Fish with male and female sexual features have been discovered on the Shannon system which may be linked to chemicals emanating from sewage treatment plants.
The discovery of simultaneous male and female germ cells in the gonad of individual fish has been confirmed in populations of roach (Rutilus rutilus) on three Shannon rivers.
Populations of brown trout (Salmo trutta) sampled by a team from Athlone Institute of Technology have shown signs of delayed spermatogenesis or late maturation of sperm.
In both cases, the irregularities were found in fish located downstream of sewage treatment plants, according to the Athlone team of Dr Cepta Broughan, Mr Jim Roche, Dr Eileen Lane and Dr Andy Fogarty.
Research students Colm McGee, Dr Sarah Brennan and Dr Miriam Kelly-Kirwan assisted with the seven-year study on 11 rivers on the Shannon system. Fish are sensitive indicators of pollution, and research abroad has established links between chemical pollution and adverse reproduction in wildlife.
Dr Andy Fogarty said endocrine disrupting chemicals (EDCs) can cause negative impacts, ranging from delayed spermatogenesis to intersex fish. EDCs can be found in one of the active ingredients of the contraceptive pill, in phtalates such as plastisicers, in organochlorine pesticides and in certain ultra-violet filters, such as sunscreens and certain cosmetics.
The Athlone team haven't located a precise source for the contamination, but EDCs are a by-product of secondary sewage treatment plants - and such links have been discovered in Britain and North America.
Advanced tertiary treatment plants can be designed to extract EDCs during water treatment, but the majority of sewage treatment plants currently in commission are of secondary level only.
The Athlone team worked with the Shannon Regional Fisheries Board on the project, using electrofishing techniques. The group presented its findings to the board earlier this week in Portumna, Co Galway.
Dr Fogarty said that the ecological consequences of the findings still had to be examined, but even delayed spermatogenesis in trout was a "very serious development".
Roach tended to be very tolerant of de-oxygenated water, which could be found below sewage plants, and this explained why the more pronounced changes were identified in this species.
"Experience abroad is that we will have to sample many more trout to determine precise changes with this species," he said. It was also unclear yet if the development was affecting actual fish populations, he said.