Three subspecies of Harbour Porpoises are currently recognized by the Society for Marine Mammalogy’s Committee on Taxonomy (2022). The Pacific Harbour Porpoise (Phocoena phocoena vomerina) occurs in the North Pacific, the Atlantic Harbour Porpoise (P. p. phocoena) has a distribution spanning the North Atlantic from North America to Europe and northwestern Africa, and the Black Sea Harbour Porpoise (P. p. relicta) occurs in the Black Sea (Viaud-Martínez et al. 2007). Harbour Porpoises around the Iberian peninsula and northwestern Africa are genetically distinct from those in the rest of the European Atlantic (Fontaine et al. 2010) and Fontaine et al. (2014) suggested they were a new subspecies, P. p. meridionalis. A distinct mitochondrial lineage has been found in one porpoise from West Greenland, which raises the possibility that another Atlantic subspecies exists (North Atlantic Marine Mammal Commission and Norwegian Institute of Marine Research, 2019).

The size of the Baltic Proper subpopulation of Harbour Porpoises is estimated to be fewer than 250 mature animals. A continuing decline in mature animals can be inferred based on information on the number of animals bycaught in fishing gear compared to the potential growth rate of the Baltic Proper subpopulation. Other threats to the reproduction and survival of Harbour Porpoises, including contaminants, noise, and prey depletion, are also ongoing for this subpopulation. All mature individuals of the Baltic Proper subpopulation belong to one subpopulation, which cannot be rescued by immigration of individuals from another subpopulation. Thus, the subpopulation meets criterion C2a(ii). This subpopulation was listed as Critically Endangered (C2a(ii)) on the Red List in 2016 and it remains Critically Endangered (C2a(ii)) in this updated assessment.

During May–October, Baltic Proper Harbour Porpoises aggregate on and around the offshore banks south of Gotland and east of Öland, and most animals are believed to be east of the island of Bornholm in the southern Baltic Sea. During November-April, detections are more spread out along the coasts and archipelagos of the Baltic Proper (Carlén et al. 2018). It is not known how far west Baltic Proper Harbour Porpoises disperse in winter, but modelling of seasonal detection rates at acoustic monitoring stations in German waters indicates that they move at least as far west as to the offshore waters northeast of Rügen (Gallus et al. 2012). This is confirmed by genetic analyses of genome-wide single nucleotide polymorphisms (SNPs; Tiedemann 2017). A tentative management border during November–April has been proposed at 13°E (ICES 2020b).

In the early 1900s, Harbour Porpoises were regularly observed in the entire Baltic Sea, including the Gulfs of Bothnia, Finland and Riga, and the Baltic Proper. However, in the latter half of the 1900s, the range was reduced considerably, and currently the species is rarely observed in the northern and eastern gulfs of the Baltic Sea (HELCOM 2022, 2023a; Koschinski 2001). 

Several genetic and morphometric studies have concluded that Baltic Proper Harbour Porpoises form a separate population distinct from those living in the Kattegat, Skagerrak and North Sea (e.g., Huggenberger et al. 2002, Wiemann et al. 2010, Galatius et al. 2012, Lah et al. 2016). This conclusion is also supported by spatial separation during the breeding season from the neighbouring Belt Sea population, which primarily is distributed in the Kattegat, the Belt Sea and the southwestern Baltic (Sveegaard et al. 2015, Carlén et al. 2018). New genomic evidence has identified a separate genetic cluster of Harbour Porpoises within the Baltic Proper with high confidence (Celemín et al. 2022). These data reveal that inbreeding seems so infrequent that the two Baltic populations (Belt Sea population and Baltic Proper population) appear to have independent demographic trajectories. Among the sampled individuals, no recent interbred individuals have been detected and the genetic data indicate potential single interbreedings a number of generations ago. Thus, genetic data show that the Baltic Proper subpopulation is demographically independent from the neighbouring population, and should it go extinct, it cannot be rescued by immigration of individuals from another population.

In 2011–2013, the abundance of the Baltic Proper Harbour Porpoise subpopulation was estimated to be 491 individuals (CV=68.0%; 95% confidence interval = 71–1,105) (Amundin et al. 2022). The proportion of mature individuals was calculated from a Leslie matrix model using population parameters available for Harbour Porpoises in the closest populations (Belt Sea, Kattegat, Skagerrak, North Sea; Hedlund 2008, Winship 2009, Kesselring et al. 2017, Hammond et al. 2019, IJsseldijk et al. 2021) using R package RLA (Genu et al. 2021). The proportion of mature animals was calculated as 0.439, which results in an estimated 216 mature individuals in the Baltic Proper subpopulation.

Based on a population viability analysis, the Baltic Proper Harbour Porpoise subpopulation is estimated to be viable, with a growth rate of 2.3% per year, under a baseline scenario of a healthy population with no bycatch (Cervin et al. 2020). The mean annual reported bycatch from 2000 to 2012 was 3.5 animals, however, this is believed to be an underestimation of true bycatch level. The true bycatch level was estimated as 7 individuals in 2017, based on the bycatch rate for the neighbouring Belt Sea population (adjusted to account for the lower density of the Baltic Proper subpopulation of Harbour Porpoises and the reported gillnet fishing effort (within ICES sub-areas 25-29)) which was then multiplied by the reported gillnet fishing effort in the same area from 2009–2017 (North Atlantic Marine Mammal Commission and Norwegian Institute of Marine Research 2019). This bycatch level is predicted to lead to a population collapse to ≤50 animals with high probability (40–100%) over the next century, assuming an intermediate or low (< 73%) fecundity (Cervin et al. 2020). Based on a dynamic production model, population abundance and annual bycatch estimates, the population is estimated to have declined by 9% from 2009 to 2017 (North Atlantic Marine Mammal Commission and Norwegian Institute of Marine Research 2019). Assuming a constant annual bycatch level of 7 individuals from 2018 to 2025, a continued decline of 12% since 2009 was predicted (North Atlantic Marine Mammal Commission and Norwegian Institute of Marine Research 2019). While it is recognised that the level of bycatch is unlikely to be constant as the population declines, ongoing reports of bycatch within the distributional range of the Baltic Proper subpopulation indicate that the level of bycatch is still likely higher than what the population can sustain (estimated to be 0.7 animals per year (North Atlantic Marine Mammal Commission and Norwegian Institute of Marine Research 2019)), resulting in a continuing decline in the population. Additionally, other threats likely to result in reduced survival and fecundity, such as underwater explosions, prey limitation, and pollution, are still ongoing within the distributional range of the Baltic Proper Harbour Porpoise subpopulation.

Subpopulation-wide trend data are lacking, with only one abundance estimate to date (Amundin et al. 2022). Comparisons of detection rates at a limited number of acoustic monitoring stations in Danish, Polish and Swedish waters suggest potential local increases at some sites since 2011–2013 (Swistun et al. 2019, Sveegaard et al. 2020, Owen et al. 2021); however, it remains unknown whether any such increases are a result of changes in population size or constrictions of Harbour Porpoise distribution. In the first half of the 1900s, Harbour Porpoises were distributed much more widely throughout the range of the Baltic Proper subpopulation (including into the Gulfs of Bothnia, Finland and Riga) than the currently recognised distribution range within the Baltic Proper. Whether this range contraction has continued in recent years remains unknown.

Although there are no reliable estimates of pre-exploitation size of the Baltic Proper subpopulation, historical data from a Polish bounty scheme (Psuty 2013; see the Threats section) indicate that around 1930, the Baltic Proper subpopulation was likely at least an order of magnitude greater than today, i.e. at least 5,000 individuals. Further, bycatch records from Sweden in 1960-1961 (Lindroth 1962; see the Threats section), show that the Baltic Proper subpopulation was still several times greater in the early 1960s than today (Lindroth 1962). Based on this, and using an exponential rate of decline within the IUCN workbook tool, the current rate of decline of the Baltic Proper subpopulation is estimated to be 64% over the last three generations (36 years).

The Baltic Sea is a semi-enclosed, relatively shallow brackish shelf sea (mean depth about 50 m), with a few basins deeper than 100 m. The deep basins suffer from frequent and prolonged anoxic conditions below the halocline (Kõuts et al. 2022), and oxygen can only be replenished by infrequent saltwater inflow from the North Sea (Mohrholz et al. 2015). There is a salinity gradient with declining salinity towards the east and north. Winter sea-ice normally covers the northern and eastern parts of the Baltic Sea.

No recent dietary information is available for the Baltic Proper Harbour Porpoise subpopulation. Based on stomach content analysis, the most common prey items of animals bycaught in the central Baltic Proper in 1960–1961 were Sprat (Sprattus sprattus), Transparent Goby (Aphia minuta), Atlantic Herring (Clupea harengus) and Atlantic Cod (Gadus morhua) (Lindroth 1962). In animals bycaught and stranded in the western Baltic in 1980–2011, Cod and Herring were the main prey of adults, while gobies (Gobiidae spp.) were also frequently consumed by juveniles (Andreasen et al. 2017). Data from German waters of Mecklenburg-Vorpommern (31 stomachs, 2013–2019) suggest that greater than 90% of the diet in terms of biomass is small-sized Cod (mostly below 30 cm), followed by Whiting (Merlangius merlangus) (Klemens 2019). It should be noted that the Porpoises in these studies have not been genetically assigned to any subpopulation and, based on the sampling locations, it is most likely that the majority of those analysed by Andreasen et al. (2017) and Klemens (2019) were from the Belt Sea population, and only a small proportion were from the Baltic Proper subpopulation.

The Baltic Proper Harbour Porpoise subpopulation shows a seasonal movement pattern (Carlén et al. 2018). Increased presence of Harbour Porpoises in the southern Baltic Sea during winter has been associated with cold air temperatures (Gallus et al. 2012) and severe ice conditions (Johansen 1929, Ekman 1938, Tägström 1940, Lönnberg 1940, Wölk 1969).

Historically, Harbour Porpoises were intentionally hunted and caught, and unintentionally bycaught in fishing gear, throughout the range of the Baltic Proper subpopulation (HELCOM 2022). Data from Polish fisheries reports show that in the area around Hel Peninsula and Puck Bay, at least 676 Harbour Porpoises were caught from 1922 to 1933. During 1934–1935, the minimum number caught was about 400, and the total number was roughly estimated to be 800 (Psuty 2013). From November 1960 to October 1961, at least 50 Harbour Porpoises were bycaught by Swedish salmon fishermen in the central Baltic Proper (Lindroth 1962).

Today, bycatch in fishing gear remains the most significant threat to the Baltic Proper Harbour Porpoise subpopulation, and the threat level is classified as high (ICES 2019). The majority (at least 97%) of the bycatch records in the Baltic Proper have been reported to occur in static nets, such as gillnets, trammel nets or other entangling nets (Berggren 1994, Skóra and Kuklik 2003, EC-DGMARE 2014). The most common types of static nets in which bycatch has occurred are semi-driftnets (anchored at one end) set for Salmonids, and bottom-set gillnets set for Cod. In addition to static nets, Harbour Porpoises are also bycaught in smaller numbers in trawls (ICES 2020a).

The Potential Biological Removal (PBR) limit for the Baltic Proper Harbour Porpoise population has been calculated to be 0.7 individuals (North Atlantic Marine Mammal Commission and Norwegian Institute of Marine Research 2019). PBR is the maximum number of animals (excluding natural mortalities) that can be lost from a population while still allowing the population to reach the defined conservation objective: that a population at the maximum net productivity level (MNPL greater than or equal to 50% of carrying capacity (K)) is able to remain there for 20 years, and that a population at 30% K is able to reach MNPL in 100 years (Wade 1998). In comparison to the PBR of 0.7 individuals, the number of animals bycaught in the Baltic Proper Harbour Porpoise subpopulation in 2017 was estimated to be 7 individuals, and records of bycaught animals still occur within the distributional range of the population. Therefore, the number of bycaught animals alone likely far exceeds the PBR limit.

Along with bycatch, pollutants are classified as a high threat to the Baltic Proper Harbour Porpoise subpopulation (ICES 2019). Biomagnifying pollutants such as persistent organic pollutants (POPs, e.g. PCBs and dioxins) and heavy metals (e.g. mercury) are of particular concern for Harbour Porpoises in the Baltic Sea. They may act as endocrine disruptors, affecting the reproductive system, thyroid gland, neuroendocrine system, immune system, and the systems that control nutrient partitioning (Rhind 2008). In Harbour Porpoises, high PCB burdens have been found to be associated with reduced immune system function, health status, and fertility (Jepson et al. 2005; Beineke et al. 2007a,b; Murphy et al. 2015). High burdens of mercury have been associated with prevalence of parasitic infection and infectious diseases (Siebert et al. 1999). Since the start of regulation of the use of PCBs in the 1980s, PCB concentrations in marine mammals initially declined worldwide, but have since stabilised at toxicologically significant levels in several European cetacean species (Law 2014, Jepson et al. 2016). A similar temporal pattern is seen in several other POPs and their toxic equivalence (TEQ) values in Baltic Herring and seabird eggs (European Food Safety Authority 2005, Jörundsdóttir et al. 2006, Miller et al. 2014). Recent data from Baltic Proper Harbour Porpoises are lacking, but PCB concentrations in animals sampled in the 1980–1990s (Kannan et al. 1993, Berggren et al. 1999, Bruhn et al. 1999, Falandysz et al. 2002) were often well over a proposed threshold for adverse health effects (Jepson et al. 2005). During the 1980–1990s, PCB levels in Harbour Porpoises from the Baltic Sea region were up to 254% higher than mean levels of PCBs in corresponding samples from the Kattegat and Skagerrak (Berggren et al. 1999). TEQ values of dioxins, dioxin-like PCBs and chloro-organic contaminants in Herring fillets sampled at 11 locations from west of the British Isles to the Latvian coast in the Baltic Proper during 1996-2004 show an increase of about 35 fold from west to east (Karl and Ruoff 2007).

Different sources of underwater noise are considered to be a medium or high threat (ICES 2019). Hearing impairment is a major problem for porpoises as they rely on their biosonar to catch prey, communicate and orientate. With respect to behavioural responses to, or avoidance of, noise, spectral characteristics of the received sounds are important. Harbour Porpoises are especially sensitive to the mid- (1–10 kHz) and high-frequency (>10 kHz) part of the spectrum at which they show behavioural reactions. Even low levels of mid- and/or high-frequency components may elicit a response to broadband sounds (Dyndo et al. 2015). Due to the low salinity in the Baltic Sea, the absorption of sound is less than in oceanic waters (Andersson et al. 2016, Andersson and Johansson 2013). This results in increased received levels of mid- and high-frequency sounds, and greater disturbance ranges, than in oceanic environments. In addition, sound channels typically occurring in the Baltic Proper under certain stratification conditions increase disturbance ranges at similar frequencies (Pihl et al. 2011). The most commonly occurring sources of impulsive noise in the Baltic Sea are sonars and seismic surveys (ICES 2022), which may cause temporary threshold shifts (TTS) in hearing sensitivity, avoidance reactions, and reduced foraging rates (Lucke et al. 2009, Sarnocińska et al. 2020, Thompson et al. 2013). Less frequent but much louder are underwater explosions, e.g. from clearance of unexploded ordnance (UXO) or construction work. In the Baltic Sea, approximately 175,000 mines are estimated to have been laid during the world wars and approximately 1,985 mine fields have been identified (Möller 2011). Explosions of UXO have been linked to blast injuries and death of Harbour Porpoises in the Baltic Sea region (Siebert et al. 2022). For the Dutch North Sea, it has been estimated that 88 explosions in a 1-year period for clearing UXO caused between 1,280 and 5,450 events of permanent hearing loss in Harbour Porpoises (Benda-Beckmann et al. 2015), indicating that mine clearance in the Baltic Sea could have a major impact on Harbour Porpoises. Offshore development is still limited but increasing in the Baltic Sea and there are extensive plans for the development of offshore windfarms (4C Offshore 2022). Activities linked to different phases of the life cycle of an offshore windfarm cause impulsive or continuous underwater noise, e.g. seabed exploration, explosions for clearance of cable corridors and wind farm areas, pile driving, turbine operation, and decommissioning. Pile driving may cause displacement and TTS in Harbour Porpoises (Brandt et al. 2011; Dähne et al. 2013, 2017; Tougaard et al. 2009). Continuous noise, such as ship noise or wind farm turbine noise, is widespread, but information on impacts at the individual- and, especially, population-level is largely lacking. However, ship noise is estimated to cause reduced communication distance by masking (Hermannsen et al. 2014) and strong behavioural reactions have been recorded (Dyndo et al. 2015, Wisniewska et al. 2018).

Prey depletion and habitat degradation are deemed to be medium threats to the Baltic Proper Harbour Porpoise subpopulation (ICES 2019). Energy density of the diet appears to be critical in determining reproductive success of female Harbour Porpoises (IJsseldijk et al. 2021), indicating that reduced quality and quantity of Cod, Herring and Sprat, which are three of the main prey species for Baltic Proper Harbour Porpoises, may impede recovery. Since the 1970s, most stocks of commercial-sized Cod, Herring and Sprat in ICES subdivisions 25 and higher in the Baltic Sea have decreased, with the Eastern Baltic Cod stock presently at the lowest level observed since the 1950s (ICES 2021). Eastern Baltic Cod has displayed a regime shift from high reproductive potential before the 1980s to low potential since then (Voss and Quaas 2022). During recent decades, the reduction in Cod status has been driven largely by biological changes, including poor nutritional condition, reduced growth and high natural mortality (ICES 2021). The largest Herring stock, the Central Baltic stock, has decreased since the 1970s (ICES 2021). During recent years, the spawning component of the smaller northern size of Herring has been dominant over the component of the larger southern size in both the landings and in the stock. The spawning stock biomass of Sprat has fluctuated considerably due to a combination of varying fishing pressure, recruitment and natural mortality (the latter linked to Cod biomass). When the Cod biomass was high in the beginning of the 1980s, the Sprat stock was low. This was followed by a rapid increase in Sprat biomass, reaching a maximum in 1996–1997, and at the same time a 40% decline in weight at age. By 2005–2017, the stock biomass had declined again by 45%, but a small increase is estimated by 2023. In addition to fishing pressure and species interactions, climate change and anthropogenic eutrophication are also predicted to be main drivers of the biomass, distribution and condition of Harbour Porpoise prey species in the Baltic Sea (e.g. Bartolino et al. 2014, Bossier et al. 2021, Voss and Quaas 2022).

Habitat loss and habitat degradation caused by high nutrient loads in combination with the hydrogeographic situation of the Baltic Sea, leading to hypoxic and anoxic conditions, may further reduce the potential for recovery of the Baltic Proper Harbour Porpoise subpopulation. Since 1993, hypoxia development in the Baltic Sea has shown two regimes, and there is indication of a third. The first regime was characterised by a three-fold increase of the hypoxic area until 1999, and the second by a stationary process until 2017 (Kõuts et al. 2021). Data showing that anoxia has reached a new stage in 2018–2019, with anoxic conditions regularly occurring in previously hypoxic areas in the southern basins of the Baltic Proper, could be the beginning of a new trend (Hansson et al. 2019).

This subpopulation is not used or traded.

The species is listed in Appendix II of CITES.

The species is listed in Annex II and IV of the Habitats Directive (Council Directive 92/43/EEC), which require that Special Areas of Conservation shall be designated and that a system of strict protection shall be established for the species. The Harbour Porpoise is listed as a protected species in 18 Natura 2000 sites completely or partially east of 13°E, i.e. east of the tentative management border of the Baltic Proper Harbour Porpoise population during November-April (European Environment Agency 2022). In three of these sites, the Baltic Proper Harbour Porpoise subpopulation’s presence is listed as non-significant, which implies that no management plan is required for the species. Four of the 18 sites are within the proposed May-October management area of the Baltic Proper subpopulation (Carlén et al. 2018), namely Ławica Słupska (PLC990001), Ostoja Słowińska (PLH220023), and Zatoka Pucka i Półwysep Helski (PLH220032) in Poland, and Hoburgs bank och Midsjöbankarna (SE0330308) in Sweden. While a management plan exists for Hoburgs bank och Midsjöbankarna since 2021, management plans are still needed for the other three areas. Despite the lack of a management plan, some fisheries regulations are in place in Zatoka Pucka i Półwysep Helski.

In July 2019, Regulation (EU) 2019/1241 on the conservation of fisheries resources and the protection of marine ecosystems through technical measures entered into force. With regards to Harbour Porpoises, the objective of the regulation is to ensure that bycatch is minimised, and where possible eliminated, so that it does not represent a threat to the conservation status of the species. The regulation includes measures prohibiting the use of drift nets in the Baltic Sea. It also provides for mandatory use of pingers (acoustic deterrent devices) in gillnet and other entangling fisheries for vessels of 12 m length or more in specified areas in the southern Baltic Sea, and cetacean bycatch monitoring in some trawl and net fisheries on vessels of 15 m length or more. However, the vast majority of the fishing vessels in the Baltic Sea using static nets are less than 12 m in length. The regulation states that the Member States shall take necessary steps to monitor and assess the effects of acoustic deterrent device use over time. Further, for vessels flying a given State’s flag, additional measures or restrictions may put in place to minimise, and where possible eliminate, bycatch of Harbour Porpoises and other listed species. Parts of the regulation also apply to recreational fisheries.

Since July 2019, there has been a ban on fishing for Cod in the Baltic Sea due to a collapse in Cod stocks. The ban was regulated by Commission Implementing Regulation (EU) 2019/1248 until the end of 2019, followed by Council Regulation (EU) 2019/1838 from January 2020. The regulations prohibit EU vessels from fishing for Cod in ICES subdivisions 24–26, with the exception that vessels of less than 12 m length are allowed to fish with several gear types in waters less than 20 m deep. These regulations are not intended explicitly to protect Harbour Porpoises, but they should have the effect of reducing the risk of Harbour Porpoise bycatch in gillnets set for Cod in the southern Baltic Proper.

In May 2020, ICES published a Special Request Advice on emergency measures to prevent bycatch of Common Dolphins (Delphinus delphis) and Baltic Proper Harbour Porpoises in the Northeast Atlantic (ICES 2020b). In the advice, ICES (2020b) concluded that to meet the management objective of achieving a bycatch level below the PBR limit for porpoises (< 0.7 individuals per year; North Atlantic Marine Mammal Commission and Norwegian Institute of Marine Research 2019), all fisheries of concern should be closed. To significantly reduce the bycatch risk, but not expecting to reach the PBR limit, a set of five measures was recommended to be implemented as a whole.

In February 2022, the Commission Delegated Regulation (EU) 2022/303 was published. This regulation amends Regulation (EU) 2019/1241 and is aimed at reducing bycatch of the Baltic Proper Harbour Porpoise subpopulation. The regulation implements several, but not all, of the bycatch mitigation measures recommended by ICES (2020b). The regulation prohibits all fishing except with pots, traps and longlines at the Northern Midsea Bank, an offshore bank with the highest acoustic detection rates of Baltic Proper Harbour Porpoises, and prohibits static net fishing in ten Natura 2000 sites and an additional area of importance for the Baltic Proper Harbour Porpoise subpopulation. It also includes measures providing for mandatory use of pingers in two Natura 2000 sites and an adjacent area. Some of the measures are seasonal. The closures entered into force in February 2022, and the mandatory pinger use in June 2022.

A major difference between the set of bycatch mitigation measures recommended by ICES (2020b) and the Commission Delegated Regulation (EU) 2022/303 is that while ICES recommended prohibition of the use of static nets without the simultaneous use of pingers within the seasonal management areas, the EU regulation does not require this. The EU Member States could not agree to large-scale pinger use due to military considerations, leaving the Baltic Proper Harbour Porpoise subpopulation without any directed bycatch mitigation measures in place in the majority of its range.

In 2023, the EU published a new Action Plan on protecting and restoring marine ecosystems for sustainable and resilient fisheries calling on Member States to adopt national measures or submit joint recommendations to the Commission to minimise bycatch (or reduce it to the level that enables the full recovery of the populations) by the end of 2023 for harbour porpoise in the Baltic Proper and the Black Sea, the Iberian Atlantic and the common dolphin in the Bay of Biscay.

In 2016, ASCOBANS adopted a revised recovery plan for Baltic Harbour Porpoises, called the Jastarnia Plan (ASCOBANS 2016). In 2020, a resolution on the conservation of the Baltic Proper Harbour Porpoise subpopulation was adopted (ASCOBANS 2020).

A descriptive evaluation of whether Harbour Porpoise bycatch is below mortality levels that enable the species to reach good status in the Baltic Sea is given in a HELCOM Core indicator (HELCOM 2023b). The mortality limit for the Baltic Proper Harbour Porpoise subpopulation is set to zero, and good status is not achieved. In 2020, HELCOM adopted a revised recommendation on protection of Harbour Porpoises in the Baltic Sea area (HELCOM 2020). In 2021, an update of the HELCOM Baltic Sea Action Plan was published, including measures to address knowledge gaps on all threats to the Baltic Proper Harbour Porpoise subpopulation, identify and implement mitigation measures for threats other than bycatch, and to develop, implement, promote and evaluate bycatch mitigation measures (HELCOM 2021).