Associations between Atlantic cod (Gadus morhua) and hydrographic variables: implications for the management of the 4VsW cod stock

Abstract

Many groundfish stocks on the Atlantic coast of North America are monitored by bottom trawl surveys which use a stratified random design. The resulting estimates of abundance often exhibit high variability within and between surveys, particularly on inter-annual time scales, and inconsistencies in the estimates of relative year-class strength over time. These inconsistencies, or year-effects, may reflect changes in the distribution or catchability of the fish that may be related in turn to environmental factors such as water temperature and salinity. The estimates of cod abundance derived from the research vessel trawl surveys conducted in July over the eastern Scotian Shelf in NAFO areas 4Vs and 4W from 1970–1993 are shown to exhibit these features. Analyses of these survey data also show that cod exhibit age- and area-specific associations with near-bottom water temperature and salinity ranges that are consistent with the properties of a Cold Intermediate Layer (CIL) water mass in the area and that inter-annual variability in the estimates of abundance is correlated with the area of the bottom found within the CIL. A model relating the variability in the CIL to the estimates of age-specific cod abundance obtained from the surveys was used to derive new survey indices of abundance. These new indices do not have detectable year-effects and may provide more consistent estimates of true relative year-class strength than the original time series.