American Crayfish invade UK Rivers

American Signal Crayfish female - Pacifastacus leniusculus - Source: Astacoides on Wikipedia
American Signal Crayfish female – Pacifastacus leniusculus – Source: Astacoides on Wikipedia

There are many invasive species in the UK. Grey squirrels, several species of deer and mink are just some, but one of the most damaging lurks beneath the surface of many of our rivers, streams, lakes and ponds – the American signal crayfish.

Introduced in the 1970s in the hope of exporting farmed animals to Scandinavia, where there is a long tradition of eating crayfish, they soon escaped and started causing havoc wherever they spread.

The signal crayfish is a voracious predator, in some streams in England they have all but wiped out all other animal life. Insects, fish, molluscs and anything else that strays within range of their pincers can all fall victim. It also carries the crayfish plague, a disease against which the signal crayfish is immune, but which is deadly to other species of crayfish, including the native European crayfish, which is now barely hanging on in a few more remote locations in the country.

Not only do these animals threaten river wildlife but also their extensive burrowing into the banks of rivers and streams can cause them to collapse. They can burrow up to a metre deep, and with their large numbers the damage they cause can be extensive.

The Secrets to their Success

Signal crayfish are long lived, living for up to 15 to 20 years. They start to breed after four years and can have two to four hundred young every year. The females will carry their eggs and young to protect them for as long as possible.

They are also very mobile. They can move overland up to a kilometre in a single night. They are able to live outside water so long as their gills stay moist for up to a month. So they can travel from pool to pool, finding new habitats as they spread.

Local predators do not make a significant dent in their numbers. While predatory fish, birds and mammals will feast upon them, they breed so rapidly they can more than make up numbers lost in this way. Their biggest predators are probably other signal crayfish – cannibalism is common.

Finally, they are tough, able to survive major injuries and regenerate the damage. If they are able to crawl away, they are likely to come back another day. Many caught crayfish will be missing a pincer or two, and multiple legs. The most extreme example I have caught is one that had no pincers and only four of eight legs!

A Nice Catch

Fortunately, they are tasty, which is why they were farmed in the first place. It’s quite easy to catch them in sufficient numbers for a meal or two. One can catch them by hand when they are crawling on a riverbed or hiding under stones that can be turned over. However, the best way is to use baited nets and traps.

Drop nets, the kind used to catch crabs by the seaside, are a good, although they will need to be pulled up at regular intervals. This can be a nice way to spend an afternoon and in good weather can keep the kids entertained.

Traps are easier to use, in that they can be left overnight and then retrieved with their catch inside. Although sometimes you will only catch one or two of the creatures, in a good spot you may well end up with dozens in a single trap.

Both types of traps can be bought easily online and are simple to use and store. It is important, however, to have the relevant permissions. Make sure you have the landowner’s and local angling club’s consent where you fish and the proper license from the Environment Agency. The license is free and can be found on their website.

Almost any bait can be used. I use cat food kept in a nylon net. Table scraps (such as discarded pork rind, chicken skin, uneaten meat) work well and fish is, of course, an excellent bait.

American Crayfish caught in a Wiltshire River – Note orange underside – Source: Author

The crayfish are most active from about May through to October. Outside these months you are unlikely to catch enough to make it worth your while. When catching crayfish ensure it is the correct species. The European ‘White Clawed’ Crayfish is protected and their claws are a dirty white colour on their underside, while the Signal Crayfish is larger with a red/orange underside to their claws. You can be fined for catching the European Crayfish, so be very sure of what you are catching.

Meal Time

Signal Crayfish can be cooked in a variety of ways. I boil them and then take the meat from their tails (and claws for the larger males). Then I use the meat in curries, stir fries and gumbo. In the US they are usually boiled with a spicy mix and then eaten in large quantities straight out of the pan (once they have cooled a bit).

Plate of Crayfish – Source: Author

It is best to kill them beforehand with a blow to the head or to put them in a freezer for half an hour before cooking, which puts them in a torpid state and when cooked they will die before waking up fully.

Finally, never cook a crayfish that has died without you having killed it. They may look alright but could have been dead a while and be carrying a lot of bacteria. Cook them as soon as you have killed them or straight out of the freezer when they are in their torpid state.

It’s sad that our native species is being endangered by an invasive one which is so damaging to the wildlife in our rivers. But we can do our bit by catching as many American Crayfish as we can. And we can have some enjoyable meals in doing so.

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