Estimating Natural Mortality Of Wild Cod From Controlled Feeding And Starvation Experiments Conducted In The Laboratory

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The contribution of individual factors to the rate of natural mortality is usually unknown because it can hardly be measured in the wild, particularly in large ecosystems. Atlantic cod experienced a decline of their nutritional condition in the early 1990s in the Gulf of St. Lawrence. Whether or not natural mortality increased as a result of that decline was determined from controlled experiments conducted in the laboratory. The maximum range of several variables reflecting the nutritional condition of fed and unfed cod >30 cm was determined. By comparing values observed in the wild and in the laboratory, we concluded that wild cod may die of inanition during the spring period. The decline in the nutritional condition of wild cod from January to April was slower however than in cod deprived of food during a similar period of time in laboratory experiments. This indicates that cod fed but did not meet their energy requirements in winter prior to spawning. In contrast, the decline in condition from April to May matched the rate observed in the laboratory experiments, indicating that wild cod probably fast at this time. Estimates of survival probability based on the frequency distribution of condition factor during the spawning period appear to be unbiased as the rate of energy mobilization observed in the field never exceeded the rate observed in the laboratory.