affect the survey catchability of different species and sizes of fish that could otherwise be confounded with changes in stock abundance. Consequently comparative fishing experiments involving the old and new equipment and protocols are used to estimate calibration coefficients that are then applied to maintain the integrity of survey time series. Here we analyze data from previously analyzed and unanalyzed comparative fishing experiments to develop indices for Greenland halibut in NAFO 4RST that combine for the first time. data from surveys in the northern Gulf of St. Lawrence (GSL) and the southern GSL (sGSL), and which extend the survey series back to 1984. Currently, the main index used in the assessment for this stock begins in 1990 and is based on the northern GSL survey, which covers most but not all of the distribution of Greenland halibut in the GSL. We then show how the abundance index from the sGSL survey can be used to predict a GSL-wide index using a heuristic model of density-dependent distribution change. This approach is then applied to the sGSL index, available since 1971, to provide a longer-term perspective on the dynamics of the stock, particularly in light of fishery exploitation rates that were particularly elevated at times from the early 1970s to the mid-1990s. Although some refinements are required to improve the approach, these new indices provide important insights on the productivity, dynamics, and exploitation history for the stock.