The Länsstyrelsen and MCR team conducted a visual and acoustic vessel-based line-transect survey for the presence and distribution of marine mammals over Hoburg’s Bank and Midsjöbankarna from 8th to 16th August 2022. The survey incorporated 1,692 km of on-track acoustic effort over this region of the Baltic Proper, of which 830 km also included visual effort.

All of the planned transects were surveyed with at least acoustic effort. Acoustic surveys were conducted using an array of hydrophone elements towed 140 m behind the survey vessel at 5-8 knots; detection of candidate porpoise clicks was conducted using the software package PAMGuard and validated by a human observer. Visual surveys were conducted from the foredeck of the vessel during daylight hours in suitable sea state conditions (sea state 3 or less) by two observers with approximate eye heights of 3.5 m above sea level. There were 10 sightings of grey seals but no harbour porpoises were seen. There were at least 12 acoustic detections of harbour porpoises.

When considering only ‘tracks’ (i.e. a sequence of ≥ three clicks with a clear bearing trail), the density of porpoises was estimated as 1.2 porpoises 1000 km-2 (95 % CI 0.3-4.4, CV = 75 %) throughout the study area; this rose to 2.5 porpoises 1000 km-2 (95 % CI 0.6-10.2, CV = 80 %) when incorporating a correction for those animals likely to have been missed. These densities applied to the whole survey block derived abundance estimates of 22 porpoises (95 % CI 6-82), rising to 47 porpoises (95 % CI 12-191), when a correction for availability was applied. It should be noted the high coefficients of variation led to the correspondingly high uncertainty as reflected in the wide confidence intervals.

Although direct comparison with previous studies is challenging, the density estimates presented here broadly align with those of the SAMBAH project for the Baltic Proper (0.5-8.3 porpoises 1000 km-2 for May-Oct). The 2022 survey was limited in scope and duration but provides a baseline upon which further surveys could build. Repeated surveys would improve the confidence of these initial estimates and allow effective monitoring of changes to local porpoise density over time.