The Baltic Proper harbour porpoise (Phocoena phocoena) population is listed by IUCN as Critically Endangered, but conservation action has been lacking for decades, partly due to very limited knowledge on important ecological parameters such as distribution range and abundance. 

This thesis provides some of the information needed for implementing effective conservation. The seasonal distribution of harbour porpoises in the study area in the Baltic Sea is modelled based on two years of passive acoustic monitoring data. Results reveal an area of high probability of detection on and around the offshore banks in the Baltic Proper south of the island of Gotland in summer (May – October). This area is likely to be the most important breeding area for the Baltic Proper population given the concentration of animals during the summer reproductive season. A summer management border is identified for the population, going from Jarosławiec on the Polish coast to the inner Hanö Bay on the Swedish coast. The abundance of the population is estimated to 491 individuals (95% CI 71-1105), which confirms the redlist status of the population and underlines the need for immediate conservation measures.  

The conservation policy of the Baltic Proper harbour porpoise and other harbour porpoise populations in Europe is discussed, detailing the latest developments including the 2020 ICES scientific advice on bycatch mitigation for the Baltic Proper harbour porpoise and noting that the legal framework for protection is mostly present, but that there is still a lack of concrete conservation action. It is suggested that this failure to protect porpoises may be a result of low motivation for decision-makers to take action which in turn is likely due to some politically difficult prioritisations having to be made, in combination with the low public recognition of the harbour porpoise as a species throughout Europe. Suggestions are made for next steps.

One technical solution to mitigating harbour porpoise bycatch is acoustic deterrent devices, pingers, which are placed on fishing nets to alert harbour porpoises to the presence of nets. However, in the Baltic there has been discussions that pingers work as dinner bells for seals, and fishermen have been hesitating to use them. Here, it is shown that high-frequency pingers do not cause an increase in seal depredation on static nets and hence that they are a viable solution for much needed bycatch mitigation within the distribution range of the Baltic Proper harbour porpoise population.