This paper proposes a formal analysis of the discarding issue including the sorting labour costs. Empirical evidences from an application to the Nephrops fishery in the Bay of Biscay show that sorting is an important time consuming activity on board and a factor of discarding. However existing literature does not explicitly take into account the sorting activity and mainly focus on quota and catch constraints. The discarding model described in the paper includes sorting as a factor explaining discards. The costs of sorting landings and the time to sort them are included in the approach. The model is developed for one species studied at a tow scale with the assumption that catches are exogenous. It is shown that taking sorting costs into account, discarding of one species may occur and that time or effort constraints can induce an over incentive to discard.