Estimating the abundance of the critically endangered Baltic Proper harbour porpoise (Phocoena phocoena) population using passive acoustic monitoring

Knowing the abundance of a population is a crucial component to assess its
conservation status and develop effective conservation plans. For most
cetaceans, abundance estimation is difficult given their cryptic and
mobile nature, especially when the population is small and has a
transnational distribution. In the Baltic Sea, the number of harbour
porpoises (Phocoena phocoena) has collapsed since the mid-20th century and
the Baltic Proper harbour porpoise is listed as Critically Endangered by
the IUCN and HELCOM; however, its abundance remains unknown. Here, one of
the largest ever passive acoustic monitoring studies was carried out by
eight Baltic Sea nations to estimate the abundance of the Baltic Proper
harbour porpoise for the first time. By logging porpoise echolocation
signals at 298 stations during May 2011-April 2013, calibrating the
loggers’ spatial detection performance at sea, and measuring the click
rate of tagged individuals, we estimated an abundance of 71-1,105
individuals (95% CI, point estimate 491) during May-October within the
population’s proposed management border. The small abundance estimate
strongly supports that the Baltic Proper harbour porpoise is facing an
extremely high risk of extinction, and highlights the need for immediate
and efficient conservation actions through international cooperation. It
also provides a starting point in monitoring the trend of the population
abundance to evaluate the effectiveness of management measures and
determine its interactions with the larger neighbouring Belt Sea
population. Further, we offer evidence that design-based passive acoustic
monitoring can generate reliable estimates of the abundance of rare and
cryptic animal populations across large spatial scales.