Maximum sustainable yield from interacting fish stocks in an uncertain world: two policy choices and underlying trade-offs

ICES Journal of Marine Science (2016), 73(10), 2499–2508. doi:10.1093/icesjms/fsw113 Original Article Maximum sustainable yield from interacting fish stocks in an uncertain world: two policy choices and underlying trade-offs Adrian Farcas1 and Axel G. Rossberg1,2,* Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft NR33 0HT, UK and Queen Mary University of London, School of Biological and Chemical Sciences, 327 Mile End Rd, London E1 4NS, UK 2 *Corresponding author: tel: +44 (0)20 7882 5688; e-mail: Farcas, A. and Rossberg, A. G., Maximum sustainable yield from interacting fish stocks in an uncertain world: two policy choices and underlying trade-offs. – ICES Journal of Marine Science, 73: 2499–2508. Received 26 December 2015; revised 26 May 2016; accepted 1 June 2016; advance access publication 28 July 2016. The case of fisheries management illustrates how the inherent structural instability of ecosystems can have deep-running policy implications. We contrast 10 types of management plans to achieve maximum sustainable yield (MSY) from multiple stocks and compare their effectiveness based on a management strategy evaluation (MSE) that uses complex food webs in its operating model. Plans targeting specific stock sizes (BMSY ) consistently led to higher yields than plans targeting specific fishing pressures (FMSY ). A new self-optimizing control rule, introduced here for its robustness to structural instability, led to intermediate yields. Most plans outperformed single-species management plans with pressure targets that were set without considering multispecies interactions. However, more refined plans to “maximize the yield from each stock separately”, in the sense of a Nash equilibrium, produced total yields comparable to plans aiming to maximize total harvested biomass, and were more robust to structural instability. Our analyses highlight trade-offs between yields, amenability to negotiations, pressures on biodiversity and continuity with current approaches in the European context. Based on these results, we recommend directions for developments of EU fisheries policy. Keywords: Common Fisheries Policy, food webs, harvest control rule, management strategy evaluation, maximum sustainable yield, structural instability, theoretical ecology. Introduction Four questions about multispecies maximum sustainable yield Fisheries management aiming to attain maximum sustainable yield (MSY) from multiple interacting stocks is considerably more complicated than single-stock management (Pope, 1976; Bulgakova and Kizner, 1986; Collie et al., 2003; Walters et al., 2005; Matsuda and Abrams, 2006; Gecek and Legovic, 2012; Houle et al., 2013; Voss et al., 2014; Thorpe et al., 2016). A priori it is not even clear what the best translation of the MSY objective for a single, isolated stock is to cases with multispecies interactions, i.e. with feeding and competitive interactions among species (for simplicity, we use “stock” interchangeable with “fish species” in this paper). Even when a management objective considering multispecies interactions is defined, attaining it can be difficult because these interactions are generally not well known. It is therefore not surprising if legislation aiming at MSY acknowledges the role of multispecies interactions in principle, but tends to play it down. Examples are §§301.a.3, 303.b.12 of the Magnuson-Stevens Act (U.S. Department of Commerce, 2007) or Article 9.3.b of the Common Fisheries Policy (EU, 2013), but see, Section 13(2) of New Zealand’s Fisheries Act (Parliamentary Council Office, 2014). In the current practice of fisheries management, multispecies interactions among managed stocks play only a minor role. Noteworthy exceptions are cases where stock assessments take account of the dependence of predation mortality on the abundances of other species (Gjøsæter et al., 2015; ICES, 2014b). C International Council for the Exploration of the Sea 2016. V This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. 1 2500 To contribute to the development of management practices mindful of multispecies interactions we ask here: Q1: How can the singe-species MSY objective be translated into the multispecies case? Q2: Which strategies are suited to achieve such objectives? Q3: How do these options compare with regards to the yields they achieve, the degree of collaboration among players required to reach the objectives, pressures on biodiversity, and their political acceptability? Q4: How much can be gained from multispecies management in comparison with management disregarding ecological interactions? First policy choice: the management objective There is no unique way of translating the single-species MSY objective to the multispecies case. Maximization of yield from one stock will generally require different strategies than maximization of yield from another. In a simple predator-prey system, for example, the maximization of yield from the prey requires culling the predator, while by not exploiting the prey yield from the predator can be maximized (May et al., 1979; Clark, 1990). The three types of high-level objective we consider here are: (i) Nash Pressure: To fish all exploited stocks at such rates that changes in the exploitation rate of any single stock cannot increase the long-term yield from that stock. (ii) Nash State: To keep all exploited stocks at such sizes (e.g. in terms of spawning stock biomass) that changes in the size of any single stock cannot increase the long-term yield from that stock. (iii) Total Yield: Maximization of the summed long-term yield from all exploited stocks. Formal mathematical definitions of these objectives are given in Supplementary Material S1. In general, they are equivalent only in absence of multispecies interactions. The objectives Nash Pressure and Nash State are two ways of implementing the idea of “maximization of yield from each stock separately”. They correspond to the Nash equilibrium outcome in game theory (Osborne and Rubinstein, 1994), where no player of a game could improve its gains by changing its moves. Nash equilibria are traditionally understood as arising naturally when players are not collaborating. For the Nash Pressure objective the players are hypothetical fleets, each targeting one specific stock, and the permitted moves changes in their exploitation rates. For the Nash State objective the players are hypothetical managers of individual stocks and their moves are the stock sizes they aim at. The Total Yield objective corresponds a situation where players chose their moves such that the total gain by all players is maximized. Attaining this objective generally requires collaboration or enforcement through governing institutions. Vari (...truncated)