Best practice II: effect of discard survival on North Sea sole and plaice

This report investigates the effects of discard survival on the current stock assessment and perception of the North Sea sole and plaice stocks. By recalculating the discard fraction of the catches and rerunning the assessment model, the stock assessment of sole and plaice is corrected for discard survivability. Secondly, all discard survival corrected assessments of both stocks are forecasted over 50 years under a landing obligation and discarding (business as usual) scenario. This simulation shows the effect of discard survival under a landing obligation and under the discarding scenario. The trend and perception of both stocks do not change when discard survivability is taken into account. But the fishing mortality, stock biomass, and recruitment are overestimated. The effect of taking into account discard survivability is a scaling depending on the characteristics of the stock (such as maturity at age) and the extent to which the part of the stock is being discarded. The effect of discard survival is greater in North Sea plaice than in North Sea sole, since the plaice is discarded more. The Fmsy reference points increase with increasing discard survivability. However, the “F-targets”, the F corresponding to the maximal yield under the landing obligation, that are calculated to simulate the “landing obligation-scenario” do not show the same trend with increasing discard survivability. The forecast simulation of North Sea sole and plaice was performed by projecting the stocks with targets for fishing mortality that maximise the yield of both stocks. This method gives insight in the effects of the discarding and landing obligation scenario on the catches, recruitment, spawning stock biomass, and fishing mortality. Differences between scenarios increase with increasing discard survivability, although differences are marginal in the simulation of sole (compared to the differences between scenarios in plaice). Mainly the catches are effected by discard survivability under the landing obligation scenario