Research Document 1998/03

Relative fishing efficiency for witch flounder of vessels and gears used in the August and September bottom trawl surveys in the Gulf of Saint Lawrence

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

Abstract

The vessel used to conduct the September survey of the southern Gulf of St. Lawrence changed from the Lady Hammond to the Alfred Needler in 1992. A comparative fishing experiment between the two vessels was conducted in the southern Gulf in August 1992. We analyzed the results of the this experiment using generalized linear models assuming a Poisson error distribution with overdispersion. We estimated that fishing efficiency of the Alfred Needler for witch flounder was 1.5 times that of the Lady Hammond. The vessel and gear used to conduct the August survey of the northern Gulf changed from the Lady Hammond using a Western IIA trawl to the Alfred Needler using a URI trawl in 1990. We estimated relative fishing efficiency of the two vessels and gears for witch flounder from comparative fishing conducted during the 1990 survey. The size distribution of witch catches differed markedly between the two vessels. We calculated relative fishing efficiency for witch flounder 24 cm or greater. Relative efficiency of the two gears and vessels did not appear to vary with length over this length range. The probability of catching witch of these sizes by the Lady Hammond/Western IIA trawl was 1.75 times the probability by the Alfred Needler/URI trawl. For witch of lengths 24 cm and greater, catch rates by the Lady Hammond/Western IIA trawl were 2.0 times those of the Alfred Needler/URI trawl. We compare unadjusted time series with those adjusted for changes in survey vessel and gear.

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