Very fishy tales, but true ones

THE fact that our lakes and rivers still contain many biological surprises was emphasised recently when it was announced that freshwater jellyfish had been discovered in Lough Derg and Lough Erne.

Tese small jellyfish, the cap is about the size of a euro coin, are natives of the Yangtze River in China but were accidentally introduced into Europe over a century ago.

This is the first time they’ve been found in Ireland, though it’s quite likely they’ve been here for some time. Jellyfish have a life-cycle that involves them living for long periods in something akin to a larval form, called a polyp. In order for this species to develop into a free-swimming jellyfish it needs water temperatures in excess of 25 degrees over a considerable period of time. It was the warm summer that made this happen, and it may not happen again for some years. They’re too small to sting humans and the biologists don’t believe they pose any significant threat to the ecology of our waterways.

The waters of Lough Leane, in the Killarney National Park, are the home of another biological oddity, the Killarney shad. This small fish is not found anywhere else in the world. Its existence has been known for a long time but it has been rather neglected by the scientific community. But a team from the University of Salford, Greater Manchester, working with a number of Irish researchers, has recently published a genetic study of Killarney shad which answers many of the outstanding questions about them.

There is a species of fish called the thwaite shad which spawns in freshwater and feeds at sea. There are still a few that spawn in Irish rivers, notably the lower Barrow, but they have become rare here and throughout their range across northern Europe. In the past they spawned in the River Laune, which connects Lough Leane to the sea. But twice, once 16,000 years ago and once 7,000 years ago, something blocked the river and trapped the fish. Those inter-bred and evolved into a new species that could survive without returning to the sea to breed.

This unique Irish fish species faces some threats and its conservation should be given a very high priority.