Effects of changes in stock productivity and mixing on sustainable fishing and economic viability

ICES Journal of Marine Science (2017), 74(2), 535–551. doi:10.1093/icesjms/fsw083 Contribution to the Symposium: ‘Targets and Limits for Long Term Fisheries Management’ Original Article Effects of changes in stock productivity and mixing on sustainable fishing and economic viability François Bastardie1,*, J. Rasmus Nielsen1, Margit Eero1, Federico Fuga2, and Anna Rindorf1 1 National Institute of Aquatic Resources (DTU-Aqua), Technical University of Denmark, Denmark Studiofuga, Verona, Italy 2 *Corresponding author: tel: þ45 35883398; e-mail: Bastardie, F., Nielsen, J. R., Eero, M., Fuga, F., and Rindorf, A. Effects of changes in stock productivity and mixing on sustainable fishing and economic viability. – ICES Journal of Marine Science, 74: 535–551. Received 15 December 2015; revised 25 April 2016; accepted 25 April 2016; advance access publication 1 July 2016. Within the new FMSY European paradigm, this paper shows how a combination of changes in fish stock mixing, non-stationarity in productivity, and constraints on unit stock concepts undermine the effective management of fisheries, especially when management reference points are not adjusted accordingly. Recent changes in stock structures, conditions and stock mixing between eastern and western Baltic cod can jeopardize the reliability of stock assessments and of the fishery economy. We modelled how different management, individual vessel decision-making, and stock growth and mixing scenarios have induced alternative individual vessel spatial effort allocation and economic performance by affecting fishing costs and by changing the relative stock abundance and size distribution. Stock mixing heavily influences profit and stock abundance for stocks that have experienced increased fishing mortality (F) levels. Western cod F has increased from a higher total allowed catches (TAC) advised in the medium-term due to the westward migration of eastern cod while eastern cod F has increased from reduced growth in the east. Greater pressures on western cod and decreased eastern cod growth and conditions greatly reduce the overall cod spawning stock biomass, thus changing the landing size composition and associated fishery profits. As a cumulative effect, fishing efforts are redirected towards western areas depending on management (quotas). However, total profits are less affected when traditional fishing opportunities and switching possibilities for other species and areas are maintained. Our evaluation indicates that current management mechanisms cannot correct for potential detrimental effects on cod fisheries when effort re-allocation changes landing origins. By investigating different economic starting conditions we further show that Baltic cod mis-management could have resulted in unintended unequal (skewed) impacts and serious consequences for certain fleets and fishing communities compared with others. Our management strategy evaluation is instrumental in capturing non-linear effects of different recommendations on sustainability and economic viability, and we show that fixed F-values management is likely not an attainable or sufficient goal in ensuring the sustainability and viability of fisheries and stocks given changing biological conditions. Keywords: agent-based modelling, Baltic cod, bio-economic fisheries model, decision making, long-term management plans, MSY Approach, spatial effort allocation, stock production and mixing. Introduction The new European Common Fishery Policy (EU, 2013) implements a Maximum Sustainable Yield (MSY) as the key fisheries management objective and states that all stocks must be exploited in accordance with this principle by no later than 2015. The relevance of using an MSY approach to manage Baltic cod (BCOD) remains questionable given that BCOD belongs to two genetically and morphometrically distinct stocks (Hüssy et al., 2016) being different in productivity levels. The eastern BCOD stock (subdivisions 25–32; Figure 1) has historically been fished around the fishing mortality leading to MSY (FMSY) from 2008 to 2013 and quantities of this stock have substantially increased since 2007 following several decades of overexploitation. This increase was largely due to improved recruitment production levels emerging C International Council for the Exploration of the Sea 2016. All rights reserved. V For Permissions, please email: 536 F. Bastardie et al. Figure 1. The Central Baltic Sea region with ICES area codings and bathymetry (in blue levels). The DISPLACE model discrete positions (underlying graph nodes; 4 by 4 km grid in IIIa, SD 22–25; 100 by 100 km otherwise) are shown together with the spatial origin of the 2012 cod landings in kg (coloured circles; for the selected vessels only) used to deduce area- and vessel-specific catch rates when combined with the deployed individual effort (Bastardie et al. 2014; see also Appendix A, catch rate). in recent years as well as due to improved compliance with the Total Allowed Catches (TACs) principle (Eero et al., 2012). Western BCOD (subdivisions 22–24; Figure 1) is by contrast estimated to be fished above FMSY, though it is also managed with long-term targets in mind (ICES, 2014). Hence, it appears that MSY principles are already in place in management systems to the benefit of eastern and western cod stocks, the broader ecosystem, and fisheries. Unfortunately, since 2014, no analytical assessment of Eastern cod has been available, and a range of adverse developments (e.g. low nutritional status levels and the disappearance of larger individuals) indicate that the stock is in distress (Eero et al., 2015). Somatic growth has presumably declined in recent years, and there seem to be signs of increased natural mortality levels, likely as a result of various different biological interactions (e.g. predation and ecosystem conditions) (Eero et al., 2015). In addition, it has not been possible to evaluate the exploitation level relative to FMSY, thus leaving the present stock status unclear (ICES, 2014). As FMSY targets assume a constant biological regime of stocks (see Larkin, 1977; Holt 2006; Longhurst, 2010, for a discussion), there appears to be a conflict between current FMSY implementation and changing underlying biological conditions of Eastern BCOD. Applying the European MSY approach to BCOD fisheries is questionable given uncertain long-term reference points that are further complicated by the spatial overlap between populations and mixed stock compositions that have changed over time. This increased abundance of Eastern BCOD in the late 2000s presented a new challenge to the management of western cod, as the two populations mix within the western management area (SD 24, Eero et al., 2014) and a separate TAC is associated with each management area. Recently, Hüssy et al. (2016) found an increased proportion of eastern cod in the western Baltic management area relative to previous levels, indicating that mixing is likely to present a gro (...truncated)