Research Document - 2012/081

Estimating the age composition of discards in the NAFO 4T American plaice fishery

By R. Morin

Abstract

Discarding of American plaice by the mobile gear fleet in NAFO Division 4T was widespread until appropriate management measures came into effect in 1994. To assess the status of the resource, it is necessary to develop a commercial catch at age that incorporates both the landed and discarded catches. This study was undertaken to estimate the size and age composition of American plaice discarded by mobile gear from 1976 to 2009. The method used to estimate discards combined length frequencies from landed catches with research survey length frequencies and retention ogives for the dominant mesh size used in the fishery. The approach was modified to take into account the distribution of the fishing fleet and the potential effect of unspecified flounder catches in the 1970s. Annual estimates of discards, as a percentage of the number of plaice caught, averaged 76% from 1976 to 1993 assuming a uniform distribution of catches within 4T, 12% after 1993. The estimated rate of discarding was comparable to at-sea observer estimates for 1976, but less than observed discarding in three of four other comparisons from 1984 and 1990-1992. On the basis of minimum mesh sizes imposed since 1993, discarding should be minimal and may be overestimated in this study. However, estimates for the 1976-1993 period may be less than what occurred in the fishery.

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