This article investigates the relationship between optimal gear selectivity and effort cost in the fishing industry We first show that optimal selectivity depends negatively on the level of effort cost, but that this relationship is not continuous. Optimal selectivity switches when real effort cost goes beyond a certain level, and this switch induces a non-marginal reduction in the level of fishing effort. In the second part of this article, we shoot, that the current level of real effort cost in the Nephrops fishery of the Bay of Biscay is far below the switch point, which makes high selectivity optimal. The discrepancy between optimal and current selectivity may be explained by the state of the fishery management and also by the fact that selectivity is hardly observable with the type of gear presently used.