Distribution and density of harbour popoises in Dutch North Sea waters

Summary

Aerial surveys were conducted to investigate harbour porpoise density and distribution in Dutch waters. Surveys were conducted following standard line transect distance sampling methodology on tracklines providing a representative coverage of the study area which ranged from the Dutch coast to about 120 km offshore thus covering about half of the Dutch EEZ. Within this overall study area two sub-areas were defined. One sub-area (from Texel to the German border) was covered in on 29 November 2008 and 3 April 2009. Density was 1.02 animals per km² during the November survey and 0.52 animals per km² during the April survey. An overall survey, covering both sub-areas, was conducted on 3 February, 18 March and 3 April 2009. The resulting density was 1.12 animals per km². This corresponds to an estimate of harbour porpoise abundance for this study area of 36 825 animals (95% C.I. 19 090 – 68 130; 0.33 C.V.). Distribution patterns of porpoises in the surveys waters were patchy and seemed variable between surveys. Applying this abundance estimate, a range of potential mortality limits were calculated for the Dutch harbour porpoises.