Friday, 15 March 2013

Porpoises Have to Be Careful in the Eastern Scheldt in the Netherlands

Mar. 7, 2013 — The unexpected conclusion of the doctoral research project on the feeding ecology of porpoises by Okka Jansen at Wageningen University is that the Eastern Scheldt in the Netherlands may be an ecological trap. She also discovered that more than just examining stomach contents is required to find out what porpoises eat. The analyses of stable isotopes and fatty acids provide different and more complete data.

Coastal waters are rich marine ecosystems. Porpoises are the most common small cetacean species in the Netherlands' coastal waters. The numbers and distribution of porpoises have considerably changed in recent decades. The question is: what is driving these changes? Within this context, doctoral candidate Okka Jansen studied the feeding ecology of the porpoise in the Netherlands' coastal waters. 

"Most studies of the feeding ecology of porpoises are based on examinations of stomach contents," says Okka Jansen. "But this method falls a long way short of the mark in telling us everything about what they eat. I performed additional research using two other techniques: analysis of stable isotopes and analysis of fatty acids in porpoises and their prey. These three methods combined give the most accurate reflection of the diet of porpoises." The analysis of stable isotopes (from bone and muscle tissue) mainly provides information about the location in which and how high in the food chain the animals have foraged. Fatty-acid analysis indicates the most probable composition of the diet of porpoises in recent months.

"The combination of these three techniques is unique in sea-mammal research," says doctoral thesis supervisor Prof Peter Reijnders. "Examination of the stomach contents only enables you to see recent food ingestion, whereas fatty-acid analyses allow you to trace back what animals have eaten over a slightly longer period, and analyses of stable isotopes (extracted from bone tissue) indicate what they have eaten over a much longer period. In this way, Okka's research showed that the diet of porpoises in the Netherlands' coastal waters consists of inshore fish species, such as gobies, smelt, and dragonet as well as schooling species from deeper waters further from the coast, which were eaten a longer time ago, such as mackerel and herring. The porpoise is in essence an opportunistic predator. By contrast, the white-beaked dolphin, which she also studied, turned out to be much more of a specialist; with the exception of extremely young animals, the staple part of their diet always consists of cod and whiting, irrespective of their habitat."

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