Some sources of variation in the assessment when introducing unaccounted mortality due to discarding practices. The case of Southern Four spot megrim stock

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Four spot megrim (Lepidorhombus boscii) is widely distributed in Atlantic waters form Iceland to Cape Bojador. In ICES Divisions VIIIc and IXa, Portuguese and Spanish ships captured this species as by-catch mainly in mixed bottom trawl fisheries. Only one exploitation unit of this species is assessed (from Divisions VIIIc and IXa) in the ICES Working Group of Hake, Monk and Megrim. Discard data are not used in the assessment of this species, though they are considered significantly high for younger ages. Discards data are available for Spanish trawlers for 1993, 1994, 1997, and the period from 1999 to 2001. A simulation using discard data was made to avoid the underestimation of mortality and recruitment. Due to the change in January 1990 in the minimum legal size for this species in this area, it was decided not to use historical information previous to this year, as it is supposed that this change could substantially affect the fleet discard pattern. Different options were considered as the way of raising: by effort (number of trips, hour of trawling) or by landings (in weight or in number). Reasons for the final choices are explained for all options considered. The way to introduce discards data in the VPA procedure for assessment was through the catch at age matrix since we considered that no fish of this species survive after being discarded. Differences in yield of Short-term and Medium-term projections were observed.