The increased number of juvenile and newborn harbour porpoise strandings along the German Baltic coasts in summer provides evidence for a possible local calving and nursing ground in this area. Calving and nursing habitats are the most important areas for management purposes.
This project aimed to describe residence patterns and diet of females and calves along the German Baltic coast, in order to achieve effective conservation measures. Nitrogen, carbon and, for the first time, sulfur stable isotopes (δ13C, δ15N and δ34S) were analysed in coastal species of fish and invertebrates in spring, autumn and winter 2015/2016 and in four different areas, to assess seasonal and geographical variation of porpoise habitat use. The structure of ecological niches was inferred within and between communities using the SIBER model, and was compared to results from stomach content analysis.
N, C and S values of fish and invertebrates strongly varied, seasonally and geographically, as a consequence of environmental factors characteristic of each sampling area. Additionally, each species presented large isotopic variability, suggesting possible intraspecific dietary specialisations. Females δ13C and δ34S signatures integrated both open-waters and coastal feeding, while juveniles presented a more coastal distribution. The absence of difference in δ15N values between age classes confirmed persistence of milk assimilation from the mothers. Niches structures and overlaps and stomach content analysis suggested gobies (Potamoschistus spp and Neogobius spp in particular), crabs and small coastal fish (ex. three-spined stickleback) as the main preys of juveniles. The integration of all three stable isotopes together permitted to well delineate even the highly dynamic food web of the Baltic German waters, confirming the use of sulfur isotopes in marine ecological studies.