Mating, fertilisation and spawning occur naturally in the ponds, and there are no free-swimming larval stages. The male deposits a spermatophore on the underside of the female who then spawns and fertilises eggs within 24 hours. The eggs are held under the female’s tail until they are ready to hatch usually six to eight weeks. The larvae develop within the eggs, which subsequently change colour from green, to brown to orange. Finally the eggs hatch and the 12 mm long juveniles remain attached to the female for one to two days prior to moving away as completely independent miniature forms of the adults.
One disadvantage is the relatively low fecundity of freshwater crayfish, between 300-1,000 per spawn; however, because of the ease with which broodstock can be obtained, it is a minor concern, especially as the female can be marketed after release of the juveniles. Under optimal pond conditions redclaw can grow from hatching to the smallest marketable size (~30 g) within four months.
Queensland redclaw usually mature at around six months of age, or 45-50 g, but it may be possible to select for later maturing animals when selecting for faster growers. Female redclaw will stop growing at maturity when the ovaries develop and enlarge. Larger females are therefore less likely to have spawned, and providing it is a heritable trait, selecting the largest females may affect selection for late maturing individuals. When best practice techniques are used, the majority of males will reach 100 g, and females 70 g, within 12 months of growout. The best of these should then be selected as broodstock for the next generation.
Redclaw Crayfish Farming Technology
In the late 1980s very little was known of the biology or farming techniques of this species. However, in
the 1990s excellent biological data and appropriate farming techniques became available from the Queensland Department of Primary Industries Freshwater Fisheries and Aquaculture Centre in north Queensland. Researchers there were able to make recommendations which incorporate cost effective, very low levels of technology, and which produce high yields.
For commercial production, the most important factors to pay attention to are stock management, water quality, feeding and the provision of shelter. Stock management is important in order to keep track of animals so that selection of the fastest growers can occur with confidence. There is no point choosing large animals if you have no record of their age. Also it is important to have like-sized animals in your pond at stocking. The greater the variability in size at stocking, the more pronounced cannibalism is likely to be.
Although redclaw will tolerate a range of water quality conditions, including what is generally referred to as poor water quality, optimum growth and commercial viability will only be achieved if water quality is managed and maintained within optimal ranges. Careful record keeping, a little knowledge of water chemistry or sound advice, and some experience are all that is required.
Redclaw are highly adaptive with respect to nutrition and are often referred to as omnivores which means they eat a variety of organic matter. Presently there are no farms feeding complete diets to redclaw as they do to marine prawns. This is in part due to the fact that no complete diets are available, and partly because commercial growth can be attained by supplemental feeding. Supplemental feeding relies on redclaw’s ability to gain some nutrition from invertebrates found in ponds and on pond floors.
Supplemental feeding involves the distribution of a low-protein, high carbohydrate pellet diet in combination with organic matter. The organic matter stimulates productivity of the detrital food chain. The supplemental pellets are either ingested directly by the redclaw or are broken down by bacterial, fungal or protozoan decomposers and are subsequently consumed by other invertebrates. These larger invertebrates make up a substantial proportion of the diet of redclaw in ponds.
Shelter must be provided in abundance. Redclaw, like all other crustaceans, moult or shed their shell as they grow. Immediately after moulting the redclaw are soft-shelled and are vulnerable to predation by their pond mates. The provision of shelter has been shown to improve survival substantially (from 15% with no shelter to 75% for the best shelter types). The best forms of shelter are mesh-type materials, such as onion bags or shadecloth, and short lengths of pipe. Discarded car tyres have also been used effectively, although these may attract a removal fee when the venture closes.
Harvesting is a remarkably simple process. A flow-trap has been used for a number of years. Redclaw respond strongly to a current in the water. This is probably the result of evolving in a billabong environment. As the water body evaporates and becomes shallower, any new inflow represents an upstream water body and the opportunity to move to better conditions. This has been exploited by farmers who slowly drain the ponds over 24 hours and introduce a flow of new water. Commonly an aluminium box is placed in the pond in the late afternoon when the pond is two-thirds empty.
The box is located towards the deep end, with a ramp pointing down towards the pond outlet, or where the last volume of water will be when the pond drains. An inflow of new water is introduced via the harvest box that is sensed by the redclaw. The redclaw then walk upstream, up the ramp and fall into the harvest box. The box is also supplied with aeration to sustain the redclaw until morning. This flowtrap reduces stress on the animals, maintains them in clean fresh water and results in a healthy premium product at market. In excess of 95% of the animals will be caught in this way.