Understanding common bottlenose dolphin depredation on artisanal fisheries in Mediterranean waters

Depredation of fisheries by marine mammals is a globally reported phenomenon. Though it is a common occurrence in Mediterranean fixed net artisanal fisheries, knowledge on its impacts and the factors influencing its probability are still lacking. Being the most commonly reported marine mammal to depredate these coastal fisheries, the common bottlenose dolphin (Tursiops truncatus) is the focus species of this study. Investigating factors that may influence depredation probability, as well as the catch amount of these fisheries is key for the development of informed management and mitigation policies. Acoustic detection of dolphins (ADD) using passive acoustic monitoring (PAM) devices attached to fishing gear was used as the proxy for dolphin presence (and thus depredation probability). Landings were photographed, measured in software, and converted into biomass via formulas specific to the species. Catch amount was expressed as catch per unit effort (CPUE), which is the total biomass of all caught species standardised in terms of net length and fishing operation duration (FOD). The influence of the CPUE, the type of gear used (trammel net or gillnet), the net length, the FOD, and the depth of the fishing site on ADD was tested by Generalised Linear Model (GLM) with a binomial distribution, while the influence of the ADD and gear type on CPUE was tested by a GLM with a gamma distribution. CPUE, gear type, and FOD showed significant effects on ADD. Higher ADD probability was associated to lower CPUE, and to gillnets as opposed to trammel nets. Increased FOD significantly increased the probability of ADD. Net length and depth at the fishing site showed no significant effects. Gear type and ADD had significant effects on CPUE, with gillnets having higher CPUE than trammel nets, and dolphin presence decreasing CPUE. Understanding how and which factors impact depredation probability and catch quantity is crucial for continued improvement of coexistence between wildlife and fisheries.