Atlantic Fisheries Research Document 1996/083

Recent changes in the distribution of Atlantic cod and American plaice in the southern gulf of St. Lawrence

By D.P. Swain

Abstract

The distribution of Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) in the southern Gulf of St. Lawrence was compared between recent years (1994 and 1995) and earlier periods of low or high abundance using data from bottom trawl surveys conducted each September since 1971. Cod distribution differed between the earlier periods of low and high abundance. In the low abundance period in the mid-1970s, cod densities were highest in shallow water in the western half of the southern Gulf. In the high abundance period in the early to mid-1980s, cod distribution expanded into the central Magdalen Shallows and eastern half of the southern Gulf, with peak densities shifting to intermediate depths. In recent years when cod abundance was again low, cod distribution shifted back to the pattern seen in the previous low abundance period, with peak densities in shallow water in western regions of the southern Gulf. An index of cod depth distribution was significantly correlated with cod abundance but was unrelated to indices of environmental conditions. These results support the hypothesis that cod distribution is density-dependent during the summer and early fall feeding season in the southern Gulf. Cod distribution differed between the current period and the previous period of low abundance in one respect: a higher proportion of cod occurred in eastern regions of the southern Gulf in recent years. In recent years, cod biomass has fallen to levels similar to those of the early to mid-1970s in western strata but has remained well above the earlier low levels in eastern strata. Plaice distribution was similar between the high abundance period in the late 1970s and the low abundance period in the mid to late 1980s. However, distribution in recent years has shifted sharply to the east. Declines in plaice biomass since 1991 have been confined to western strata.

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