Report of the International Whaling Commission (1996)


This paper reviews and summarises published and unpublished information on harbour porpoises in Irish waters and presents results of recent research. Harbour porpoises have been recorded from all Irish coasts and have always been considered a coastal species. However, information from bycatch data suggests that it also regularly occurs offshore. Stranded and bycaught animals were used to examine reproduction, composition of prey species in the stomach contents and contamination of total and methyl mercury, organochlorines and radionuclides. Preliminary analysis shows that males reach sexual maturity at a length of between 1.3 and 1.4m. All females examined were immature. The most frequently recorded prey type was Trisopterus spp., followed by whiting Merlangius merlangus, poor cod T. minutus and herring Clupea harengus. Cephalopods recorded included Loligo forbesi and sepiolids. Prey remains from the stomachs of bycaught and stranded animals were similar, with fewer Clupidae and whiting recorded from the bycaught animals. Bycaught animals were not found to be feeding on the target species of the fishery they were caught in. Contamination levels of total and methyl mercury, 17 organochlorincs and Cs-137 and K-40 were compared with porpoises from other geographical regions. Metal and organochlorine levels were generally lower than those recorded from other studies. Geographical differences in concentrations of Cs-137 were found, with samples from the Irish Sea having the highest concentrations. A review of published and unpublished records of incidental capture revealed 43 records of harbour porpoise bycatch in Irish waters. Most porpoises (98%) were caught in gillnets with 26 (63%) of these being caught in static gillnets and 12 (29%} in tangle nets. As part of an observer-based study of marine mammal bycatch in a gill net fishery in the Celtic Sea, it was estimated that the total annual bycatch of harbour porpoises was between 1,825 and 2,049 (95% CI 657 – 3,361) for the combined Irish and UK fleets.