Atlantic Fisheries Research Document 1996/135

Implications of Increasing Otter Trawl Mesh Size to Avoid Catching Immature Cod: Effects on Spawning Biomass, Fishing Effort and Catch Rates

By A.F. Sinclair; G.A. Chouinard

Abstract

A simplified single species, single fishing gear (otter trawl) simulation using data from the southern Gulf of St. Lawrence cod fishery was used to examine the implications of changing the selectivity pattern of a commercial fishery to avoid catching cod below the length of 95% maturity. The length at 95% maturity of the cod stock was 47 cm. Prior to the closure of this cod fishery in 1993, the regulated codend mesh size was 130 which had and L50 (length of 50% retention) of 45 cm. It may be necessary to change to a mesh with an L50 in the order of 60 cm to avoid catching immature cod. The fishing mortality at F_0.1 for the larger mesh size was more than twice that for the 130 mm mesh, implying proportionally higher target fishing efforts in a fully operational fishery. However, there was no improvement in spawning stock biomass (SSB) over a 5-year simulation period fishing at F_0.1 with the larger mesh, and SSB declined when fishing at F_0.1 with the 130 mm mesh. There was a slight increase in SSB if the larger mesh was used with an F of about half its F_0.1 level. Catch rates of the larger mesh was initially only 40% those of the 130 mm mesh, but they doubled over the 5-year simulation period. These simulations provide general indications of the direction of changes in population status and fishing success under different mesh selection options.

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