Factors regulating survival of northern cod (NAFO 2J3KL) during their first 3 years of life

Abstract

Mean instantaneous mortality was estimated for northern cod (NAFO Divisions 2J3KL) during the first 3 years of life for the period 1962-1987. Mortality was a density-dependent function of the adult population, increasing non-linearly with abundance. Residual mortality reached maximum values in 1970-1971, in association with cold, low salinity water of the Great Salinity Anomaly. Juvenile cod ≤2 years old were distributed towards the inshore and did not migrate offshore seasonally with larger juvenile cod (≥3 years) and adults. Geographic distributions of 1- and 2-year-old cod were density-dependent, where the distribution expanded in direct proportion to year-class abundance. There were no local increases in density as year-class abundance increased. These results imply a spatial limit on preferred seabed habitats for small cod. When 2-year-old cod were more abundant and distributed more widely onto the shelf they experienced a significant increase in cannibalism. Older juvenile cod, age 4, did not expand their geographic range with increasing year-class abundance, but did increase in density within local areas of high abundance, implying that this age group was not dependent on seabed habitats. The offshore persistence within local areas of high abundance may be associated with other factors, such as prey availability. Three-year-old cod appeared to be in transition between the habitat-dependent distributions of younger cod and the habitat-independent distribution of older cod. We hypothesize that increased cannibalism occurs at higher adult population sizes during their summer feeding migration to the inshore. In addition, increased cannibalism occurs on cod ≤2 years old belonging to more abundant year classes owing to a wider geographic distribution into less preferred habitats that overlap with the overwintering distribution of adults and large juveniles. Our results are consistent with the hypothesis that recruitment in cod is largely established during the egg and larval stages linked to environmental effects, while density-dependent processes operate to moderate survival during subsequent juvenile stages prior to recruitment.