A review of possible methods for controlling nuisance populations of alien crayfish
As alien crayfish continue to spread in many countries throughout the world, there is a need to find a means to control or eradicate those established populations that have become a nuisance, because of their biological and physical effects on the freshwater environment. Such methods should conform to established criteria so as not to make the situation worse. Five broad categories of control are recognized: legislative, mechanical, biological, physical and chemical. Legislative control is in operation in many countries, but is rarely effective. Mechanical methods include the use of traps, fyke and seine nets, and electro-fishing. Each can be used effectively to control nuisance populations if used intensively enough, but eradication by such methods is unlikely. Biological control includes the use of fish predators, disease and microbial insecticides. Of these, only predaceous fish have been used so far. In some cases, there is a clear inverse relation between the presence of fish predators and crayfish and introduction of such fish may lead to a reduction in crayfish numbers, but the situation is not so clear cut in others. Crayfish plague is lethal to non-North American crayfish and even to North American crayfish if they are put under stress. If more specific strains of the disease could be developed then this could be a very effective control agent. Microbial insecticides such as Bti are effective against insect pests and there might be potential in developing a strain against crayfish. Physical methods include draining ponds and lakes, diverting streams and rivers, and creating obstacles such as weirs and electrical barriers. A problem exists with controlling burrowing crayfish during drain-downs as they are known to be able to survive for long periods of time in their burrows. Chemical methods include the use of biocides, surfactants and pheromones. Organophosphate and organochlorine insecticides can be effective against crayfish, but there have been few large scale studies and there are often objections to their use because of problems with bioaccumulation and biomagnification. Baytex and Dipterex have been used effectively in recent experiments in France. Fourth generation pyrethroids are effective in killing crayfish at very low doses, e.g. deltamethrin is effective at levels allowable in European drinking water. The anthelminthic drug, ivermectin, is effective in killing fish lice on farmed salmon but has yet to be tested against crayfish. Rotenone is used as a piscicide in fisheries management, but some crayfish can withstand levels 500 times that required to kill fish, so its use 246for eradicating crayfish would prove very expensive. Surfactants (wetting agents) have been found to reduce oxygen consumption of crayfish, but are unlikely to prove effective in eradicating crayfish. The use of sexual attractants (pheromones) has not been used as a method to control crayfish, but they might have potential if they could be isolated. No single method is likely to be effective in controlling or eradicating nuisance crayfish populations in all situations. Multiple approaches are needed for each situation and perhaps for each species. Quick kills could be achieved with known organophosphate and pyrethroid insecticides, but in most situations authorities are likely to require more environmentally friendly mechanical, physical and biological methods to be used.