Use constraints: Data can be used freely given that the source is cited (following creative commons license CC-BY). The source should be cited as: “HELCOM HOLAS 3 Dataset (2023).

The waters included in HOLAS-III (identical to the HELCOM area) are inhabited by two separate populations of harbour porpoises, namely the Belt Sea population primarily inhabiting the Kattegat, the Belt Seas and the Western Baltic, as well as the Baltic Proper population primarily inhabiting the Baltic

Proper. As a result of the differences in conservation status, and the different amounts and types of data available on distribution for each of the two populations (in the Belt Sea area and Baltic Proper), they were handled separately when preparing the HOLAS-III maps. Furthermore, due to the huge difference in the abundance of the two populations, it was decided to change the HOLAS-II distributions maps that showed the relative density of porpoises, to maps of areas of importance for harbour porpoises. This way, the critically endangered Baltic Proper population would not weigh less in the HOLAS-III assessment, simply due to its low abundance. This change meant that new categories needed to be defined, and these are now defined as “higher” importance, “medium” importance, “lower” importance and “no/limited data”. These categories were chosen due to concern about the lowest category for the Baltic Proper population, and to underline that the categories are relative and not absolute. The low abundance of this population is of such high concern, that each individual must be considered of high importance and since individual porpoise detections/incidental sightings do occur in basically all of the Baltic Sea (east of the Belt Sea population management unit), it could be argued that all of the Baltic Sea (the Baltic Proper, the Gulf of Finland and the Gulf of Bothnia) is of high importance and certainly not of low importance. Furthermore, the most comprehensive study in the Baltic Proper, the SAMBAH project, is now rather old since data collection took place in 2011-2013 (Carlén et al. 2018, Amundin et al. 2022), therefore our knowledge of the distribution may be outdated and areas of importance may have changed. Consequently, the old HOLAS-II categories “high” and “low” were renamed as “higher” and “lower” importance.

The importance map for the Baltic Proper harbour porpoise population was based on the SAMBAH results, where the probability of detection ≥ was considered as medium importance and ≥ as higher importance. Data from national monitoring programmes and research projects were used to complement the SAMBAH data in areas where this type of data could provide better detail, given the comparably low spatial resolution of the SAMBAH data. Areas of higher and medium importance were added through expert judgment based on these data.

The maps for both harbour porpoise populations were merged into one importance map for the species (Figure 3.1). The delimitation between the two populations was placed at 13.0 ̊E based on the same reasoning as in the ICES advice (2020). The border between population areas is sharply visible in the final map. This does not reflect a true sharp border of the importance for harbour porpoises, however with the current knowledge of population distribution, this is the best possible option. It is the opinion of all the experts that participated in creating this map, that the combined map of important areas for the two populations is the best solution for illustrating the distribution since we have no or limited information on the presence and distribution of the Baltic Proper porpoises in the Belt Sea and vice versa.

Source: https://dce2.au.dk/pub/TR240.pdf

Data sources:

Telemetry, passive acoustic monitoring and visual surveys from research projects for the Belt Sea. Passive acoustic data from the SAMBAH project (data collected between May 2011 and May 2013), research projects (Hel Marine Station, University of Gdańsk in years 2013-2018) and national monitoring data (data collected from the Polish State Monitoring Programme between 2016 and 2018 and from the Finnish national monitoring programme since 10/2016) for the Baltic Proper.

The levels of importance are based on:

Belt Sea: Data from telemetry and visual surveys.

Baltic Proper: Passive acoustic monitoring (SAMBAH) in combination with national expert judgment based on information obtained from the Finnish national passive acoustic monitoring program and passive acoustic research projects in Polish coastal waters.

Raster representing abundance class with following values:

1 = Higher importance, 50% isopleth (Belt Sea) and areas of ≥ 20% probability of acoustic detection (Baltic Proper) =

0.5 = Medium importance, 75% isopleth (Belt Sea) and areas with a detection probability of 10-20% (Baltic Proper)

0 = Lower importance, areas outside isopleths (Belt Sea) and the rest of the Baltic Sea (Baltic Proper)

No or limited data (shapefile) = no data from SAMBAH or other monitoring studies