Questionnaire and experimental surveys show that dolphins cause substantial losses to a gillnet fishery in the eastern Mediterranean Sea

Common bottlenose dolphins (Tursiops truncatus) are known to depredate fishing gear, resulting in damage to the catch and/or the gear itself. The extent of this damage and estimated financial loss varies between areas, metiers, and survey methods. We quantified losses due to bottlenose dolphin depredation in a coastal gillnet fishery in the Thermaikos Gulf, Greece, in terms of CPUE reduction, catch damage, and gear damage. Experimental fishing effort was carried out over two seasons (2020 and 2021), along with concurrent questionnaire surveys over the second season (2021). Depredation frequency (similar to 35%) and CPUE changes due to depredation (a significant decrease of 45-50%) were similar for both sampling schemes. The number of damaged fish in experimental hauls increased significantly with dolphin depredation, but did not fully account for the sizeable loss in marketable catch, indicating that large numbers of fish were removed from the nets entirely. Damage to experimental nets increased with dolphin presence and group size, with an average of 0.59% of net surface area damaged per depredation event. Both datasets point to annual economic losses of over euro5000 per vessel in this fishery, while the similarity between direct observations and self-reported losses highlights the usefulness of frequent questionnaire surveys.

Automated summary

In this study, losses caused by bottlenose dolphin depredation in a coastal gillnet fishery in Thermaikos Gulf, Greece, were quantified. Experimental fishing efforts and questionnaire surveys were conducted over two seasons to measure the frequency of depredation, changes in catch per unit effort (CPUE), and damages to catch and gear. The results showed that depredation frequency was approximately 35%, resulting in a significant decrease in CPUE by 45-50%. The number of damaged fish in experimental hauls significantly increased with dolphin depredation, suggesting that a large number of fish were completely removed from the nets. The study also revealed that the presence and group size of bottlenose dolphins were associated with increased damages to the experimental nets.