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The diet of harbour porpoises bycaught or washed ashore in Belgium, and relationship with relevant data from the strandings database
Haelters, J.; Kerckhof, F.; Toussaint, E.; Jauniaux, T.; Degraer, S. (2012). The diet of harbour porpoises bycaught or washed ashore in Belgium, and relationship with relevant data from the strandings database. Royal Belgian Institute of Natural Sciences. Management Unit of the North Sea Mathematical Models. Marine Ecosystem Management Unit: Oostende. 46 pp.

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Keywords
    By catch; Diets; Fish; Fishing gear; Prey; Stomach content; Stranding; Phocoena phocoena (Linnaeus, 1758) [WoRMS]; ANE, Belgium, Belgian Coast [gazetteer]; Marine

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  • Haelters, J., more
  • Kerckhof, F., more
  • Toussaint, E.

Abstract
    The harbour porpoise Phocoena phocoena is currently the most abundant marine mammal in Belgian waters. Large-scale changes in the distribution of this top predator in the North Sea have occurred during the last decades, possibly caused by changes in food availability. An analysis of the strandings database 1970-2011, containing data on 737 harbour porpoises, revealed that throughout the year two peaks in strandings occurred: one in spring, for an important part caused by the strandings of animals incidentally bycaught in fishing gear, and a second one during summer. Most of the stranded animals were juveniles, with a higher percentage of males than females. As could be expected, bycaught animals were on average heavier than equally sized naturally died animals, and they had a thicker blubber layer. In naturally died animals the blubber layer was thicker in winter than in summer, illustrating the insulating function of the blubber layer besides its function as an energy storage.

    We investigated the stomach content of 64 harbour porpoises washed ashore or bycaught in Belgium between 1997 and 2011. Ten of the stomachs were empty. Fish contributed to most of the prey remains. In total we found the remains of 19 fish species belonging to 10 families. The numerically most important prey items in juveniles were gobies (Gobiidae). Reconstructing the original weight of the prey items revealed that gobies constituted the most important prey by weight, but that larger sandeels (Ammodytidae) and to a lesser extent gadoids (Gadidae) were also important. In adults the majority of prey items were gobies and sandeels, but the reconstructed weight of the stomach content revealed that sandeels and gadoids constituted by far the most important prey. Surprisingly, clupeids (Clupeidae) did not contribute much to the diet, although the return of the harbour porpoise is often linked to an increase in herring Clupea harengu stocks in the southern North Sea. Also no twaite shad Alosa fallax were found, although this diadromic fish is common again in the area. A small number of smelt Osmerus eperlanus was found in some of the recently stranded harbour porpoises. Smelt is a diadromic fish of which densities are increasing from very low levels. The fact that juvenile harbour porpoises had apparently fed on large quantities of small bottom fish may help to explain why they were more prone to bycatch than adults. The analysis revealed that a gradual shift occurs in the feeding habits of harbour porpoises while becoming adults: from small benthic fish towards larger fish taken from the water column.

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