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In this study, we analyse seasonal aspects of harbour porpoise strandings in the Sea of Azov and discuss factors affecting the stranding rate. Data on 633 strandings were obtained from monitoring of a 35-km long area of the south coast of the Sea of Azov in 1999–2013. A distinct peak of strandings fell in July and August: it depended on the bycatch peak and calving season. Stranding rates depended neither on weather conditions nor on the seasonal fishing activities (including IUU fisheries). Moreover, stranding peaks in the neighbouring Black Sea were also tied to the calving season rather than to the fishing activities. We suggest that the seasonal mortality patterns are indirectly determined by nutritional stress: in Atlantic, winter-stranding oceanic populations and summer-stranding inner-sea populations occur that also possibly differ in the seasonal dynamics of body mass, weaning time or duration of mother–calf association, and dentine structure. In a typical summer-stranding population, summer is the season of nutritional stress, parturition, independent foraging of yearlings and lactation of nursing females, which leads to the risky foraging behaviour near gillnets. Another possible factor of increased bycatch is the seasonal habitat preference, corresponding to the gillnet preferences. Therefore, stranding and bycatch seasonality of porpoises can largely be explained by the aspects of their life history and foraging behaviour rather than by weather conditions and fisheries. This supports the time–area closure strategy as an adequate conservation measure, which would consider minimizing the conflict of interest with fisheries.

Seasonality of strandings and bycatch of harbour porpoises in the Sea of Azov: the effects of fisheries, weather conditions, and life history