The Baltic harbour porpoise

The Baltic Proper harbour porpoise population is classified as critically endangered by the International Union for Conservation of Nature, IUCN. It is genetically and morphometrically different from its closest neighbours in the Belt Sea, showing that the Baltic Proper harbour porpoise is a separate population and should be treated as its own management unit.

After a significant decrease in numbers from the 1960’s and forward, in the beginning of the 21st century no one knew how many animals were left in the Baltic, and there was no knowledge on the distribution of animals in the Baltic Proper. The only information came from observations reported to different national bodies and collected at the HELCOM harbour porpoise database.

Reported observations occur mostly along the coasts, since this is where most people spend their time. 

Without information on where the largest part of the population is, it is very difficult to take effective conservation measures. To gain more information, authorities and scientists from all EU countries around the Baltic Sea decided to start a common project using new acoustic methods to survey the distribution and abundance of harbour porpoises in the Baltic Sea. The project, which started in 2010, was named SAMBAH, Static Acoustic Monitoring for the Baltic Sea Harbour Porpoise. SAMBAH estimated the number of harbour porpoises left in the Baltic Proper population to approximately 500 animals. The project could also show what areas are most important to porpoises during different seasons, and could thereby also show where the Baltic Proper harbour porpoise reproduce. This was previously completely unknown, and this new knowledge was instrumental in designating relevant areas for protection of porpoises. The SAMBAH results were extremely important for conservation actions taken for the Baltic proper harbour porpoise since. However, many years have passed since the SAMBAH survey and there is a need to carry out a new survey of the population. We need to know how the population status is changing and if distribution has changed since the last data collection. 

Photo credit: Anthony Pierce / Alamy Stock Photo

Based on the results from SAMBAH, in December 2016 the Swedish government designated a large Natura 2000 area south of Gotland, Hoburgs bank och Midsjöbankarna. There are other Natura 2000 areas designated for the Baltic Proper harbour porpoise, especially in Poland and Germany, but this area is very likely the most important area for this population.The area is more than 1 million hectares, and includes most of the area where the Baltic Proper harbour porpoises are thought to give birth to their calves and mate in the summer. To protect the porpoises here is very important to give the population a chance to recover. In 2021, the management plan for this area was finalised, and although this plan does not contain any set conservation measures for the area, it is an important document. 

During 2022 static net fisheries will be closed in the entire area, through an EU delegated act. Around the Northern Midsea bank, a smaller area within the Natura 2000, all fisheries except with pots, traps and longlines will be closed. With effective conservation measures like this, this area could be instrumental for the recovery of the Baltic Sea harbour porpoise. We are now hoping that measures regulating underwater noise will also be taken in this area.

The most severe threat to harbour porpoises is bycatch in fisheries. Because porpoises need to get to the surface to breathe, they drown if they get caught in a fishing net. The nets with the highest risk of bycatch are large mesh nets used to catch for example cod and flatfish, as well as salmon nets. Porpoises rarely get caught in trawls or other active gear, so set nets are the primary threat. 

In 2019, the European Commission asked ICES for scientific advice on how to minimise bycatch of harbour porpoises in the Baltic Proper. The ICES advice was published in May 2020 and proposes that:

• Natura 2000 areas designated for the Baltic Proper harbour porpoise should be closed for static net fisheries.
• Pingers should be used on static nets used outside of protected areas. Pingers emit sound signals scaring the porpoises or warning them of the presence of nets.
• Pingers should only be used as an intermittent measure to avoid bycatch, and in the long-term, static nets should be replaced by alternative gear such as pots or traps that do not cause marine mammal bycatch.

There are problems with all of these methods and different methods may be suitable in different areas, so it is important to have a good dialogue with stakeholders to adapt measures.

Because harbour porpoises use echolocation, i.e. sound, to orientate themselves, find prey and communicate, they are sensitive to underwater noise. The level of noise in the oceans has increased significantly during the last decades, with noise from shipping, dredging, construction, leisure boats and jet skis. With increased noise, porpoises have problems hearing the echoes from their own echolocation clicks, which makes it harder for them to find prey. It also becomes more difficult for porpoises to hear each other. Their high frequency sounds travel only short distances underwater, and with increased noise levels it becomes even shorter. This can cause calves to lose their mothers, or females and males not finding each other when it is time to mate. Noise can also disrupt important behaviours such as hunting or nursing of calves. To decrease the impact of underwater noise in especially sensitive areas we can:

• Restrict shipping traffic;
• Move shipping routes;
• Introduce speed limits for, or completely ban leisure boats;
• Prohibit construction of for example wind farms, or enforce strict rules on how construction must be carried out;
• Prohibit use of active sonars;
• Prohibit detonations of explosives, or, if some activities are strictly necessary, make mitigation methods such as bubble curtains obligatory.

Photo credit: Nature Picture Library / Alamy Foto Stock