The Polish fisheries administration registered incidents of harbour porpoises caught by fishermen from fishing bases within the country’s borders between 1922 and 1938. These data are unique in the Baltic Sea region. To date, they only have been known from summary reports that have been cited repeatedly in papers regarding the habitat preference and sizes of historical populations of harbour porpoises in the Baltic Sea. Lacking other sources of information, archival data had a great importance in the delimitation of Special Areas of Conservation (NATURA 2000) designated for harbour porpoise in Poland. Analysis of archival source materials suggested discrepancies between fisheries inspectors’ reports and the published data. These new data provided detailed information on harbour porpoise bycatch, including the time of capture, gear type utilized, and the location of fishing grounds where harbour porpoises were most frequently caught. In summation, fisheries inspectors registered 691 individual harbour porpoises. The animals were registered primarily in the spring, from March to April, during targeted salmonid catches throughout the Gdańsk Bay region and the open sea off the Hel Peninsula coast. The majority of caught animals were recorded from the Hel fishing district. Contrary to conclusions to date, these instances cannot be attributed to the bounties offered for the “elimination of pests.” This paper also addresses and discusses the dependence on the number of registered harbour porpoise catches over a series of years and the maximum ice cover on the Baltic Sea as the factors influencing harbour porpoises’ periodic declines (Teilmann & Lowry, 1996).