Overexploitation of round sardinella may lead to the collapse of flat sardinella: What lessons can be drawn for shared stocks

Abstract

This paper describes the fisheries and presents quantitative stock assessments for two stocks of Sardinella in the waters of NW Africa. More robust stock assessments are now possible because of improved sampling in Mauritania and Senegal. Round sardinella (S. aurita), which has a wider distribution than flat sardinella (S. maderensis), is the most important species targeted by the different artisanal and industrial fleets. The industrial fleet is located beyond 13 miles from the coast, taking mainly adults. The average size in the offshore fishery, although it has decreased, remains above 26 cm. The species is also exploited by an artisanal fleet using small purse seiners. Two types of assessment model were applied: a production model and size-based model. The abundance indices were consistent, and showed a continuous decline, increasing in rate in recent years. The Just Another Bayesian Biomass Assessment (JABBA) model (applied for the first time) appeared to be appropriate for the abundance indices (Root Mean Squared Error (RMSE) 18.3–19.2 % for the two models). The results of the two models are consistent in showing increasing fishing mortality and declining biomass for both stocks over the time series. According to the JABBA model, the situation for the round sardinella stock is now critical (biomass at approximately 5 % of unexploited biomass (B0) and 10 % of biomass capable of producing maximum sustainable yields (Bmsy), with fishing mortality 2.5 times above Fmsy), and the flat sardinella is also overfished and suffering massive overfishing (B estimated at 18 %B0 and 25 %Bmsy, with fishing mortality more than 11 times above Fmsy). The results of the Length-based Bayesian Biomass (LBB) model are much more optimistic. It still concludes that both stocks are suffering overfishing (round sardinella F at ∼1.4 times Fmsy proxy, flat sardinella ∼2.2 times Fmsy proxy), but estimates that only the flat sardinella is overfished (B 28%B0 and 80 %Bmsy), while the round sardinella stock is not overfished (B 40 %B0 and 120 %Bmsy). However, these results are not consistent with the abundance index from acoustic surveys (used in the JABBA model) and overall are thought to be less robust. Signs of heavy exploitation have been observed for round sardinella for several years. This situation has been exacerbated since the arrival in late 2016 in the Mauritanian zone of efficient semi-industrial purse seiners targeting small pelagics. The various indirect assessment models used show a difficult stock situation for the round sardinella, on which many fishing communities in the area depend. Abundance indices from acoustic survey confirm the state of overexploitation of this resource. This situation has directed effort towards the flat sardinella, which is characterized by a more coastal distribution and limited migration. This species has been showing signs of overexploitation since 2019. We highlight this result for flat sardinella, which was previously not thought to be overexploited, and propose that managers should also consider how the estimated maximum sustainable catch and fishing mortality can be appropriately distributed between the different fleets in different countries, to ensure that future industry planning in the countries of the region is compatible with the sustainability of these critical stocks. This is a very complex task, as it involves not only discussions between neighboring countries, but also difficult decisions within countries about the economic and socio-economic importance of different fisheries sectors. It is essential that these reflections start immediately.