By catch, as an indicator of pressure, is based on the number of animals found dead by fishermen in nets. The data is collected by the Sea Fisheries Institute - National Research Institute in Gdynia (MIR-PIB). By-catch refers directly to the trends calculated from data for the last 10 years. Since 2006, there has not been recorded a single case of by-catch of the harbour porpoise. Additionally in Poland, from 2011 to 2016, a total of 22 porpoise reports from by-catch were recorded (data from 2011-2015 based on the WWF report, data from 2016 based on SMIOUG data). It has been assumed that by-catch at the level of one individual is a significant threat to the porpoise population in this region (Harkonen et al. 2013, HELCOM 2017d). Since there are no threshold values agreed on a regional level the assessment of criterion D1C1 was not assessed.

In HELCOM, pressures on mammals regarding by-catch were not parametrized as indicators. Since 2006 4 individuals of grey seal caught in 2011-2016 were reported (MIR-PIB 2017). This can be attributed to a small number of reported, caught animals in relation to the high fishing effort. The number recorded in the report is in contrast to the available data collected in the WWF/SMIOUG database (www.fokarium.pl) - in the period 2011-2017 more than 200 dead seals were found on the beaches, some of which had traces indicating the possibility of by-catch. In the case of porpoise, the by-catch was defined as a critical threat to the species, in particular for the Baltic Sea population. Since there are no threshold values agreed on a regional level the assessment of criterion D1C1 was not assessed.