fisheries scienceWhite-clawed crayfish & crayfish plague

 Issue 19 (Nov-Dec 2017)   Off the Scale magazine


It’s fair to say that many anglers and water lovers know very little about crayfish in general. They are usually quite cryptic and shy in their behaviour and most only ever get fleeting glances. In fact, many people are totally unaware that we have these crustaceans – which can look to some like mini-lobster – in our waterways at all! But, as you will read here, they play vital roles in our freshwater ecosystems and really are very important.

In Ireland we have a single species, the white-clawed crayfish (Austropotamobius pallipes), one of some 16 species now found across Europe. Many anglers who have fished abroad or even in the UK will be aware of American Signal crayfish (Pacifastacus lenisculus), an invasive species with bright red claws which has spread rapidly since its escape from the food farming industry decades ago. Signal crayfish are the nemesis of the white-clawed, which are native to Ireland, the UK and much of Western Europe – we will find out why later on.

Crayfish distribution Ireland
Current white-clawed crayfish distribution in Ireland & Northern Ireland. Shaded red areas indicate crayfish plague outbreak locations in 2017 (image/data courtesy of NBDC)

Habitats and feeding

White-clawed crayfish are typically found in streams, rivers and canals with good water quality and high oxygen levels but are also found in some lake systems. Like all invertebrates, crayfish do not have a skeleton on the inside like we do – instead they have what is termed an ‘exoskeleton’, literally meaning one on the outside. Their shell is made primarily of calcium and therefore they need to live in waters where calcium is plentiful, or else shell formation and growth would be problematic. Calcium-rich waters are alkaline and have an above average pH (7 being pH neutral, like most tap water) and are classically found flowing over and in limestone areas throughout the country. Basically, limestone waters are richer in aquatic life as they can support more invertebrates, which means more food for practically everything else, including fish of course.

“Crayfish are quite secretive creatures which are mostly nocturnal. They, therefore, need plenty of places to hide during the day to avoid being eaten…”

Crayfish are quite secretive creatures which are mostly nocturnal. They, therefore, need plenty of places to hide during the day to avoid being eaten. Such places are collectively called ‘refugia’ (i.e. refuges) and can be anything from large stones and boulders, to cracks in underwater stonework, sunken logs and tree roots. The key characteristic a crayfish is in the market for is a home that will not move or be washed away in high flows or during floods. Juvenile crayfish, which are very small, are even more vulnerable to predation and often hide out in dense weed beds. White-claweds also sometimes live in burrows in the bankside, which they dig out with their claws. Areas with steep or vertical marginal slopes, such as those often found in our canal networks, must be like the Ritz for crayfish. They avoid areas of fine silt as this clogs their gills and makes oxygen uptake difficult.

white clawed crayfish
Crayfish require structures that don’t move with floods or fast currents to live and hide under such as large boulders (photo credit: Ecofact)

White-clawed crayfish grow slowly and can live for up to 10 years, becoming mature after 3 or 4. Few crayfish reach a (body) length greater than 10 or 12cm and grow through moulting, whereby the old shell, now too small to fit inside, is discarded and a new one grown. Moulting depends on age and sex; mature males moult twice a year (in early and late summer), mature females moult only once (in late summer) and juveniles may moult several times in a season. Males are usually larger than females and are fiercely territorial, frequently fighting with other males. Common battle scars include crayfish with only one claw after the victor has literally ripped it off during the encounter!

female white clawed crayfish
Female crayfish carry 50-100 eggs on their undersides through the winter before they hatch in mid-summer. When doing so they are termed “berried” and do not feed or grow

Crayfish group up for breeding in the autumn (October – November) when water temperatures are around 10°C. Below this temperature crayfish do not feed nor breed. Males fertilise the eggs, usually less than 100 per crayfish, which the female carries around with her on her underside through the winter and spring, before hatchlings appear in early to mid-summer. Females carrying eggs are said to be ‘berried’, as the eggs look like small berries stuck to her abdomen. About half of the eggs survive to this stage and these hatchlings remain attached to their mother until their second moult. It is only at this stage that the female can resume her own moulting and feeding as the young become independent.

Crayfish are omnivores, meaning they will eat both live and dead plant and animal matter. Snails, crustacean and insect larvae, fish eggs and fry, fallen leaf litter and submerged vegetation are all fair game. As such, white-clawed crayfish play an important role in ‘cleaning up’ aquatic habitats and recycling nutrients, and their impact of grazing on weedbeds can be quite surprising for such a small creature. There is some research to suggest that white-claweds may well prey on the invasive zebra mussel in Irish waters where they now co-exist. As mentioned above, crayfish primarily feed at night to reduce risk of predation and are often visible under torch light in the shallow margins.

Importance of crayfish

The ecological importance of white-clawed crayfish should not be understated and they form a key part of the diet of many other animals. These high-protein packages are a delicacy if you are an otter, and some of you may have come across conspicuously placed otter droppings (“spraint”) full of broken crayfish shells. Dragonfly larvae, which are among the largest insect larvae found in our waters, are significant predators of juvenile crayfish. Crayfish are also eaten by a host of water birds such as herons and, when small, kingfishers, and form an important part of the diet of many fish species in Ireland including perch, pike and eels, as well as bream, roach and their hybrids, tench and carp, where present.  Interestingly, many of the larger wild brown trout found across the Irish midland rivers, such as the River Suir, would appear to be so large due largely to the crayfish portion of their diet.

“The ecological importance of white-clawed crayfish should not be understated and they form a key part of the diet of many other animals”

So, as you can see, white-clawed crayfish are a very important species in our freshwaters. Not surprisingly they are afforded legal protection under both Irish and European law. This species is listed on Annex II of the EU Habitats Directive – the key environmental legislation protecting habitats and species across Europe – which means that Special Areas of Conservation (SACs) are established specifically to conserve this species. Any environmental or development works carried out along or beside waters containing crayfish are legally obliged to do all they can to prevent damage to the species.

white clawed crayfish
Crayfish play a key role in grazing submerged weed beds and recycling decaying leaf matter. Juveniles also hide in weed to avoid predators


Populations of white-clawed crayfish across Western Europe have reduced dramatically in the last 150 years due to human disturbances such as overfishing, habitat destruction, pollution and the introduction of foreign crayfish species, notably Signal crayfish which are larger, produce more offspring and generally out-compete them for food and habitat. These trends make our Irish populations especially important as Ireland is the last great stronghold for white-clawed crayfish in Western Europe. However, unfortunately all is not well in our waters…

The threat of invasive species is particularly high for Ireland and Northern Ireland, being a small, largely ecologically-isolated island with a more limited range of native species than elsewhere in Europe. In the UK (mostly England), Signal crayfish have reached huge abundances throughout and have seen off many of the last remaining populations of white-clawed crayfish. There is serious concern that Signal’s will soon reach Irish shores. On top of this, the issue of illegal trapping and poaching of native crayfish in Irish waters is an increasingly large problem. Crayfish are relatively easy to catch in baited traps and entire populations can effectively be wiped out in smaller waterbodies. Widespread declining water quality is also a cause for concern.

Crayfish plague disease

No doubt many of you in Ireland will have seen or heard mention of the alarming crayfish plague over the course of this summer. Crayfish plague (Aphanomyces astaci) is a highly infectious fungus-like water mould that causes 100% mortality in every white-clawed crayfish that contracts it. There is no natural immunity and no vaccine to cure it. The plague attaches to thin areas of cuticle (outer shell) as a spore and then grows through the tissues, leading to death in around two to three weeks. The free-swimming spores then transmit directly from the infected or recently dead crayfish to other areas. Spores are easily spread accidentally by anglers and other water users (e.g. canoes, kayaks, boats) through wet gear. Plague also affects the normal vision of the crayfish and so it may be seen crawling around in broad daylight where it usually wouldn’t. Crayfish plague originated in North American crayfish species and it is their introduction to Europe that has caused many of the white-clawed populations to decline.

Although there was a recorded outbreak of crayfish plague in the Boyne catchment in the 1980s, and a further isolated incident in the River Bruskey, Co. Cavan in 2015, Ireland has remained relatively free from the impacts of this infection until this year (2017). Over the course of this summer, crayfish plague was confirmed first in the River Suir and then quickly spread to the nearby River Barrow. The Limerick Deel* and Lorrha River in Tipperary (tributary of Lough Derg) have also suffered outbreaks and wide-scale population declines have occurred, and continue to occur. Preliminary genetic analysis of samples from these locations has proven that the outbreaks in the Suir and Barrow are unrelated to the one on the Lorrha, meaning it is likely that there have been at least two different sources of infection. A single source of crayfish plague infection would be worrying enough but more than this is serious cause for concern.

“relevant authorities… have done little to raise awareness of the crayfish plague problem let alone… prevent further spread”

It cannot be disputed that Ireland’s record for biosecurity – the prevention of non-native species spreading to and within our shores – is very poor. Indeed, in recent years alone we have seen the spread of zebra mussels, Asian clams and Lagarosiphon weed, all highly invasive species capable of causing significant ecological and financial damage to aquatic habitats. Then we have the big, wet elephant in the room; roach, which have directly caused widespread and irreparable changes to Irish fish communities since their spread starting in the early 1970s (a story for another issue, perhaps). And these are just the aquatic invasive species, not mentioning the ream of terrestrial ones! The introduction and subsequent (and continuing) spread of crayfish plague throughout Irish rivers epitomises many of the huge problems Ireland has in dealing with biosecurity and its environment in general.

crayfish plague awareness
A crayfish plague awareness sign along the River Barrow. Great to see but is it really enough?

Here we are talking about an infection that causes 100% mortality in white-clawed crayfish, a native and highly important, keystone species; an infection so deadly that it can wipe out entire populations in a matter of weeks! And yet, one can’t help but ask what has really been done to prevent more damage occurring? It is the opinion of all at this magazine that the relevant authorities – namely the National Parks & Wildlife Service (NPWS) as well as Waterways Ireland and Inland Fisheries Ireland – have done little to raise awareness of the crayfish plague problem let alone implement tangible management actions to prevent further spread. To date, the only real action taken, bar a few awareness posters along certain rivers, is the operation of a ‘voluntary ban’ on anglers and watercraft users moving equipment from infected sites elsewhere. There are no fines promised, no sanctions on boat traffic, no angling bans – nothing. Hopefully this will change in the very near future but sadly it does not look good for Irish white-clawed crayfish populations.

But what can you, as an angler, do to help this fascinating and endearing species that is so important to our aquatic habitats? For a start, report any sightings of dead crayfish or crayfish acting strangely (roaming shallow margins in daylight, for example) to the NPWS. Most importantly simply clean, disinfect and dry all your wet gear (nets, weigh slings, buckets, boots etc.) after a fishing trip. Use hot water where possible and a cheap household sterilising fluid, such as Milton, before drying, ideally in strong sunshine. It may sound simple but it could just save a population of crayfish and, in turn, your favourite fishing spot.

*Deel results were unavailable at time of publication

crayfish plague awareness
(poster updated and correct as of August 2019)