Gauging the impact of fishing mortality on non-target species

The most obvious effect of fishing on non-target species is direct mortalit y. To quantify this effect on the vulnerability of species requires measure ment of the current fishing mortality rate and of the tolerance of the spec ies to fishing mortality. These are difficult to estimate for the little-st udied non-target species. We describe two potential methods for estimating current fishing mortality rate when data are limited. Their application is illustrated for dab (Limanda limanda) and grey gurnard (Eutrigula gurnardus ), two common non-target species in the North Sea. We also develop approach es to define tolerance levels for fishing mortality for little-studied and rare species, based on the phi potential jeopardy level: the fishing mortal ity that causes a phi reduction in spawning stock biomass per recruit relat ive to the unexploited situation. We propose that for non-target species, m odels founded on basic knowledge of life history parameters, and on general ly established relationships between these parameters, may offer the only p ractical approach. (C) 2000 International Council for the Exploration of th e Sea.