Population structure of harbour porpoises in the Baltic region: evidence of separation based on geometric morphometric comparisons

The harbour porpoise is seriously depleted and threatened with extinction in the Baltic Sea. It is usually assumed that Baltic porpoises form a separate population unit, although the evidence for this has been disputed lately. Here, a 3-D geometric morphometric approach was employed to test a number of hypotheses regarding population structure of the harbour porpoise in the Baltic region. 277 porpoise skulls from Denmark, Sweden, Finland, Germany and Poland were measured with a suite of 3-D landmarks. Statistical analyses revealed highly significant shape differences between porpoises from the North Sea, Belt Sea and the inner Baltic Sea. A comparison of the directionalities of the shape vectors between these units found differences that cannot be attributed to a general, continual shape trend going from the North Sea to the inner Baltic Sea. These vectors indicate a morphological adaptation to the specific sub-areas. Such adaptation may be the result of the topographic peculiarities of the area with variable topography and shallow waters, e.g. in the Belt Sea porpoises, there may be a greater reliance on benthic and demersal prey. The present results show that isolation by distance alone is an unlikely explanation for the differences found within the Baltic region and thus support previously reported molecular indications of a separate population within the inner Baltic Sea.