Research Document 1998/06

Assessment of 4T American plaice in 1996 and 1997

By R. Morin, G.A. Chouinard, I. Forest-Gallant, and G.A. Poirier

Abstract

Provisional landings of American plaice in NAFO Division 4T have averaged 7285 tonnes since 1965. In 1996, landings reached their lowest level in that time period, at 1381 tonnes, increasing to 1724 tonnes in 1997. The decline in landings in 1996 was attributed largely to numerous restrictions on the fishery, including a record number of closures due to cod by-catch. The annual total allowable catch (TAC) of 4T plaice was maintained at 10000 tonnes from 1977 to 1992, then dropped to 5000 tonnes from 1993 to 1995. In 1996, the TAC was set at 2000 tonnes, increased to 2500 tonnes in 1997. The competitive mobile fleet of vessels less than 45 feet attained their allocations in 1996 and 1997, but all other fleet sectors failed to attain their allocation. Seines were the dominant gear in the fishery, contributing approximately 75% of the annual landings in 1996 and 1997. Commercial plaice catches and fishing effort have increasingly concentrated in the eastern part of 4T since 1993, in unit areas 4Tf and 4Tg. Commercial catch rates have declined progressively over time in western 4T, but have maintained a relatively high level through most of the 1990s in eastern 4T. Plaice catch rates in the 4T research survey have declined to their lowest level for the third consecutive year, reaching 131 plaice per standard tow. Since the survey began in 1971, catch rates have averaged 376 plaice per tow. Survey data indicate that the stock reached a maximum in 1977 (1127 per tow), but declined in the late 1970s and has fluctuated at a low level since then. Analyses of catch-at-age data from the survey indicate that total mortality between ages 5 and 13 continues to be at a high level (0.49 in the 1995-1997 period). Other analyses of the survey catch-at-age indicate that year-classes were remarkably strong in the early 1970s, but have declined in abundance since the mid-1970s. A length-based index of fishing mortality (F), calculated from the ratio of commercial to survey catches of non-discarded plaice (?30 cm), suggests that F during 1997 was at a high level relative to the pattern observed since 1976. Both research surveys and commercial catches indicate that plaice are more abundant in eastern 4T than western 4T; however, analyses of mortality, year-class abundance, growth and population genetics indicate that 4T plaice form a single stock unit.

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