Acoustic detection of bottlenose dolphin depredation on nets and implications for conservation

Abstract

Understanding the depredation process is pivotal to the management of conflicts between wildlife and human activities. Depredation by marine mammals on fisheries occurs when animals ‘steal’ fish from the fishing gear, causing catch loss and damage to gear and catches. The common bottlenose dolphin is a vulnerable species and is the main depredator of coastal static net fisheries in the Mediterranean Sea. The nature and extent of the phenomenon is still poorly understood since evidence is often incomplete or inconsistent mostly due to the different methods used to measure depredation. Here, passive acoustic monitoring devices were used to estimate the consistency between dolphin acoustic detection in proximity to the net and direct dolphin observations and catch measurements at different sites in Sardinia (north‐western Mediterranean Sea). Further, some factors influencing depredation (fishing duration and fishing net length) and changes in catch amount and composition were evaluated. The frequency of dolphin detection based on direct dolphin observations was significantly lower than that based on acoustic monitoring; the latter was related to the amount of catch. Thus, the measurement of catch was more realistically related to depredation than direct dolphin observations. Further, dolphin depredation was related to the duration of the fishing operation. The measure of depredation and catch loss provided here can be used to realistically estimate the damage suffered by fishers and provide preliminary insights to guide the development of an approach for sustainable coexistence between dolphins and fisheries.

Areas of spatial overlap between common bottlenose dolphin, recreational boating, and small-scale fishery: management insights from modelling exercises

“…Top predators for which the predation risk is generally low, such as the bottlenose dolphin in the Mediterranean Sea, develop strategies of resources and habitats use based on the optimization of their foraging success ( Mannocci et al, 2014 ). Thus, the present result can be explained by both the depredation behavior of the bottlenose dolphins on fishing nets, a widely spread feeding strategy in the Mediterranean Sea ( Reeves, Read & Notarbartolo di Sciara, 2001 ; Bearzi, 2002 ; Brotons, Grau & Rendell, 2008 ; Lauriano et al, 2004 ; Lauriano et al, 2009 ; Revuelta et al, 2018 ; Pardalou & Tsikliras, 2020 ; La Manna et al, 2022 ; “Isola Piana: risultati del monitoraggio 2022 e confronto con le annualitá precedenti”; G La Manna, M Moro Merella, A Ruiu, G Ceccherelli, 2022, unpublished data), and the likely sharing of the most productive zones within the study area between dolphins and fishers ( Pennino et al, 2015 ; Carlucci et al, 2016 ).…”

“…Further, conflict with fishers due to the depredation on fishing nets can impair dolphin conservation status ( Read, Drinker & Northridge, 2006 ; Read, 2008 ) owing to retaliatory measures (intentional injury and killing) taken by fishers. The economic crisis faced by small-scale fishery have exacerbated fishers’ perceptions of dolphins as competitors ( Lauriano et al, 2004 ; La Manna et al, 2022 ; “Isola Piana: risultati del monitoraggio 2022 e confronto con le annualitá precedenti”; G La Manna, M Moro Merella, A Ruiu, G Ceccherelli, 2022, unpublished data). This is particularly relevant in Sardinia where a depredation frequency of dolphins on gillnets and trammel nets higher than 50% has been recently estimated ( La Manna et al, 2022 ).…”

“…The economic crisis faced by small-scale fishery have exacerbated fishers’ perceptions of dolphins as competitors ( Lauriano et al, 2004 ; La Manna et al, 2022 ; “Isola Piana: risultati del monitoraggio 2022 e confronto con le annualitá precedenti”; G La Manna, M Moro Merella, A Ruiu, G Ceccherelli, 2022, unpublished data). This is particularly relevant in Sardinia where a depredation frequency of dolphins on gillnets and trammel nets higher than 50% has been recently estimated ( La Manna et al, 2022 ). Sardinia is also one of the Italian regions with the largest concentration of boat berths, tourist ports and marinas ( ONN, 2011 ).…”

Exploring odontocete depredation rates in a pelagic longline fishery

“…There is low strength of evidence of the efficacy and commercial viability of bycatch and depredation mitigation approaches for odontocetes in pelagic longline gear. This includes mitigation approaches such as spatiotemporal fisheries management, including quasi-real time dynamic (fleet communication, move-on rules, habitat suitability modelling, real-time acoustic tracking) and static approaches [ 9 , 15 , 22 , 24 , 26 , 27 ], input controls such as limits on soak and haul duration [ 24 , 28 ], and output controls such as bycatch thresholds [ 29 ]. The same applies to gear technology approaches such as:…”