Possible management options for Procambarus clarkii include the elimination or reduction of populations via mechanical, physical, chemical or biological methods; the restocking of native crayfish populations threatened by the crayfish plague fungus and interspecific competition with alien species; the development of plague-resistant strains of native crayfish; and the use of legislation to prohibit the transport and release of alien crayfish.Preventative measures: Legislation designed to prevent the spread of crayfish has proven difficult to enforce due to the presence of conflicting social motivations such as the desire to propagate the species for recreational or commercial purposes. Political barriers, particularly in Europe, may also hinder conservation goals. For example the free trade policy backed by the European Union has hindered the attempts of European countries to prohibit the importation of live crayfish from other countries within the EU (Holdich et al, 1999).
Physical: Reduction of P. clarkii populations may be possible through physical control methods. However, eradication is unlikely unless the population is particularly restricted in range and size. All physical methods have environmental costs, which should be weighed up against the environmental benefits of employing them. Mechanical methods to control crayfish include the use of traps, fyke and seine nets and electro-fishing. Continued trapping is preferable to short-term intensive trapping, which may provoke feedback responses in the population such as stimulating a younger maturation age and a greater egg production. Bait, such as roach, bream, bleak or white bream, may increase the number of crayfish caught in traps, although freshwater fish should be avoided to prevent spread of the crayfish plague fungus, which may be transmitted on their scales (Gherardi & Panov, 2006; Westman, 1991; Alderman et al, undated in Holdich, Gydemo and Rogers, 1999; Kerby et al, 2005). A fair population reduction of P. clarkii by removal was achieved in Lake Naivasha, Kenya using traps and removal from floating vegetation in attempts to promote recovery of native macrophytes (Smart et al, 2002). Further control methods include the drainage of ponds, the diversion of rivers, or the construction of physical or electrical barriers to limit its spread (Kerby et al, 2005).
Chemical: Chemicals that can be used to control crayfish include biocides such as organophosphate, organochlorine, and pyrethroid insecticides; individual crayfish are differentially affected depending on their size, with smaller individuals being more susceptible (Gherardi & Panov, 2006). Furadan 5G, active ingredient carbofuran, has also been found fatal to P. clarkii in Kenya (Rosenthal et al, 2005). Since no biocides are crayfish-specific other invertebrates, such as arthropods, may be eliminated along with crayfish, and may subsequently have to be re-introduced. There is cause for concern about toxin bioaccumulation and biomagnification in the food chain, although this is less of a problem with pyrethroids. Another chemical solution lies in the potential to use crayfish-specific, or even species-specific, pheromones to trap P. clarkii (Gherardi & Panov, 2006).
Biological control: Possible biological control methods include the use of fish predators, disease-causing organisms, and use of microbes that produce toxins, for example, the bacterium Bacillus thuringiensis var. israeliensis (Pedigo, 1989, in Holdich et al, 1999). Only the use of predaceous fish has been used successfully; eels, burbot, perch and pike are predators are all partial to crayfish (Westman, 1991, in Holdich et al, 1999). Pike are being reintroduced into Massaciuccoli Lake, Italy to help control P.clarkii (Schleifstein & Fedeli, 2003). The amount of cover, type of fish predator used and location are all important variables in determining the success of such an approach, and in general reduced coverage is correlated with increased predation rates.
Integrated management: The application of 20 Gy x-rays ionizing radiation to males has been found to reduce the size of testes and alter spermatogenesis. Reproductive success decreased and hatchlings were reduced by 43% in a test study (Alquiloni et al, 2000).
