Can the common fisheries policy achieve good environmental status in exploited ecosystems: The west of Scotland demersal fisheries example
Introduction
The exploitation of fish stocks in European waters is regulated by the Common Fisheries Policy (CFP). Since its creation in the 1970s this long-standing policy has been through several reforms, the latest of which took effect on January 1st 2014 (EC, 2013). The latter reform proposed a new framework to manage European fisheries, and amongst several new initiatives, it highlighted a need to move from traditional singe-stock management towards an ecosystem approach to fisheries (EAF) (Prellezo and Curtin, 2015). EAF originated from the principle of sustainable development and aims to achieve both human and ecosystem well-being (Garcia et al., 2003). The implementation of EAF can vary between an Ecosystem Approach to Fisheries Management (EAFM) in which ecosystem aspects are given consideration when taking management decisions, to Ecosystem-Based Fisheries Management (EBFM) in which ecosystem health becomes a management goal included in trade-offs when pursuing competing management objectives (Patrick and Link, 2015). Most importantly, EBFM prioritises the wellbeing of ecosystems over economic and social objectives since ecosystem wellbeing is considered a prerequisite for the last two objectives (Murawski et al., 2008).
While the new CFP advocates for the implementation of some form of EAF, it remains largely unclear how to include conservation objectives within management measures in practice (Prellezo and Curtin, 2015). The CFP currently aims to fish at levels consistent with achieving Maximum Sustainable Yield (MSY) for all exploited stocks (EC, 2011). In northern European waters, these fishing levels are proposed by the International Council for the Exploration of the Sea (ICES) which delivers annual scientific advice for the management of northern European fish stocks. This advice provides biological reference points for each stock, including the level of fishing mortality (F) needed to achieve MSY (FMSY). FMSY is defined by a single-stock approach, meaning that it is calculated individually for a stock based on its own status only, regardless of the status of other stocks. However, this contradicts EBFM (Prellezo and Curtin, 2015), where the interactions between species should be taken into account when defining safe harvest levels for fish stocks. In fact, while FMSY has long been considered a desirable exploitation level for single stocks (Schaefer, 1954), its performance in mixed fisheries, where several stocks are caught simultaneously by the same fleet, has been challenged (Walters et al., 2005), largely due to the fact that it is virtually impossible to apply FMSY simultaneously to all stocks in mixed fisheries (Kumar et al., 2017; Larkin, 1977). Nevertheless, despite this criticism recent empirical studies have shown that the current MSY approach has succeeded in leading European fish stocks towards recovery (Cardinale et al., 2013; Fernandes and Cook, 2013). This suggests that the traditional single stock FMSY values for European stocks may not be too far off the harvest levels needed to achieve sustainable mixed fisheries, potentially facilitating the transition towards EBFM. For example, Froese et al. (2008) have shown that EBFM can be achieved by improving existing single-stock management.
In addition to the traditional advice and corresponding single stock FMSY values, ICES now also provides FMSY ranges for most stocks in European waters, which consist of upper (FMSY upper) and lower (FMSY lower) F boundaries around FMSY within which fishing mortality is deemed sustainable (ICES, 2016a, 2015). These ranges are a recent addition to the ICES advice and were requested by the European Commission in order to develop long-term management plans with quantifiable targets. FMSY ranges should be precautionary and also ensure that they deliver no more than a 5% reduction in long-term yield. Whilst they do not originate from a proper multispecies approach such as the one used by the mixed fisheries advice (ICES, 2017), the FMSY ranges do provide some leeway around the single stock FMSY values which are usually difficult to apply simultaneously to all stocks. In mixed fisheries, the Total Allowable Catch (TAC) derived from FMSY for the least abundant stock is most likely to be reached before the TACs of more abundant stocks are exhausted. Such a situation typically leads to over-quota discarding, a practice no longer allowed as the landings obligation is phased in for European fisheries (EC, 2015a). As a result, it has been proposed that in mixed fisheries the most vulnerable stock with the lowest FMSY should determine the limit of exploitation for all other stocks caught in the same fishery (EC, 2011). However, such an approach is likely to result in a ‘choke species’ scenario leading to the under-exploitation of other stocks and ultimately jeopardising the fishery (Baudron and Fernandes, 2015).
Another regulation in European waters is the Marine Strategy Framework Directive adopted in 2008 (EC, 2008) which states that all member states should reach Good Environmental Status (GES) by 2020 (EC, 2009). Although achieving GES differs from achieving EBFM, GES measures the performance towards most of the biological and environmental attributes of EBFM (Ramírez-Monsalve et al., 2016). GES is defined by 11 descriptors. Descriptors 1 (biodiversity), 3 (commercial species), and 4 (food webs) directly relate to fisheries and are, therefore, particularly relevant for EBFM. In order to integrate these GES descriptors into an EBFM framework, indicators are needed to inform whether GES criteria are met for each descriptor. Developing meaningful ecosystem indicators can be challenging due to a lack of relevant data. However, ecosystem indicators for descriptors 1, 3 and 4 can be derived from biomass and/or catch data which are available for most species in ecosystems found in northern EU waters (Coll et al., 2016; Gascuel et al., 2016; Kleisner et al., 2015; Reed et al., 2017). In addition, the information a single ecosystem indicator can provide is limited: it is therefore preferable to use a portfolio of indicators to fully assess each descriptor (Samhouri et al., 2009). Lastly, GES indicators also need to be informative. Ideally, what constitutes GES should be defined for each indicator in order to assess whether an ecosystem has reached GES or not based on indicator values. For example, Link (2005) proposed reference points for some ecosystem indicators, in which case the examination of indicators’ trends relative to the reference point values can then be used as a basis for management recommendations (Jennings and Rice, 2011). However, not all ecosystem indicators have clearly defined reference points, and these reference points are not transferable across ecosystems with different characteristics (Heymans et al., 2014).
EBFM can benefit from ecosystem modelling in order to explore policy options where management objectives (e.g. diversity, abundance of non-target species, etc.) involve multiple species and their trophic interactions which cannot be assessed with single-species models (Christensen and Walters, 2005). Plagányi (2007) reviewed available ecosystem models spanning a wide range of complexity levels from minimum realistic models to whole ecosystem ones. This latter category includes Ecopath with Ecosim (EwE), a food web ecosystem model (Christensen and Walters, 2004a). EwE is the most applied tool for modelling marine ecosystems (Colléter et al., 2015) and can be used to investigate marine policy issues such as GES (Piroddi et al., 2015). However, it is crucial to demonstrate that a model can replicate historical trends in ecosystems in order to make plausible predictions in response to novel situations before any management decision can be based upon it (Christensen and Walters, 2005). Of the vast number of EwE models that have been published, only a few have been calibrated using historical time series of data and even fewer have been employed for management purposes (Heymans et al., 2016). One EwE model fulfilling these two criteria was recently published for the west of Scotland ecosystem (Alexander et al., 2015; Serpetti et al., 2017).
The west of Scotland ecosystem (WoS) located in ICES Division VIa is home to numerous valuable species of finfish and shellfish that support four fisheries: an inshore crustacean fishery targeting the valuable Norway lobster (Nephrops norvegicus); a mixed demersal fishery targeting cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) on the continental shelf; a fishery for monkfish (Lophius piscatorius and Lophius budegassa), hake (Merluccius merluccius) and saithe (Pollachius virens) in the deeper waters of the shelf edge; and a pelagic fishery targeting mainly mackerel (Scomber scombrus) and herring (Clupea harengus) (ICES, 2016b, 2016c, 2016d, 2016e, 2016f, 2016g). In 2014, these fisheries contributed to 35% of the total value of all commercial species caught in Scotland, totalling £182.5 million (The Scottish Government, 2015) and are, therefore, important for the Scottish fishing industry. However the WoS fisheries are currently facing several management issues. First, the stocks of cod and whiting are depleted and their Total Allowable Catches (TACs) have been set to zero since 2012 and 2006 respectively (ICES, 2016c). Secondly, the extensive bycatch of juvenile gadoids by the crustacean fishery is thought to jeopardise gadoid stocks, whiting in particular (ICES, 2016c). Thirdly, the population of grey seals (Halichoerus grypus), a top predator in the WoS, has been increasing steadily over the last two decades (SCOS, 2015). While Alexander et al. (2015) suggest that excessive exploitation rates rather than an increase in predators were to blame for the collapse of cod and whiting, increased predation from seals seems to have offset the reduction of fishing pressure on VIa cod (Cook et al., 2015) and is likely to hamper the recovery from low stock sizes (Cook and Trijoulet, 2016). The complexity of the WoS food web and the mixed fisheries it supports, coupled with management challenges and the availability of an ecosystem model, makes the WoS an ideal case study to assess the performance of EBFM in achieving specific management goals such as GES.
Here, we reviewed and updated the EwE model for WoS with the latest data available and repeated the calibration procedure to extend the hindcasting period from 1985 to 2013. We used this model to explore the FMSY ranges of the demersal stocks by performing forward simulations of every possible combination of fishing mortalities within these ranges. Additional exploitation scenarios were performed to investigate the impact of juvenile whiting bycatch by the crustacean fishery and grey seals predation. For each scenario, ecosystem indicators related to GES descriptors 1, 3 and 4 were calculated. Outputs from the models were analysed to assess whether the single stock FMSY and/or FMSY ranges implemented by the CFP could achieve GES in WoS the demersal fishery. Management measures required to recover the cod and whiting stocks were also identified.
Section snippets
The model
The model was built using EwE software version 6.5 released in July 2016 (www.ecopath.org). EwE consists of two components: (i) Ecopath, a mass-balance model accounting for energy transfers in the ecosystem which depicts a ‘snapshot’ of the ecosystem in a given year; and (ii) Ecosim, the dynamic component which allows for temporal simulations based on Ecopath. Ecosim is based on the foraging arena theory (Ahrens et al., 2012), and each prey-predator interaction is defined by a vulnerability
Hindcast
Once the updated Ecopath model was successfully balanced, PREBAL (Link, 2010) diagnostics were carried out and confirmed that: the biomass slope on a log scale declines by ca. 5–10% with increasing trophic levels; predator/biomass ratios are <1; and vital rates decline with increasing trophic levels (Appendix B). These diagnostics suggest that the Ecopath model is ecologically sound (Link, 2010). The structure of the updated Ecopath food web is depicted in Fig. 3, and the final balanced model
Discussion
The results from the model simulations suggest that the single stock FMSY values currently advised by ICES, if applied to all stocks in WoS, would likely recover cod whilst achieving catches on par with historical levels for most species. This management scenario would also lead to an increase in whiting SSB, but would fail to recover this stock to within safe biological limits, suggesting that the current FMSY value for whiting in ICES area VIa is incompatible with this stock’s recovery. In
Conclusion
Using a food web ecosystem model to simulate management scenarios accounted for prey-predator interactions whilst investigating biodiversity and food web indicators related to GES descriptors. Our results suggest that the single stock FMSY values currently advised by ICES would recover the VIa cod stock, providing that FMSY is applied to all stocks in VIa, but would fail to recover the VIa whiting stock. The exploration of alternative management scenarios led to the identification of the
Acknowledgements
Alan R. Baudron, Niall G. Fallon and Paul G. Fernandes were funded by the Horizon 2020 European research project MareFrame (grant No. 613571). Natalia Serpetti and Johanna J. Heymans were funded by the Natural Environment Research Council and Department for Environment, Food and Rural Affairs under the Marine Ecosystems Research Programme (MERP) (grant No. NE/L003279/1). We thank two anonymous reviewers for their insightful comments.
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