Research Document 1998/64

Considerations on the demise or otherwise of Northern cod

By G. Winters and J. Brattey

Abstract

Perspectives on the current status of northern cod depend upon the interpretation of recent, decadal changes in its population dynamics. In particular, the unexpected decline of northern cod in offshore areas in the early 1990's has yet to be resolved in terms of a comprehensive hypothesis that can account for all anomalous events. Three major hypotheses have been put forward to explain the apparent collapse viz. (1) STOCK MISMANAGEMENT HYPOTHESIS-------the stock has been in continuous decline since the mid-1980's because of high fishing mortalities that were not detected by offshore RV surveys and resulted in persistent overestimation of stock biomass levels, (2) CATASTROPHIC MORTALITY HYPOTHESIS-----massive increases in natural mortality of idiopathic origin devastated the stock in the early 1990's, and therefore the offshore surveys correctly tracked stock dynamics and (3) MIGRATION CHANGE HYPOTHESIS-----a major change in offshore migrations occurred in the early 1990's such that the overwintering now occurs in the inshore; i.e. offshore RV surveys adequately measure offshore, but not total abunance of northern cod. In this communication we have examined a wide variety of indices, particularly from the inshore, in an attempt to evaluate and eliminate hypotheses that are inconsistent with available data. We have concluded, from the congruence in the inshore trap indices, inshore gillnet indices, the Spans offshore index, exploitation and population estimates from inshore and offshore taggings, that northern cod abundance was stable and consistent with the offshore surveys at least up to the early 1990's. The Stock Mismanagement Hypothesis can therefore be rejected on the grounds that it lacks an evidential basis. With respect to the Migration Change Hypothesis, this hypothesis can only be rejected if Sentinel catch-rates of northern cod are shown to be an invalid measure of biomass. We have concluded, from a consideration of potential biasses in Sentinel catch-rates along eastern Newfoundland and Labrador and from comparisons with implied biomass equivalence of sentinel catch-rates in Div. 3Ps, that these inshore indices represent significant (and perhaps substantial) concentrations of cod and therefore the Migration Change Hypothesis cannot yet be rejected. An alternative hypothesis, along with supporting evidence, is presented which places Sentinel catch-rates in the context of an expansion of historical links between northern cod and a resurgent 3Ps cod stock.

Complete document (PDF, 2,655 KB)

Accessibility Notice

This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.