Research Document 1998/04

Status of witch flounder in NAFO Division 4RST, January 1998

By D. Swain, G. Poirier and R. Morin

Abstract

Provisional landings of witch flounder in the Gulf of St. Lawrence (NAFO divisions 4RST) were 498 t in 1996 and 528 t in 1997, slightly above the low 1995 value but well below the longterm (1960-1990) average of 3066 t. Landings were almost entirely by mobile gear, mostly (86-96%) seines. Landings were mostly from areas 4Tf and 4Tg (75%) and 4Rd (15%). Witch-directed effort by seines has remained steady in 4T since 1991 but declined sharply in 1994 in 4R where it has remained low to the present. The size composition of landings has remained fairly stable in recent years, with peak catches in the 35-40 cm range. The lengths of fish landed have increased slightly since 1988 but are well below the lengths landed in the late 1970s and early 1980s. A composite abundance index for the entire Gulf of St. Lawrence was constructed by combining data from the September surveys of the southern Gulf and the August surveys of the northern Gulf, adjusting for variations in fishing efficiency between day and night and between vessels and gears. The index was restricted to lengths of 24 cm and greater due to changes in the size selectivity of gears. The index suggested a gradual decline in abundance since the late 1980s. Declines in abundance were progressively greater for larger sizes. This was reflected in large declines in trawlable biomass. Biomass in 1994-1997 was estimated to be 30% of that in 1987-1990 at lengths of 30 cm and greater, and 21% of the earlier level at lengths of 40 cm and greater. Biomass declines did not appear to be spatially uniform. In the eastern 4T area, biomass in 1994-1997 appeared to be similar to that in 1987-1990. The ratio of catch at length divided by the RV index of population abundance at length (relative F) did not indicate any time trends in fishing mortality. Abundance of small fish (length < 15cm) appears to have been relatively high in 1990 and 1991, low from 1992 to 1996 and high in 1997.

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