As every year, International Porpoise Day is celebrated on the third Sunday of May. The observance of this Day is one of the elements of the implementation of the Bonn Convention (Journal of Laws 2003, No. 2, item 17) and the resulting ASCOBANS Agreement (ASCOBANS 1992), of which Poland is a signatory. Baltic Porpoise Day aims to promote knowledge of these marine mammals and draw attention to the threats to the Baltic population.
The Baltic harbour porpoise (Phocoena phocoena) is not the only representative of the porpoise family (Phocoenidae). Besides it, there are six other species, often under protection (www1).
Porpoises are closely related to dolphins but differ in body structure. Porpoises are smaller and their body is more curvaceous. The head is more distinctly oval without the ‘beak’ characteristic of dolphins, and the dorsal fin is short and triangular (except for two species that do not have this fin: the finless porpoise (Neophocaena phocaenoides) and the Yangtze finless porpoise (Neophocaena asiaeorientalis) (www1)), while in dolphins it is long, and sickle shaped. Porpoises are also shyer and interact less frequently with people or boats. In addition, they do not jump above the surface of the water or flick their fins making a lot of fuss as their cousins tend to do (www2).
The Baltic harbour porpoise is the only cetacean found in the Baltic Sea. It inhabits the northern hemisphere, residing mainly in shallow, coastal waters. Adults can reach up to 180 cm in length and weigh up to 70 kg. It is dark grey or black on the back, which gradually becomes lighter on the sides (Lockyer 2003, Jefferson et al. 2015). In the wild, it can live to be about 15 years old, although a case has been reported where an individual lived a record 24 years (Klinowska 1991, Lockyer 2003). A key sense for the harbour porpoise is echolocation, a system of determining the location of objects using the acoustic echo phenomenon (Villadsgaard et al. 2007). With this, it tracks obstacles, finds food, and communicates with other individuals. Porpoises most commonly use sound pulses of 120-130 kHz, called ‘clicks’ (Kastelein et al. 2002).
The Baltic sub-population, numbering around 500 individuals (SAMBAH 2016), is listed on the IUCN Red List of Threatened Species as Critically Endangered (CR) (Hammond et al. 2018). The abundance of this species, through its mobility and shy nature, is hard to estimate. The SAMBAH surveys are the only such accurate estimates of harbour porpoise occurrence, but they were conducted 10 years ago (2011-2013) and it is difficult to determine what the exact population status is today (SAMBAH 2016). The most recent harbour porpoise surveys conducted by the Museum of Natural History in Sweden (Naturhistoriska riksmuseet) between 2017 and 2020 at the same stations as in the SAMBAH project showed that the frequency of detection of this critically endangered population in the Baltic has increased over the last decade, with an average increase of 2.4% per year. However, it is important to note that while this is positive news, the rate of population growth is well below what is possible for a healthy population of porpoises in the absence of threats (www3). Currently, the most dangerous to this species, especially in the Bay of Puck, is by-catch, i.e., accidental entanglement in fishing nets. These nets are made of fine synthetic fibres that form an imperceptible obstacle. The meshes of the nets are so large that the mouth of a porpoise that falls into the net is irretrievably trapped in it. The entangled animal cannot swim to the surface to take a breath and drowns (Pawliczka 2022).
Another threat to this marine mammal is noise, as porpoises that use echolocation are highly susceptible to acoustic disturbance. Noise is mainly generated by marine transport. However, as the renewable energy sector advances, offshore wind farm construction is also a source of noise in the marine environment. The process of building an offshore wind farm includes what is known as piling, which is the driving of monopile structures into the seabed, emitting enormous noise (Jeppesen 2019). This noise can cause overfishing or interfere with the echolocation used to find food or to communicate between individuals. Near to works, noise can even cause temporary or permanent hearing loss (OSPAR Commission 2008, Bergstöm et al. 2014).
To prevent negative impacts on harbour porpoises, minimisation measures are used to reduce underwater noise intensity as much as possible. This can be achieved, for example, using air curtains, the use of acoustic marine mammal deterrent devices (pingers) installed around the construction work area, or starting construction work at low noise levels (the so-called soft-start method), which allows marine mammals to escape before underwater noise reaches maximum levels. In addition, developments in offshore wind farm technology allow for structures that do not generate as much noise, e.g., gravity foundations (Verfuss et al. 2015, Lüdeke 2017, Tougaard and Mikaelsen 2018, Jeppesen 2019, Norro 2020).
Marine pollution is another major threat to porpoises, affecting their health and reproductive success (www2). Persistent organic pollutants (POPs), i.e.: polychlorinated biphenyls (PCBs), pesticides, and trace metals found in the marine environment, bioaccumulate in marine ecosystems. In marine mammals, these compounds accumulate in adipose tissues. In a study by Van den Heuvel-Greve et al. (2021) demonstrated that porpoises with PCB levels above the highest threshold of 41 mg/kg were more likely to die from infectious diseases and/or debilitation. In addition, PCBs entered the body of calves (young porpoise) with their mother’s milk, increasing the potential for toxicity in juveniles.
The primary way to help porpoises survive is to enforce existing species protection and conservation laws. Less pollution and less underwater noise can only help. So can reducing dangerous nets for marine mammals. Also important is the activity of scientific centres that continuously monitor the condition of these animals (Prof. Krzysztof Skóra Marine Station on Hel), and the activity of environmental organisations such as WWF. They raise public awareness and thus play an important role in supporting effective conservation measures.
- ASCOBANS 1992, Porozumienie o ochronie małych waleni Morza Bałtyckiego i Północnego
- Bergstöm L., Kautsky L., Malm T., Rosenberg R., Wahlberg M., Åstrand Capetillo N., Wilhelmsson D., 2014, Effects of offshore wind farms on marine wildlife – a generalized impact assessment. Environ. Res. Lett. 9, 12 s., doi:10.1088/1748-9326/9/3/034012
- Dz.U. 2003 nr 2 poz. 17 – Convention on the Conservation of Migratory Species of Wild Animals, Bonn, 23 June 1979.
- Hammond P.S., Bearzi G., Bjorge A., Forney K.A., Kaczmarski L., Kasuya T., Perrin W., Scott M. D., Wang J. Y., Wells R. S., Wilson B., 2016, Phocoena phocoena (Baltic Sea subpopulation). The IUCN Red List of Threatened Species e.T17031A98831650
- Jefferson T. A., Webber M. A., Pitman R. I., 2015, Marine Mammals of the World. A comprehensive Guide to their identifications. Elsevier, Second Edition, 326-329
- Jeppesen M., 2019, Oddziaływania transgraniczne, aneks do raportu OOŚ dla farmy wiatrowej Kriegers Flak, Data: 01 październik 2019 r., Historia wersji: wersja ostateczna, Vattenfall, 18 ss.
- Kastelein R. A., Bunskock P., Hagedoorn M., Au W. W. L., de Haan D., 2002, Audiogram od a harbour porpoise (Phocoena phocoena) measured with narrow – band frequency modulated signals. Journal of the Acoustical Society of America, 112, 334-344
- Klinowska M., 1991, Harbour porpoise (88-101) [w]: Dolphins, Porpoises and Whales of the World. The IUCN Red Data Book, Switzerland and Cambridge, UK: IUCN, Gland
- Lockyer C., 2003, A review of methods for defining population structure in the harbour porpoise (Phocoena phocoena). NAMMCO Scientific Publication, 5, 41-70
- Lüdeke J., 2017, Offshore Wind Energy: Good Practice in Impact Assessment, Mitigation and Compensation. Journal of Environmental Assessment Policy and Management Vol. 19, No. 1, 1750005, 31 ss
- Norro A., 2020, An evaluation of the noise mitigation achieved by using double big buble curtains in offshore pile driving in the Southern North Sea. Chapter 2 (w:) Environmental Impacts of Offshore Wind Farms in the Belgian Part of the North Sea: Empirical Evidence Inspiring Priority Monitoring, Research and Management. Series ‘Memoirs on the Marine Environment’. (red.) Degraer S., Brabant R., Rumes B., Vigin L. Brussels: Royal Belgian Institute of Natural Sciences, OD Natural Environment, Marine Ecology and Management, 19-27
- OSPAR Commission, 2008, Assessment of the environmental impact of offshore wind farms. Biodiversity Series, 35 ss
- Pawliczka I., 2022, Morświny, Phocoena phocoena (Linnaeus 1758) w Zatoce Puckiej. Zatoka Pucka. Tom III. Aspekty świata ożywionego, 310-323
- SAMBAH 2016, Final report covering the project activities from 01/01/2010 to 30/09/2015. LIFE Project Number Life08 NAT/S/000261
- Tougaard J., Mikaelsen M. A., 2018, Effects of larger turbines for the offshore wind farm at Krieger’s Flak, Sweden. Assessment of impact on marine mammals. Aarhus University, DCE – Danish Centre for Environment and Energy No. 286, Scientific Report, 112 ss
- Van den Heuvel-Greve M. J., Van den Brink A. M., Kotterman M. J. J., Kwadijk C. J. A. F., Geelhoed S. C. V., Murphy S., Van den Broek J., Heesterbeek H., Grone A., IJsseldijk L. L., 2021, Polluted porpoises: Generational transfer of organic contaminants in harbour porpoises from the southern North Sea. Science of The Total Environment
- Verfuss U. K., Sparling C. E., Arnot C., Judd A., Coyle M., 2015, Review of Offshore Wind Farm Impact Monitoring and Mitigation with Regard to Marine Mammals. Chapter 147 (w:) The Effects of Noise on Aquatic Life II, (red.) Popper A.N., Hawkins A., Advances in Experimental Medicine and Biology 875, 1175-1182
- Villadsgaard A., Wahlberg M., Tougaard J., 2007, Echolocation signals of wild harbour porpoises, Phocoena phocoena.
www1 – https://porpoise.org/, access May 2023
www2 – https://www.ascobans.org/en/species/phocoena-phocoena, access May 2023
www3 – https://www.nrm.se/en-GB/forskningochsamlingar/miljoforskningochovervakning/varforskning/overvakningavtumlare.9008742.html, access May 2023
Photo. 1 and Photo. 2 – Baltic Porpoise Monument in Gdynia (photos by K.Dobosz)