Trophic models: What do we learn about Celtic Sea and Bay of Biscay ecosystems?
Introduction
Fishing activities do not only decrease the abundance of targeted species, they also affect their competitors, prey and predators (Olsen et al., 2004, Worm et al., 2006). The community structure and functioning could therefore be affected through trophic relationships (Daskalov et al., 2007, Tremblay-Boyer et al., 2011). Moreover, in a context of global change, direct and indirect impacts of fishing activities are cumulative and act in synergy to increase resource instability (Cheung et al., 2009, Pereira et al., 2010). In this context, implementing an Ecosystem Approach to Fisheries Management (EAFM) is an urgent necessity (FAO, 2003). EAFM aims to apply the principles of sustainable development to the fisheries sector and to move beyond the traditional single-species approaches in fisheries management (Collie and Gislason, 2001, Gascuel et al., 2011a, Gascuel et al., 2011b, Walters et al., 2005) by accounting for interspecific interactions, habitat quality and global change in fisheries management. The ultimate goal is to maximise human welfare from an economic, social and environmental perspective (Garcia, 2003, Garcia and Cochrane, 2005). The necessity of this approach is now recognized as self-evident to all fishery scientists and stakeholders, but the main challenge lies in its implementation.
From this point of view, ecosystem modelling is an important tool for studying, evaluating and predicting the potential effects of fisheries on the exploited ecosystems and for exploring further the structure and trophic functioning of marine ecosystems (Plagányi, 2007). Among existing ecosystem models, trophic models like Ecopath with Ecosim (Christensen and Pauly, 1992, Polovina, 1984, Walters et al., 1997) have become widely used tools for EAFM (Christensen and Walters, 2004, Pauly et al., 2000). Ecopath provides a snapshot representation of the resources in the ecosystem and their interactions in a specific period. It is used to analyse interspecific relations and direct and indirect impacts of fishing activities on the whole food web (Christensen and Pauly, 1992). Ecosim is the time-dynamic version of Ecopath and uses mass-balance results from Ecopath for parameter estimation. The model is fitted to observed data and used to evaluate the relative effects of fishing, trophic relationships and environmental disturbances on observed dynamics (Christensen et al., 2005, Christensen and Walters, 2004). EcoTroph (Gascuel, 2005, Gascuel and Pauly, 2009) is a recent approach increasingly used for modelling aquatic ecosystems (e.g. Colléter et al., 2012, Gasche and Gascuel, 2013, Prato et al., 2016, Valls et al., 2012). It is based on a simple representation of ecosystems structure using trophic spectra that summarizes the trophic functioning as a continuous flow of biomass surging up the food web, from low to high trophic levels due to predation and ontogeny processes.
In European Union waters, as a result of the overexploitation of some major stocks, yields began to decrease everywhere since the mid-1970s (Gascuel et al., 2016). Intensification of fishing effort and extension of exploitation to a wider part of the ecosystems were insufficient to compensate the decrease in abundance of exploited fish stocks and landings halved over the past 40 years (Ibid.). In the late 1990s, the overall fishing pressure reached its highest values and most stocks showed an alarming state of depletion. In addition, ecosystem indicators suggest a degradation of the health status in most of the European seas. Over the past decade, the fishing pressure decreased significantly, mainly due to more restrictive catch quotas, but no clear recovery in total biomass and ecosystem indicators is yet apparent (Gascuel et al., 2016).
The Bay of Biscay and the Celtic Sea, which are among the main European fishing zones in terms of landings, illustrate the situation described above. These ecosystems experienced a constant increase of fishing effort and an unprecedented increase in fishing capacity until the 1990s, reaching an excessive level in fishing power (Mesnil, 2008). As a consequence, several studies have shown a significant reduction of biomass with increasing impacts on all compartments of the ecosystem (e.g. Gascuel et al., 2012, Gascuel et al., 2016, Guénette and Gascuel, 2012, Pinnegar et al., 2002, Rochet et al., 2005).
In previous studies dealing with trophic modelling (Guénette and Gascuel, 2009, Bentorcha, 2014, Bentorcha et al., 2017), the Bay of Biscay and the Celtic Sea were considered as a single ecosystem. However, they are considered distinct ecosystems not only in the European Marine Strategy Framework Directive (European Commission, 2008), but also for the implementation of EAFM in European seas (STECF, 2012). Therefore, in this study, the two ecosystems were modelled separately, as case studies to improve ecosystem-based fisheries management in Europe. We show how these models can be used to (i) understand the trophic functioning of ecosystems and associated fishing-induced changes in the food web, (ii) build global diagnoses of the ecosystem impact of fishing and (iii) explore scenarios of ecosystem-based fisheries management. We compared the Bay of Biscay and the Celtic Sea in terms of changes in structure and trophic functioning. Ecopath models were built for 1980 and 2013 in a scientific advice-oriented approach and using all outputs of the International Council for the Exploration of the Sea (ICES) stock assessment and survey data as input. Then, indicators of fishing impacts on the food web were estimated using EcoTroph. In both ecosystems, Ecosim models were fitted to time series of biomass, abundance indices, catch and fishing mortalities over the 1980–2013 period. Several fisheries management scenarios were used with the best fitted model, namely the Bay of Biscay model, examining the impact on ecosystem biomass and trophic indicators of the good environmental status of ecosystems as defined by the Marine Strategy Framework Directive (MSFD; European Commission, 2008).
Section snippets
Study sites
The two trophic models represent the continental shelves, from the coast to the 200 m isobaths, of the Celtic Sea and the Bay of Biscay ecosystems (Divisions VIIe-j and VIIIab, respectively, according to the classification of ICES) (Fig. 1). These two areas are characterized by distinct oceanographic and ecological features and by specific fish assemblages (Borja et al., 1998, Koutsikopoulos and Le Cann, 1996, Pingree et al., 1981, Planque et al., 2004, Varela, 1996). The total area is 232,360 km2
Current state of ecosystems
Based on the 2013 Ecopath models, the total biomass of the Bay of Biscay ecosystem is 229.6 t·km− 2 including trophic level 1 and 85.6 t·km− 2 for animals only (TLs ≥ 2) (Table S2 in the Supplementary material). The Celtic Sea ecosystem displays a higher biomass of 461.6 t·km− 2 for all trophic levels, and 118.4 t·km− 2 for animals only (Table S2). The accessible biomass (B*), the fraction of ecosystem biomass accessible to fisheries, is 7.3 and 8.3 t·km− 2 for the Bay of Biscay and the Celtic Sea
Models limitations and perspectives
Ecosystem models, especially trophic models, have a key role to play in improving scientific knowledge on exploited marine ecosystems, and in the development of management scenarios for an ecosystem approach to fisheries (Coll et al., 2015). Despite the many uncertainties inherent to their uses, EwE and EcoTroph models provide an opportunity to represent marine ecosystems in a simplified form, to test the consistency of the available data and to explore the long-term development of complex
Conclusion
In the present study, we showed how trophic models, especially Ecopath with Ecosim as well as EcoTroph, are key tools to implement EAFM. In particular, they make it possible to establish diagnoses of the exploitation status at the scale of the entire food web. They provide insights into how trends in biomass and catch depend on the fishing impacts, environmental changes and predator-prey interactions. Moreover, Ecosim simulations enable the assessment of the impact of every fleet on the food
Acknowledgements
We thank the editors and anonymous referees for their constructive and useful comments.
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