Improving Redclaw Crayfish (Cherax quadricarinatus) Aquaculture: Assessment of Invasive Impacts and Production of All-male Broods
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Fisheries and Allied Aquacultures
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The Australian redclaw crayfish (Cherax quadricarinatus) was first considered for aquaculture in the United States in 1990 and is quickly becoming an important species for aquaculture worldwide. Redclaw are valued as both a food source and as an ornamental aquarium species. Ecologically-sound aquacultural development requires the development of environmentally low-impact, yet cost-effective culture techniques. Because this species is exotic, its potential impacts on native fauna should be addressed before the development of an aquaculture industry in the Southeastern United States. Several aspects of aquaculture for the redclaw were assessed in a series of experiments which evaluated: 1) the impacts of adult redclaw on the native crayfish Procambarus acutissimus, 2) effective anesthetics and their ability to improve survival of juvenile redclaw exposed to physical stress, and 3) the effectiveness of androgenic gland ablation in sex-reversal of stage III juvenile redclaw crayfish with the ultimate goal of producing all-male broods. A 24-week competition study (July – December) was conducted in outdoor tanks containing 9 native crayfish and a range of redclaw densities (0, 2, 6, 10, 16, and 20 per tank) with a constant number of shelters and identical feeding rates among all tanks. The experiment ended when all redclaw died in mid-December. Redclaw appeared to affect average size of P. acutissimus, but did not affect survival. Results supported previous studies that suggest redclaw cannot overwinter in most of the United States. Even though redclaw invasion duration is likely limited by cold winter temperatures, they may have significant, negative effects on growth of male crayfish. However, reduced growth of female crayfish was not observed, decreasing the likelihood of indirect effects on egg production. The brooding season of redclaw and P. acutissimus overlapped, suggesting the potential for competition between YOY of both species, but these interactions were not addressed in this study. Food preference, assimilation efficiency, and relative ability to acquire food resources should be studied to further evaluate the effects of redclaw competition on native North American crayfish species. Anesthesia has useful applications in handling, transport, and surgery of many aquaculture species. In this study, the effectiveness of different anesthetics was evaluated for their ability to produce heavy sedation and improve survival of surgery-stressed juvenile redclaw. Isoeugenol (Aqui-S) and ice bath anesthesia were both effective anesthetics for juvenile redclaw. Juvenile redclaw achieved heavy sedation in < 10 minutes with Aqui-S concentrations between 5 and 15 ml/L. Crayfish that were exposed beyond heavy sedation exhibited decreasing survival and increasing time to recovery with increasing exposure duration and increasing Aqui-S concentration. Ice bath anesthesia produced heavy sedation for juvenile redclaw at temperatures between 8 °C and 17 °C; lower temperatures were required for smaller crayfish to reach heavy sedation. Seventy-two hour, post-ablation survivorship was high for all juvenile crayfish regardless of age-class or use of anesthetic. Therefore, anesthesia was not used in the subsequent ablation evaluation study. To evaluate the effectiveness of ablation in producing sex-reversed, neofemale broodstock crayfish, androgenic gland ablation was tested on pre-release juvenile redclaw. Mating a normal male and a sex-reversed male (neofemale) together should result in the production of all-male progeny, which will greatly improve mono-sex male redclaw crayfish culture. Androgenic gland ablation was carried out on 70 stage III juvenile redclaw, ~1 week pre-release, from three broodstock females (N=210). Ablated juvenileswere grown out for three months along with non-ablated, control crayfish (N=90) in individual containers. Survival and growth rates were high and did not differ between ablated and control crayfish, showing that crayfish can be feasibly ablated at a very early age. The number of males (49% ± 6%) and females (50% ± 7%) did not differ in the control group, but the ablated group contained de-masculinized, unilateral males and eunuchs, that lacked external sexual characteristics. The ablated group was comprised of 24% ± 15% males, 64% ± 17% of which were unilateral males, 26% ± 12% eunuchs, and 48% ± 6% females. The proportion of female crayfish in the ablated group was not significantly greater than the control group, indicating that sex reversal was unsuccessful. However, the presence of de-masculinized males in the ablated group showed that ablation did have an effect on development of external sexual morphology of males. Future studies should attempt ablation at earlier age classes and at cooler culture temperatures to increase the probability of neo-female production.