Research Document - 2008/018

Viability of the southern Gulf of St. Lawrence cod population

By D.P. Swain and G.A. Chouinard

Abstract

The cod population in the southern Gulf of St. Lawrence is currently at a low level of productivity. Unusually high natural mortality of adult cod is the most significant factor in the low productivity of this stock. Slow individual growth also contributes to the low productivity. In contrast, recruitment rate (recruits per unit of spawner biomass), though lower than the unusually high rates observed between the mid 1970s and early 1980s, is currently relatively high compared to the rates observed in the 1950s and 1960s. Stochastic projections of the southern Gulf cod population were conducted, taking into account recent variability in rates of recruitment and growth and uncertainty both in the natural mortality rate of adult cod (M) and in the current levels of cod abundance at age. Projections were made based on each of the four population models examined in the 2008 assessment of this stock. All projections led to the conclusion that the population is not viable at its current level of productivity. Assuming that this low level of productivity persists into the future, the population is expected to steadily decline even with no fishery. Based on the accepted assessment model (which estimates M to be 0.59 in recent years), the population is certain to be extirpated (defined here as a spawner biomass less than 1000 t) within 40 years with no fishery and in 20 years with a Total Allowable Catch (TAC) of 2000 t (assuming a catch equal to the TAC or produced by a fishing mortality rate of 3, whichever is less). A substantial increase in productivity would be required for the population to be viable. Assuming an immediate decline in M from 0.59 to 0.4, spawner biomass is expected to be stable if there is no fishery. However, even at this higher productivity, a fishery with a TAC of 2000 t would result in extirpation in about 50 years.

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