Ecosystem structure and fishing impacts in the northwestern Mediterranean Sea using a food web model within a comparative approach

Highlights

• We characterized the structure and functioning of the NW Mediterranean Sea with an ecological food web model.

• The model highlighted the dominance of the pelagic fraction and the importance of the benthic–pelagic coupling.

• Several food web indicators showed that the NWM ecosystem was heavily impacted by fishing.

• The trawl fleet had the greatest impact on the ecosystem.

• Compared Mediterranean ecosystems shared common features in structure and functioning.

Abstract

We developed an ecological model to characterize the structure and functioning of the marine continental shelf and slope area of the northwestern Mediterranean Sea, from Toulon to Cape La Nao (NWM model), in the early 2000s. The model included previously modeled areas in the NW Mediterranean (the Gulf of Lions and the Southern Catalan Sea) and expanded their ranges, covering 45,547 km², with depths from 0 to 1000 m. The study area was chosen to specifically account for the connectivity between the areas and shared fish stocks and fleets.

Input data were based on local scientific surveys and fishing statistics, published data on stomach content analyses, and the application of empirical equations to estimate consumption and production rates. The model was composed of 54 functional groups, from primary producers to top predators, and Spanish and French fishing fleets were considered. Results were analyzed using ecological indicators and compared with outputs from ecosystem models developed in the Mediterranean Sea and the Gulf of Cadiz prior to this study.

Results showed that the main trophic flows were associated with detritus, phytoplankton, zooplankton and benthic invertebrates. Several high trophic level organisms (such as dolphins, benthopelagic cephalopods, large demersal fishes from the continental shelf, and other large pelagic fishes), and the herbivorous salema fish, were identified as keystone groups within the ecosystem. Results confirmed that fishing impact was high and widespread throughout the food web. The comparative approach highlighted that, despite productivity differences, the ecosystems shared common features in structure and functioning traits such as the important role of detritus, the dominance of the pelagic fraction in terms of flows and the importance of benthic–pelagic coupling.

Introduction

Global marine catches increased from the 1950s to a maximum of 90 million tonnes at the end of the 1980s. For many years, the total reported global landings have stagnated at around 80 million tonnes per year (FAO, 2012), with another 20 million tonnes of additional illegal/unreported catch (Agnew et al., 2009). This stagnation has occurred despite increased fishing efforts and efficiency, and the geographic and bathymetric expansion of fishing activities (Pauly and Palomares, 2010, Swartz et al., 2010, Watson and Morato, 2013, Watson et al., 2013). The overall limits of sustainable exploitation have long been exceeded (Pitcher and Cheung, 2013, Worm and Branch, 2012).

The Mediterranean Sea has been strongly influenced by human activities since ancient times (Lotze et al., 2011). However, since the industrial revolution these impacts have grown exponentially due to technological improvements in fishing fleets and the demographic explosion in the area (Lotze et al., 2011). The high impact of fishing is evidenced by several stock assessments, indicating that most demersal and pelagic stocks are fully exploited or overexploited (Colloca et al., 2013, GFCM, 2013). In addition, the Mediterranean Sea has been altered by other human effects such as habitat loss and degradation, pollution, climate change, eutrophication, aquaculture and the introduction of alien species (Coll et al., 2010, Coll et al., 2012).

The current situation of exploited resources and marine ecosystems shows that management based on a single species, the dominant one in the last 50 years, has not been sufficient to ensure the proper management of exploited resources. Therefore, there is a pressing need to move towards an Ecosystem-based Approach to Fisheries (EAF) (Cochrane and de Young, 2008, Garcia et al., 2003).

The EAF framework has aroused a great interest among the scientific community and new tools have been developed in recent decades, including ecological models and ecological indicators (Link, 2011, Plagányi, 2007). These tools provide a framework for integrating ecological and ecosystem issues into management programs by assessing, for example, the impacts of interactions between species and fisheries and their implications in marine fisheries management.

In the Mediterranean Sea, several ecological models using the Ecopath with Ecosim approach (EwE) (Christensen and Walters, 2004) have been developed (Coll and Libralato, 2012). These models have been used for a diversity of applications such as analyzing fishing impacts (Coll et al., 2006a, Coll et al., 2007), comparing ecosystem structure and functioning traits (Hattab et al., 2013, Tsagarakis et al., 2010), exploring management options (Fouzai et al., 2012), assessing the impact of aquaculture (Forestal et al., 2012), evaluating the effects of the environment (Coll et al., 2008, Piroddi et al., 2010) and exploring the effects of invasive species in the food web (Daskalov, 2002).

The NW Mediterranean Sea is exploited by French and Spanish fleets and is one of the most impacted regions in the basin (Coll et al., 2012, Micheli et al., 2013). Previous ecosystem models developed in the study area indicated intense exploitation of the marine ecosystem (Banaru et al., 2013, Coll et al., 2006a) and changes in the trophic structure of the ecosystem (Coll et al., 2008).

In this study, we developed a food web model in a large area of the NW Mediterranean Sea including the Gulf of Lions and part of the Balearic Sea. Previously, various smaller models were developed in this area (Banaru et al., 2013, Coll et al., 2006a), but the new model includes a larger area considering important hydrodynamic events that enhance the connectivity between the two regions. Also included were several submarine canyons of the area, which are mainly located in the Gulf of Lions and the northern part of the Balearic Sea. These canyons play an important ecological role and are associated with important fishing activities (Würtz, 2012). From a fisheries point of view, there are also some ‘shared stocks’ in the area, which motivated the development of the larger model. Shared stocks include the important commercial populations of hake (Merluccius merluccius) and anchovy (Engraulis encrasicolus).

With this new modeling effort we aimed to develop a useful modeling tool to contribute to the fisheries management in the region through an ecosystem approach. In this study, we present the new ecological model and use it to update previous knowledge of the area by:

a) Characterizing the structure and functioning of the ecosystem during the early 2000s;

b) Assessing the impact of fishing activity on the ecosystem using ecological indicators; and

c) Comparing the results with other food web models developed in several areas of the Mediterranean Sea and adjacent waters of the Gulf of Cadiz.