Determining suitable thermal regimes for early instar redclaw juveniles, Cherax quadricarinatus (von Martens, 1868) (Decapoda, Parastacidae), for a proposed nursery phase

Modern, intensified aquaculture typically involves three production phases; hatchery, nursery, and grow-out. For redclaw crayfish aquaculture however, such delineation has been ill-defined. Farming of redclaw was initiated based on the putative beneficial physical and biological attributes of the species, which suggested production methods would be relatively simple. The simple approach proved to be inefficient and only partially effective, which hindered industry development. Hatchery technology now exists to supply seed stock for grow-out, but hatchery production is variable, and the performance of hatchery reared juveniles is inconsistent. A nursery phase has been proposed between hatchery production and grow-out of approximately 3 weeks duration, sufficient to allow 2 or more moults. An important primary parameter in the proposed nursery phase is the thermal regime that will support optimum survival and growth. This study quantified the effect of temperature on the growth and survival of redclaw juveniles for a 22-day nursery phase. Temperature had a statistically significant effect on the survival of juveniles, whereby, the high temperatures were associated with high mortality, and the lower temperature treatments were associated with very low mortality. Survival was 98 to 100% for craylings held at temperatures between 18 degrees C and 22 degrees C, and between 0% and 6% for craylings at temperature treatments of 25 degrees C to 32 degrees C. Mortalities within treatments between 25 degrees C and 30 degrees C, primarily occurred from day six to day eleven, corresponding with the initiation of moulting. Change of mass of crayfish was significantly higher with increasing temperature between 18 degrees C and 22 degrees C and at individual weights that suggest they had completed a moult. This study suggests a water temperature of 22 degrees C is optimal for survival and growth in a nursery phase

Evaluation of four practical diets on the growth and survival of juvenile redclaw, Cherax quadricarinatus (von Martens, 1868)

Redclaw, Cherax quadricariatus (von Martens) has shown promise as an aquaculture species but commercial development has been constrained by variability of production, perhaps due to poor survival and growth of early craylings. Diet contributes to survival and growth and previous studies have determined requirements for larger redclaw, but little research exists for the early craylings. An experiment was performed to evaluate survival and growth of early instar redclaw using four diets; Frippak (commercial shrimp diet), CSIRO compound diet, bloodworms, and on-grown Artemia. Bloodworms and Artemia produced significantly higher survival of craylings over two weeks than the two other diets. Artemia diet had a significantly higher weight increase than bloodworms or the CSIRO diet, but not Frippak, which did not differ from bloodworms or CSIRO. Biomass was significantly higher when fed Artemia. High mortality in the Frippak and CSIRO diet treatments were not wholly attributable to nutritional deficiencies as the manufactured diets became less physically accessible, potentially reducing intake, leading to difficulties completing ecdysis and eventually death. This study concluded that Artemia and bloodworms promoted highest survival, and Artemia and Frippak the highest weight gain. The best combination of survival, weight gain and biomass was with the Artemia diet

Morphology and weight-length relationships for the first six instars of Cherax quadricarinatus (von Martens, 1868)

Cherax quadricarinatus (redclaw) aquaculture has not achieved the industry growth that had been predicted in the early days of development. Provision of quality juvenile crayfish seed stock has been identified as a critical factor in overcoming industry development inertia. Hatchery technology has been developed to produce independent craylings and a nursery phase is now being developed to nurture the delicate craylings to a more advanced and robust juvenile stage, suitable for pond stocking. As part of the nursery development, clear definitions of the successive stages from egg through the first several instars, are required. Although some morphological descriptions of the early stages of redclaw have been made, the characteristics and nomenclature for stages from egg through to an advanced juvenile need clarification. A naming system is proposed (Egg, L1, L2, J1, J2 and J3) for the first six instars from egg, based on gross morphology and allometric relationships. Egg volume, wet weight and ocular carapace length (OCL) were analysed through linear regression. Based on these variables, the size of each instar is defined. Descriptions and photographs of the six instars provide a visual reference for identification. Five of the six instars had a significant relationship between wet weight and OCL. Both significantly increased for each successive instar. Lyophilized (dry) weight was not significantly different between stages until after instar J1, where endogenous feeding begins. The growth of the first four instars, in terms of wet weight and OCL but not dry weight, suggests an extra endogenous source of nutrition in addition to the yolk supply. Branchial water uptake during ecdysis has been proposed as a route for dissolved organic matter or small particulate matter to be acquired by the crayfish during the lecithotrophic, non-exogenous feeding stages, as an additional food source that might further enhance growth and survival

Do elasmobranch reactions to magnetic fields in water\ud show promise for bycatch mitigation?

Elasmobranchs are under increasing pressure from targeted fisheries worldwide, but unregulated bycatch is perhaps their greatest threat. This study tested five elasmobranch bycatch species (Sphyrna lewini, Carcharhinus tilstoni, Carcharhinus amblyrhynchos, Rhizoprionodon acutus, Glyphis glyphis) and one targeted teleost species (Lates calcarifer) to determine whether magnetic fields caused a reaction response and/or change in spatial use of experimental arena. All elasmobranch species reacted to magnets at distances between 0.26 and 0.58 m at magnetic strengths between 25 and 234 gauss and avoided the area around the magnets. Contrastingly, the teleosts showed no reaction response and congregated around the magnets. The different reactions of the teleosts and elasmobranchs are presumably driven by the presence of ampullae of Lorenzini in the elasmobranchs; different reaction distances between elasmobranch species appeared to correlate with their feeding ecology. Elasmobranchs with a higher reliance on the electroreceptive sense to locate prey reacted to the magnets at the greatest distance, except G. glyphis. Notably, this is the only elasmobranch species tested with a fresh- and salt-water phase in their ecology, which may account for the decreased magnetic sensitivity. The application of magnets worldwide to mitigate the bycatch of elasmobranchs appears promising based on these results