World Aquaculture 57 of juveniles to support limited demand has been achieved by maintaining sexually mature redclaws in earthen ponds from which juveniles are periodically harvested, the model is land and labor intensive and is not the most efficient way to supply seed organisms in a sustainable manner. Juvenile redclaw survival of 5 to 10 percent obtained from reproduction ponds (Jones 1995a, Masser and Rouse 1997) is not space and resource efficient for producers. This problem cannot be solved by stocking younger individuals directly into grow out ponds because survival remains low and production becomes more difficult to predict. For the redclaw industry to further develop, survival during the hatchery-nursery phase must increase dramatically, average size of juveniles at stocking should be larger and overall juvenile production should be better controlled and streamlined. In the present study, we evaluated three hatchery-nursery protocols in an attempt to better understand hatchery/nursery phase survival and growth of juvenile Australian redclaw crayfish (Cherax quadricarinatus). First, we investigated the effect of water volume on production of juvenile redclaws. Second, we evaluated the effect of female stocking density on juvenile growth and survival. Third, we investigated the effect of nursery period and broodstock density on juvenile survival and growth. Study Methods All experiments were performed at the AGY Redclaw Hatchery and Farm of Megar S.A. de C.V. in Soto La Marina, Tamaulipas, Mexico. Based on the stage of development of redclaw eggs published by Yeh and Rouse (1994) and Jones (1995a) and on practical experience at the hatchery, a stage development table specific for the region was prepared (Table 1) and used in the present study. Female broodstock redclaw used in the present work were selected from three 2,000-m2 reproduction ponds stocked at 1.5 females/m2 at a 1:3 male to female ratio. Ponds were pre-conditioned for redclaw by adding one terracotta block/m2. Each terracotta block had 4 cavities, which resulted in four individual hideouts per square meter. A bundle of onion sac mesh per 4 m2 was also provided as refuge for juveniles. Water was pumped from a local stream, replacing evaporative water losses. Females at egg stage 5 were selected and held in flow-through systems prior to initiating the experiments. Females were harvested with extreme caution to prevent egg dislodging. A random sample of thirty gravid stage-5 females had their eggs removed gently with the aid of a toothbrush and a pair of forceps and then females were individually weighed. Total eggs/female and eggs per unit weight of female were calculated. Hatching success was assumed to be 95 percent based on Yeh and Rouse (1994) and all calculations of juvenile survival were performed based on this assumption. Water Quality Dissolved oxygen (DO) concentration (> 5 pp) and temperature (24.9- 28.7 ºC) were measured twice daily using a YSI 55 DO meter (Yellow Spring Instrument Co., Yellow Springs OH, USA). Ammonia-N concentration was measured twice weekly using a Lamotte® Freshwater AquaculEmpty broodstock pond at AGY Redclaw Hatchery. Table 1. Morphological characteristics and duration (days) of successive stages of fertilized egg and larval development of redclaw, Cherax quadricarinatus. Egg stage Morphological Approximate characteristics of egg duration 1 Olive green to Khaki 10 2 Yellow 5 3 Orange 5 4 Red, eyes not visible 7 5 Red, eyes and pereiopods visible 7 6 Gray-released 5 ture kit and remained within acceptable limits for indoor production of redclaw crayfish according to Masser and Rouse (1997). Experiment 1: Effect of water volume on juvenile production Females with stage-5 eggs were randomly taken for the experiment and individually weighed so production of juveniles/g of female could be calculated. The females were then randomly divided into six groups of 9 individuals and 3 groups of 18 individuals and each group stocked into one of nine circular tanks (1.9 m diameter) supplied with water from an outside reservoir at approximately 20 L/hr. Each tank contained plastic containers that held female redclaws and allowed juveniles to drop through after release to prevent predation. Tanks were also provided with four submerged air diffusers for aeration and water mixing. Experimental treatments were: 1) 9 females/tank and an average water depth of 10 cm (9FV1); 2) 9 females/tank and an average water depth of 20 cm (9FV2); 3) 18 females/tank and a water depth of 20 cm (18FV2). Each tank contained onion mesh bundles to provide refuge for the juveniles. The size of refuge was directly related to the volume of water to keep mesh per unit of water at a constant value. Once all females spawned and
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