Despite earlier warnings it does seem that a lot of anglers still welcome the invasion by this American crustacean. Some may even deliberately stock a few crays into their local river or lake in the mistaken belief that they will add a valuable source of protein for the resident fish and help them to grow bigger faster.
Crays are now resident in the majority of waters in the south east of the country and probably further afield too. Many of these waters are several miles from the nearest known commercial source which almost certainly indicates deliberate and illegal stocking. However it is known that signals will walk from one watercourse to another and they are extremely difficult to contain. The downside is that crays eat an enormous amount of fish eggs and fry, and of course they can spread crayfish plague to our own White Clawed Crayfish, now seriously at risk in many parts of the country.
When Signal numbers reach a critical mass, they can and will eat the entire year spawning or, at best, leave less spawn than is required for a viable regeneration. Once this happens the overall fish population of the water will start to decline. Signal crays can move PDQ when they want to and they are quite capable of catching and devouring fingerlings, so it’s just not the eggs and fry that are taken. They will also eat water snails, frogspawn and tadpoles and virtually anything else small enough to catch, chomp up and eat, provided it is fresh. They are partial to young tender vegetation too and so their foraging can have a disastrous impact on a water’s plant life. So much so that cray infestations can leave many waters without enough plants to support eggs, fry and other water life essential to sustain a healthy habitat.
Once a cray population gains a foothold, numbers in any given water will multiply rapidly and fish stocks, amphibians, invertebrates and plants will start to dwindle. Eventually, the balance gets out of kilter and the crays start to dominate. They become main predator on top of the food chain. In the worse cases, all that’s left in a water will be an awful lot of crays and a few large specimen fish. These large fish may feature more regularly in catches, giving the impression of a healthy fishery. However, this is simply because catches of smaller fish will have become rare. When the older mature fish eventually die from natural causes, there will be nothing coming on to replace them. Like the rabbit and cane toad foolishly introduced to Australia, signal crayfish may soon prove to be an equal ecological disaster here!
Signal crayfish originate in North America where they are sometimes called crawfish. There are several other cray species native to North America too, and some of these are also finding their way into UK waters. In the States, crays are farmed for human consumption and fish baits. They share habitats there with alligators, snapping turtles, racoons, largemouth bass and a host of long legged water birds, all of which feed avidly on crays and help keep their numbers in check. Crays in England on the other hand have very few natural enemies. Otters will take them, but otter numbers here are so small they can be ignored. Mink will eagerly eat them and a few indigenous fish might take the odd smaller hardbacks, but at this stage in the crays development, they are not everyday tucker. So nationally, crayfish numbers are rapidly growing.
Wherever crayfish of any species have been artificially introduced, they have managed to cause major environmental catastrophes. Italy, Spain, Sweden, France and several other European countries have learned costly lessons. The only partially successful introduction has been in parts of Africa where parasites living in the bodies of water snails were finding their way into human hosts. The crays ate the water snails and the parasite numbers rapidly reduced. However, crayfish are not indigenous to Africa and the African people will not eat the meat. So there are no effective population controls. Many African watercourses are now suffering the conditions described above and the crays, far from being an asset, are regarded as a major environmental pest.
Indeed, in some parts of North America, particularly in residential areas where natural cray predators are few, certain cray species have long been recognised as a major environmental pest. They dig burrows from below the water line at an angle of 45 degrees or so and emerge through a hole in the lawn. They then come out on the lawn at night to feed. The holes weaken the banks which can often collapse causing erosion and subsequent shallowing of water ways as well as loss of land for the occupier. There are many specialist pest control operations in the USA earning a good living trying to eradicate crayfish from people’s gardens. They tend to use chemical treatments too which poison the crays but which present a risk to other wildlife.
Crayfish are of course extremely adaptable. They will switch their diets from protein to vegetable as the need dictates. Indeed, crayfish exist happily in many watercourses where fish are now completely absent. Some fish, notably chub and pike will occasionally eat hard-backed crays but crays are most vulnerable to predation when they have just shed their outer shell and before the new one hardens. This stage is called the moult (Americans spell ‘molt’) and crays which have moulted are soft, like jelly babies. Crays will moult a dozen times or so before they reach maturity and when they moult they increase in size by a
