Research Document 1998/18

Environmental influences on the productivity of cod stocks: some evidence for the northern Gulf of St. Lawrence, and required changes in management practices

By J.D. Dutil, M. Castonguay, M.O. Hammill, P. Ouellet, Y. Lambert, D. Chabot, H. Browman, D. Gilbert, A. Fréchet, J.-A. Gagné, D. Gascon, and L. Savard

Abstract

The mechanisms linking climatic changes and Atlantic cod (Gadus morhua) stock production in the northern Gulf of St. Lawrence are examined through a review of projects conducted at MLI. The average temperature of Gulf waters varies between years and severe winters particularly in the late 1980's and early 1990's caused a marked decline of the CIL temperature. Temperature fluctuations are known to occur in deep waters as well which are also characterized by low oxygen pressures potentially lethal to cod. These climatic changes resulted in a shift in the distribution of cod and this may have had a negative influence on growth, condition and maturation. Declining energy reserves in the autumn result in poor condition in the spring so that females produce a smaller amount of eggs at a relatively higher somatic cost which exposes post-spawners to death through starvation. Preliminary results suggest that eggs produced by poor condition females are less viable due to their poor quality. The impact on larvae hatching from those eggs remains to be documented, but nevertheless larval growth and swimming activity were found to be correlated. Between 1980 and 1996, size at age followed changes in climatic conditions. Poor condition occurred in cod with slow growth rates which may mean that death from poor condition may be limited to cod exhibiting slow growth rates. Northern Gulf cod thus have a low productivity which is further impaired by a high rate of mortality due to predation by seals. Stock management decisions must factor in latitudinal and temporal variations in productivity.

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