Harbour porpoise Phocoena phocoena abundance in the southwestern Baltic Sea

Abstract

Harbour porpoise Phocoena phocoena is the only cetacean species resident in the Baltic Sea. At least 2 different subpopulations occur in this area, with a presumed demarcation line in the western Baltic Sea. Aerial surveys were conducted during different seasons in the years 2002 to 2006, to obtain estimates of porpoise abundance for the southwestern Baltic. Within the survey area, 3 strata were defined. In total, 46 surveys were completed for these strata. Abundance estimates and corresponding confidence intervals (CI) were calculated for each stratum separately for each survey. Density between areas varied seasonally and spatially, with the western study area (Kiel Bight) generally showing the highest density. Whenever all 3 strata were covered within 25 consecutive days, abundance for the overall survey area was calculated. Ten surveys met these criteria. Overall abundance in the western Baltic varied between surveys, with the lowest value in March 2003 (457 ind.; 95% CI: 0 to 1632), and the highest estimate in May 2005 (4610 ind.; 95% CI: 2259 to 9098). The results of all other surveys were in the range from 1352 to 2905 ind., with largely overlapping confidence intervals. Applying the only available bycatch estimates (which include no measure of precision) currently available for the German Baltic region, the percent of porpoise bycatch in the western Baltic lies within a range of 1.78 to 17.94% of the local population. To increase the precision of bycatch rates, more detailed bycatch estimates for this region are needed. Nevertheless, the results indicate that bycatch is a major threat to harbour porpoises throughout the western Baltic Sea.