Feeding ecology of harbour porpoises: stable isotope analysis of carbon and nitrogen in muscle and bone

Abstract

Harbour porpoises are the most common small cetaceans in the North Sea and Dutch coastal waters. To study their trophic level and feeding location, stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) were analysed in muscle and bone samples collected from 157 porpoises stranded along the Dutch coast (2006–2008). In addition, samples from 30 prey species were analysed. Prey samples showed high δ¹⁵N values in species of higher trophic level. In addition, geographic differences in isotopic composition were found, with higher δ¹⁵N and δ¹³C values in prey from more southern, coastal and estuarine areas. Based on muscle δ¹⁵N values, we found neonatal enrichment and that larger porpoises, in particular males, seem to feed on lower trophic level species, compared to smaller individuals. Also bone δ¹⁵N values show that larger animals had fed on lower trophic levels in distant times. Porpoises from the Eastern Scheldt reveal distinct δ¹³C values in muscle, but not in bone. This shows that these animals had foraged in the Eastern Scheldt for a longer time period but were not born there. Seasonal variation in bone δ¹⁵N and δ¹³C values revealed two distinct groups of porpoises along the Dutch coast, a winter group (mainly males) that migrated from neighbouring regions and a Dutch subpopulation in summer. These results furthered our insight about shifts in trophic level and feeding location of harbour porpoises from the southern North Sea over time.

Keywords:

• Phocoena phocoena
• δ¹³C
• δ¹⁵N
• trophic ecology
• The Netherlands

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

Samples were collected by staff and volunteers of the Dutch strandings network, coordinated by the National Museum of Natural History (now NCB Naturalis) in Leiden. The authors would like to thank M.F. Leopold (IMARES), C.J. Camphuysen (NIOZ), T. Jauniaux (Liège University), A. Gröne (Utrecht University) and L. Wiersma (Utrecht University) for organizing and executing the post-mortem examinations that have provided samples and information on the animals. K. Das and G. Lepoint are F.R.S.-FNRS Research Associates. L. Michel was a F.R.S.-FNRS Research Fellow. We would like to thank G.O. Keijl (NCB Naturalis) for providing long-term data of porpoise stranding records and R.S.A. van Bemmelen for creating the map of strandings localities. We also thank C. Smeenk, R.W.P.M. Laane and M.J. van den Heuvel-Greve for their valuable comments on the manuscript. This work was funded by the Dutch Ministry of Agriculture, Nature and Food Quality (LNV), BO Project 4308201019.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark