Abstract
The exploitation of fishery resources acts as a driving force on cetaceans both directly, by determining their fishing mortality or injury as by-catch species, and indirectly, through the lowering the availability of their prey. This competitive overlap between fishing and cetaceans often results in inadequate solutions so that in some cases there have been cases of intentional cetacean culling to maximize fishing production. A modelling approach applied to investigate the ecological roles of cetaceans in the food web could prove more effective to integrate ecological and fishing aspects and to provide suggestions for management. The comparative analysis carried out in the Gulf of Taranto (Northern Ionian Sea, Central Mediterranean Sea) showed that fishing exploitation provides impacts on the investigated food web greater than those due to cetacean predation. Trawling was estimated to be the most negatively impacting fishing gear considering the mortality rates and consumption flows. On the other hand, the striped dolphin was the main impact on the food web due to its highest consumption flows. Analysis showed a negative and non-selective impact on the exploited species due to the fishing gears, while the odontocetes proved to select their prey species and provide a positive impact in the assemblage. In particular, while the fishing gears are primarily size selective, targeting mostly large and economically valuable fish, the odontocetes seem to follow a co-evolution process with their prey, developing a specialization in their resources, providing control of the meso-consumers and ensuring a trophic stability in the ecosystem.
Highlights
The reduction of the conflicting interactions between cetaceans and fisheries represents a target in the management strategies for the conservation of marine ecosystems as well as their regulating and provisional services (Read 2008; Boulton et al 2016)
The goal of this study is to provide new information on the ecological role of the cetaceans and to assess the cetacean-fishery interactions in the Gulf of Taranto (Northern Ionian Sea, Central Mediterranean Sea) through a food web modelling approach based on Ecopath with Ecosim (EwE) (Christensen et al 2008)
Trawlers represent about 21% in the Gulf of Taranto is characterized by a marked anthropogenic presence and intense fishing exploitation (Carlucci et al 2016), the occurrence of cetacean species, such as the striped dolphin (Stenella coeruleoalba), the common bottlenose dolphin (Tursiops truncatus), the Risso’s dolphin (Grampus griseus), the sperm whale (Physeter macrocephalus) and the fin whale (Balaenoptera physalus) makes this area eligible for the establishment of a Specially Protected Area of Mediterranean Importance (SPAMI), according to the SPA/BD Protocol (Carlucci et al 2018b, c; Maglietta et al 2018; Azzolin et al 2020)
Summary
The reduction of the conflicting interactions between cetaceans and fisheries represents a target in the management strategies for the conservation of marine ecosystems as well as their regulating and provisional services (Read 2008; Boulton et al 2016). The assessment of cetacean-fishery interactions is generally based on reports from fishermen, experimental surveys and acoustic monitoring, or a combination of the above, which must be considered a reductive approach often adopted to assess the negative economic losses caused by cetaceans (Snape et al 2018 and reference therein). Some studies have shown that the conflicting interactions between fishing gears and cetaceans determining economic losses are generally low and are often affected by specific conditions derived from both ecological species features and environmental variability (eg., Silva et al 2011).
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