Assessment of cetacean–fishery interactions in the marine food web of the Gulf of Taranto (Northern Ionian Sea, Central Mediterranean Sea)

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.

Addressing Cetacean–Fishery Interactions to Inform a Deep-Sea Ecosystem-Based Management in the Gulf of Taranto (Northern Ionian Sea, Central Mediterranean Sea)

“…In particular, the common bottlenose dolphin Tursiops truncatus usually occurs within the continental shelf [46][47][48], while the striped dolphin (Stenella coeruleoalba), the short-beaked common dolphin (Delphinus delphis), Risso's dolphin (Grampus griseus), the Cuvier's beaked whale (Ziphius cavirostris), the sperm whale (Physeter macrocephalus) and the fin whale (Balaenoptera physalus) are mainly distributed in offshore waters on the continental slope [46][47][48][49][50][51][52][53][54][55][56][57]. Most of these species act as top-down controls on the entire food web [35], activating trophic cascades up to the middle trophic levels [36]. Moreover, this area has been suggested as a critical habitat for the common bottlenose, the striped and Risso's dolphins [46,47,49].…”

“…The Gulf of Taranto covers a wide portion of the Northern Ionian Sea (Central Mediterranean Sea) and for over 10,000 km 2 , it is characterized by depths ranging from 200 to over 1000 m. In the last decade, numerous studies have described the gulf as a hot spot of cetacean biodiversity distributed from the coastal shelf to deep waters [34]. In particular, the odontocetes have been described as top-down controllers of the entire food web of the gulf and promoters of trophic cascades up to the middle trophic levels [35,36]. Recently, several pressures affecting cetaceans in the Gulf of Taranto have been assessed and fishery activity has been found to be one of the major sources of indirect impact due to prey depletion and habitat degradation [5].…”

“…This study provides a focus on the feeding activity occurrence and the trophic interactions between odontocetes and the fishery in the marine food web of the Gulf of Taranto, by zooming in on cetaceans' prey of commercial interest. In particular, the quantification of trophic impacts is estimated using an already implemented food web mass-balance model for the Gulf of Taranto [35] that integrates information on the bathymetric displacement of both cetaceans' prey and fishing activity. Moreover, consumption fluxes and trophic impacts exerted by cetaceans and fisheries were analyzed by aggregating the species (or species groups) described in the trophic model into ecological domains (or compartments), in order to explicitly consider spatial management needs.…”

Small Scale Fisheries, Dolphins and Societal Challenges: A Case Study in the City of Volos, Greece

“…Currently, dolphins are regarded in many countries as conservation icons [13][14][15], and any interactions with humans is subject to animal welfare concerns [16]. Certain species and populations are likely to mitigate climate related changes [17], including those with a specific site-fidelity and home range [18].…”

“…Thus, the likely interaction between fisheries and cetaceans may rely on food availability with respect to environmental features. Various research methods employed in the field, such as interviews [20] and food-web modeling [15], reveal that cetacean-human interactions generally intensify in times of overexploitation of resources from the part of the fisheries, as in the case of overfishing [21][22][23]. Such food-resource niche overlap suggests a rather over the top competition, with severe potential ecological and economic losses that can arise from marine mammal population decline [24].…”