Is the Northern European hake, Merluccius merluccius, management procedure robust to the exclusion of reproductive dynamics?

Abstract

The purpose of the present study was to investigate the robustness of the management procedure (MP) of Northern European hake to alternative indices of reproductive potential (RP) based on our recently improved understanding of hake reproductive biology using a Management Strategy Evaluation (MSE) framework. MSE simulations allow the testing of the robustness of conventional management systems to uncertainties or different hypothesis about underlying population dynamics. For this purpose, four different reproductive potential (RP) indices of increasing biological complexity (i.e. Spawning Stock Biomass (SSB) using constant maturity; SSB MAT adding variation on maturity, Female Spawning Biomass—FSB, and Total Egg Production—TEP) were estimated and tested. The inclusion of more biological information affected the perception of the population dynamics, the biological reference points (BRPs) as well as the perception of the stock in relation to those biological reference points. In this study, the probability of a wrong perception, i.e. the “perceived” population is above reference limits while the “true” population is overfished, was 13, 5, 3 and 3% for the different RP investigated in the time period between 1978 and 2008. This probability was around 0% in all cases studied for the projected period of 2009–2040. Our results show that (i) when including more information about reproductive biology in the simulation, the number of years below the BRPs are higher in the “perceived” population than in the “true” population, (ii) for the historic period the probability of a wrong perception is diminished when alternative reproductive indices are included when simulating both, ‘true’ and ‘perceived’ populations, and (iii) during the initial years of the projected period, although the perception of the population dynamics in relation to BRP is different between SSB WG scenario and alternative RP indices, the outcomes of the management advice of the SSB WG are more restrictive because the wrong perception is conservative, i.e. when “true” population is above B pa the SSB WG is below. Thus, it can be concluded that the MP for European hake is robust to the different hypothesis about alternative RP indices. However, the results of the present study also showed that for the historic period, when the population level was close to BRP level, the perception of the stocks status differs between alternative RP indices. Thus, it would be convenient to include the reproductive biology of the species in the Harvest Control Rules (HCR) and Long Term Management Plan (LTMP) of this stock, especially when the level of the stock is close to BRP level.