Research Document - 2007/069

Review of the structure, the abundance and distribution of American plaice (Hippoglossoides platessoides) in Atlantic Canada in a species-at-risk context

By Busby, C.D, M.J. Morgan, K.S. Dwyer, G.M. Fowler, R. Morin, M. Treble, D. Maddock Parsons, and D. Archambault

Abstract

The structure, abundance and distribution of six stocks of American plaice (Hippoglossoides platessoides) in Canadian waters were reviewed by Fisheries and Oceans Canada (DFO) under the terms of reference for the Committee on Status of Endangered Wildlife in Canada (COSEWIC) species-at-risk. The American plaice population structure was evaluated to ascertain if the current management units, which are primarily based on life history traits of the species, are indicative of ‘Designatable Units’ (DUs) at the species level. Life history traits, survey distributions, including the location of spawning fish, abundance indices and biological information (length at age, age/length at maturity and year-class strengths) were inconclusive in assigning DUs to the management areas. Therefore, information for each area is presented separately and abundance trends are modeled for progressively grouped combinations of stock components, culminating with one for the Canadian northwest Atlantic. All stocks underwent declines and in stocks where data was sufficient for analysis, the overall model suggested declines in the range of 86.4 to 94.8%. Exploitation is likely the main cause of decline, although increase in natural mortality played a role in some areas. Since the rapid decline all stocks have remained at low abundance levels. Three spatial indices were calculated: the design-weighted area of occupancy (DWAO), the minimum area occupied by 95% of the stock (D95) and the Gini index of aggregation.

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