Crayfish plague hits the Shannon

  12th November 2018

An outbreak of crayfish plague has been confirmed on the River Al, a small tributary of the River Shannon in Athlone, Co. Westmeath. A small number of dead white-clawed crayfish (Austropotamobius pallipes) were reported on the river last week and subsequent DNA analysis by the Marine Institute has confirmed they died from crayfish plague (Aphanomyces astaci) , a highly infectious fungal infection which causes 100% mortality in this declining native species.

The fungus attaches to thin areas of outer shell (cuticle) as a spore and then grows through the tissues, leading to death in all cases within 2-3 weeks. The swimming spores then transmit directly from the infected or recently dead crayfish to other areas downstream. Spores are easily spread accidentally by anglers and other water users (e.g. canoes, kayaks, boats) through wet gear, and might also be spread inadvertently by water birds and other organisms. The fungus affects the normal vision of the crayfish and so it may be seen crawling around in broad daylight where it usually wouldn’t, although dead crayfish are the obvious sign of infection. Crayfish plague originated in North American crayfish and it is their introduction to Europe which has caused many white-clawed populations to decline. To date no non-native crayfish have been discovered in Irish waters.

“Crayfish are considered a vital part of healthy ecosystems where they occur, recycling organic matter and nutrients as well as providing a rich food source for a whole host of animals, including fish”

White-clawed crayfish, which are native to Irish waters, have suffered widespread declines through their European range in recent decades and are offered strict legal protection as an Annex II species under the EU Habitats Direction as a result. The species is also afforded protection under the Irish Wildlife Act (1976, amended 2000). Ireland represents one of the last remaining strongholds for the species. Crayfish are considered a vital part of healthy ecosystems where they occur, recycling organic matter and nutrients as well as providing a rich food source for a whole host of animals, including fish.

The Al River rises in the townland of Crosswood approximately 3km east of Athlone and flows in a south westerly direction for a distance of approximately 5 km eventually entering the river Shannon downstream of the weir at Golden Island. As with other infected sites, it is unclear at present how the plague was been introduced to the river but this outbreak has intensified fears for the conservation of the species in Ireland’s major river system.

Ireland was officially considered free from crayfish plague until 2015, when an isolated case was identified in County Cavan. Last year (2017) large-scale outbreaks occured on the rivers Suir, Barrow, Limerick Deel and Lorrha (Tipperary). Regulatory authorities were heavily criticised for not doing more to prevent further spread of this deadly virus, with waterside signage and voluntary bans on water sports and angling at infected sites the only measures implemented. Public awareness campaigns on the issue were deemed relatively ineffective by many stakeholders. This most recent outbreak in a Shannon tributary is yet another reminder of how fragile our aquatic ecosystems are and that all anglers and water users need to be extra vigilant in the cleaning (hot water), disinfecting and drying of their wet gear/equipment before moving between water bodies (please see poster below). If you observe any peculiar crayfish behaviour then please contact the NPWS, Waterways Ireland, Inland Fisheries Ireland or this magazine immediately.

Further analyses are still ongoing to establish if there may be any links between the River Al and previous outbreaks of crayfish plague.

Learn more about why crayfish are so important to aquatic ecosystems and fish HERE

For more advice on preventing crayfish plague see HERE