Cetacean stranding rate correlates with fish stock dynamics: research of harbour porpoises in the Sea of Azov

The dynamics of the endangered population of the harbour porpoise (Phocoena phocoena) in the Azov Sea is currently unknown. It can be, however, estimated by stranding analysis. In 1999–2014, the porpoise stranding rates were regularly monitored at the southern coast of the Azov Sea, particularly at the uninhabited abraded coast of the Tarkhan Cape. Specifically, the general trends and annual fluctuations in strandings were compared to the catch reports of the Azov anchovy (Engraulis encrasicolus), an important prey for porpoises. It was observed that the fluctuations in stranding rates closely correlated with the population dynamics of the anchovy stock. A cosine function, based on the data from 1999–2012, correctly predicted maximum strandings in 2013 and their substantial decline in 2014. The function worked particularly well, when possible biases affecting carcass preservation, such as discovery rate and drift conditions, were reduced. In certain environments and over established time periods, the cetacean stranding rate can be an indicator of population trends. The use of stranding rates as such indicator may be verified by external factors, including the dynamics of prey stocks.