Harbour porpoises and common dolphins are being persistently contaminated by pollutants in the Irish marine environment, according to a new study partly funded by the Heritage Council and the Marine Institute's Fisheries Research Centre.
Research on blubber and liver tissue taken from 12 harbour porpoises (Phocoena phocoena) and eight common dolphins (Delphinus delphis) has identified the pollutants.
Female harbour porpoises carried the highest levels of organochlorine (OC) pesticides in blubber, while it was most concentrated among male dolphins in the liver.
The study was carried out by Ms Maria Smyth, who did a thesis on the subject, in collaboration with Dr Eugene Nixon of the Marine Institute's Fisheries Research Centre in Dublin, Dr Simon Berrow of the Shannon Dolphin and Wildlife Foundation in Kilrush, Co Clare, and Dr Emer Rogan of the Aquaculture Development Centre at the Department of Zoology and Animal Ecology in University College, Cork (UCC).
The marine mammal tissue was made available to the researchers, after being caught accidentally in fishing nets in Irish waters. The value of this co-operation with fishermen is highlighted in the paper, as the time and cause of death are already known when the animal is examined. "Strandings just wouldn't give us this information, and we really appreciate the help given to us on this," Dr Berrow said.
The study identified concentrations of 11 organochlorine pesticides and 10 individual polychlorinated biphenyls (PCB) in the blubber and liver of the animals.
The predominant DDT metabolite had broken down into pp-DDE, which means the DDT inputs were historic, according to Dr Berrow. However, the ratios of DDT to DDE suggested there are limited new sources of DDT in the Irish marine environment, even though DDT is now banned.
Concentrations of ICES 7 PCB - the seven standard PCBs - were similar in both species. Contaminant levels were generally similar to those reported from Scotland, but lower than reports from Scandinavia.
Comparisons between species and locality are further complicated by the fact that concentrations of pesticide residues may vary with a range of physiological factors, including age, sex, maturity and body condition.
The paper says the prevailing westerly weather and currents off the Irish coast suggest that pollutant burdens should be among the lowest in Europe.
Previous studies on Irish mammal species are scarce. In 1998, research on a group of white-sided dolphins stranded in Killala Bay, Co Mayo, found concentrations of organochlorines similar to those recorded in other studies of this species in the Atlantic - suggesting that organochlorine contamination is ubiquitous, and is absorbed from different areas.
The paper points out that the role of persisent organochlorines in the health of marine mammals is uncertain, but there has been growing concern about the impact of these and other pollutants since an article in Nature in 1986 found a link between PCBs and reproductive failure in common seals.
The death from a morbilli virus of hundreds, if not thousands, of striped dolphins in the Mediterranean in 1990 to 1992 was believed to have been precipitated by elevated PCB concentrations in the dolphins' blubber, and a number of studies have shown elevated levels of organochlorines in a wide range of cetacean species.
Polychlorinated biphenyls and organochlorines in caught harbour porpoises and common dolphins from Irish coastal waters by Maria Smyth, Simon Berrow, Eugene Nixon and Emer Rogan is published in Proceedings of the Royal Irish Academy, Volume 100B, December 2000.