Research Document - 2002/082

Review of population structure, distribution and abundance of cod (Gadus morhua) in Atlantic Canada in a species-at-risk context

By R. K. Smedbol, P. A. Shelton, D. P. Swain, A. Fréchet, and G. A. Chouinard

Abstract

The structure, abundance, and distribution of 10 cod stocks in Atlantic Canada were reviewed under four terms of reference. Cod population structure was evaluated in the context of "evolutionarily significant units". The review did not provide evidence to indicate the existence of ESUs within current management units, therefore all subsequent analyses were undertaken at the level of current unit stocks. In general, all 10 stocks across Atlantic Canada exhibited some level of decline during the available time series of abundance. Seven stocks underwent declines that reached at least 90% for at least some portion of the time periods examined. Two stocks underwent both declines and recoveries, depending upon the time interval used in the analysis. Exploitation was considered to be the main cause of abundance decline in all stocks, although unfavourable environmental changes and elevated natural mortality (from either poor fish condition or seal predation) have been proposed as possible factors in the decline or lack of recovery within several management units. Mature population estimates of all cod stocks in Atlantic Canada were at least two orders of magnitude greater than the COSEWIC threshold of 10 000 mature individuals. Three indices were calculated: the area of occupancy, the minimum area occupied by 95% of the stock, and the Gini index of aggregation. No general pattern or trend in geographic distribution was evident. Most stocks that underwent a large (at least 90%) decline in abundance also demonstrated at least some decrease in area occupied and a corresponding increase in the degree of aggregation. Several stocks showed little change in occupancy. Evidence exists that suggests the loss of a population (spawning) component on the Eastern Scotian Shelf (4VsW). No evidence was presented for an increase in the degree of population fragmentation within management units.

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