The semi-enclosed tidal bay Eastern Scheldt in the Netherlands: porpoise heaven or porpoise prison?

Abstract

Harbour porpoises (Phocoena phocoena), the smallest of cetaceans, need to consume quantities of prey that amount to ca. 10% of their own body mass per day. They mostly feed on small fish, with the main prey species differing geographically. The δ¹³C muscle signature of harbour porpoises sampled in the Eastern Scheldt, SW Netherlands, has indicated that animals tend to stay here for some time after they entered this semi-enclosed basin, and that they thus must feed on local prey. A relatively low primary production and low local fish biomass raises the question what there is for harbour porpoises to feed on in the Eastern Scheldt. This study reveals that there are no big differences between biological or stranding parameters of harbour porpoises found dead in the Eastern Scheldt compared with the adjacent North Sea (the “Voordelta”), but some differences in diet were found. Still, despite the low fish biomass in the Eastern Scheldt, no evidence of excessive harbour porpoise starvation was found. The main prey species for juvenile porpoises, both in the North Sea and in the Eastern Scheldt, were gobies. Gadoids were important prey for adults in both regions. Gadoid prey was supplemented by gobies and sandeels in the North Sea, and by squid and estuarine roundfish in the Eastern Scheldt. Our results demonstrate that harbour porpoises that stay in the Eastern Scheldt for a longer period of time may develop specialised feeding skills, to cope with the relatively poor prey base. Juveniles on the other hand, must settle for small and lean prey (gobies and small sepiolids) and may face competition from adults.