Chesapeake blue crab found on Dublin beach ‘threatens’ native species

The National Biodiversity Data Centre believes it may have been released into the wild

Mystery surrounds the arrival of a “Chesapeake blue crab”, one member of a species native to the waters of the western Atlantic Ocean and the Gulf of Mexico, which turned up on Dollymount Strand recently.

While the crab is not much to look at in terms of alien invaders, the National Biodiversity Data Centre has warned it is larger and more competitive than native crabs, and the female can lay up to six million eggs a year.

Once in competition with the smaller Irish native crabs the American version - also known as the American blue crab, would be likely to take over, scientists fear.

The Biodiversity Data Centre said the appearance of the crab on Dollymount strand,where it was photographed last month by Ruth McManus, is the first recorded appearance of the crab on these shores.


How it got here is a bit of a mystery, the centre said. The Chesapeake blue, no doubt used to the warmer more luxuriant waters of the massive Chesapeake Bay, doesn’t really like colder climates, and the centre says it hopes the “Dollymount One” is something of a one-off.

A second report was submitted by a member of the public on March 9th - this time of a blue crab claw which was found on Dollymount Strand. The biodiversity centre is hoping the first and second sightings were of the same crab.

In this instance, the Irish climate may be on our side. Because the Chesapeake blue crabs operate in much warmer waters they tend to go into a torpor in our chilly waters, and do not operate as efficiently, said Dave Wall, citizen science officer with the centre.

“In fact they don’t reproduce in waters that are less than 26 degrees,” Mr Wall told The Irish Times. While it seemed unlikely, Mr Wall did say it was possible that temperatures in rocky pools around the island’s shores could hit 26 degrees, in the course of an Indian summer.

It is not thought the Dollymount One has million of brothers and sisters waiting to be discovered here.

“It is more likely that someone released it, thinking they were doing the right thing,” perhaps after getting it at a market, said Mr Wall.

He stressed people should not release non-native species into the marine eco system. In the warmer waters of the Mediterranean, the Chesapeake crab had virtually taken over, pushing out native crabs as well as species that prey on the native crabs. “So it can disrupt a whole food chain,” he added.

In Ireland US lobsters - which again are bigger then the native examples - have already started to be picked up in lobster pots by fishers and the biodiversity centre is worried about the impacts there.

The cautionary tale is exemplified by the near eradication of the red squirrel in some areas, by the non-native grey squirrel. The grey squirrel was introduced into Ireland in 1911 when six pairs were released at Castle Forbes, Co Longford.

The National Biodiversity Data Centre which collects and analyses data on Ireland's biological diversity, has asked anyone who thinks they have seen a blue crab to photograph it and sent the photo to the centre - at - for verification.

“But never, ever release a non native into the wild”, said Mr Wall.

Tim O'Brien

Tim O'Brien

Tim O'Brien is an Irish Times journalist