Yabbie aquaculture potential: early Hawkesbury trials on Cherax destructor

Assessing yabbie aquaculture potential against a known checklist#

Carroll opens by quoting an observation from Smith, made as early as 1911, that freshwater crayfish might one day matter to Australians the way lobster did in America and England, and that farming them in ponds seemed worth investigating. Nearly seventy years later, that investigation had barely begun, even as public interest in crayfish was rising sharply. Hawkesbury Agricultural College had collected around 500 names and addresses from people enquiring about crayfish, and 170 people had attended two weekend schools on freshwater crayfish held at the College in 1979 and 1980.

The paper sets out to test whether that enthusiasm was justified by examining Cherax destructor against the criteria Bardach and colleagues proposed for any organism being considered for aquaculture: it must be marketable, resistant to pollution and disease, able to breed easily in captivity, produce hardy eggs and larvae, sit low on the food chain, tolerate crowding, and grow quickly. Each of these is tested in turn using investigations carried out at the College, mostly by undergraduate students over single semesters, a design worth bearing in mind when weighing the figures reported against each criterion.

The animal and where it fits among Australian crayfish#

Freshwater crayfish belong to the Superfamily Astacoidea, split into three families, two confined to the northern hemisphere and one, the Parastacidae, found in Australia and other parts of the southern hemisphere. According to Riek, Australia hosts nine genera and 97 species, among the greatest diversity found anywhere, while the tropics and Africa have none at all.

Within this diversity, Cherax destructor stands out for its distribution rather than its size. Astacopsis gouldi can reach lengths of several centimetres more than the largest freshwater crayfish elsewhere and weights of two to three kilograms, but grows slowly and is vulnerable to overharvesting, while Euastacus armatus is prized for flavour but yields little edible meat relative to its overall size. Cherax destructor, by contrast, has adapted to an unusually wide range of habitats across South Australia, Victoria, New South Wales, Western Queensland and the Northern Territory. It turns up in natural streams and ponds as well as farm dams, bore drains and irrigation canals. Johnston identified the best natural yabby fisheries in New South Wales as occurring around Buronga, Menindee, Balranald, Bourke and Condobolin, a distribution pattern explored further in a separate study of growth differences among yabby populations. Adults typically weigh 60 to 100 g, though they can reach 280 g, and shell colour varies widely, which the paper notes often leads to mistaken claims of separate species.

How the Hawkesbury trials were set up#

Investigations drew on facilities at Hawkesbury Agricultural College, including an air-conditioned fish laboratory with around 60 aquaria, outdoor circular fish tanks, two crayfish ponds, and farm dams scattered across the College's 1,400 ha property. Breeding experiments used 320 l glass aquaria with a 0.74 square metre floor area, filtered through a bottom sand bed via air-lift pumps, with water heated by thermostatically controlled fish tank heaters when required. Stocking began at six females and two males per tank, and across six experiments a total of 102 female crayfish were used, under three temperature regimes: room temperature, a constant 25 degrees Celsius, and water varying between room temperature and 25 degrees Celsius.

Feeding trials compared five diets, including natural feed, commercial trout pellets, and formulated diets based on soybean meal, fishmeal or meatmeal, each with a crude protein content of around 36 percent other than the natural feed treatment. These were run in triplicate in small outdoor ponds stocked at seven crayfish per square metre over a 26 week period between May and October, though effective growth only occurred over 15 weeks once water temperatures rose above 16 degrees Celsius. Separate experiments tested the effect of bottom substrate and shelter on survival of juvenile crayfish, and taste testing programmes run by D'Mello assessed sensory qualities of cooked meat using panels of up to 100 people.

Breeding results and the role of water temperature#

Water temperature emerged as the dominant control on breeding. When temperatures were falling and below 17 degrees Celsius, there was no breeding activity at all. Rising and warmer temperatures stimulated gonadal development, and in experiments started in July or August, berried females, those carrying eggs under the tail, appeared within a few days, compared with several weeks when trials began in March or April. The paper suggests this points to an interaction between rising temperature and daylength patterns, and notes that a further experiment using controlled daylength was under way at the time of writing.

Temperature also shaped egg development, hatchling size, growth rate and the interval between egg batches. In one experiment run at a constant 25 degrees Celsius, the average interval between berryings was 60 days; a large proportion of females were berried at least once during an eight month period from March to November, with some berried three times and others four times, and one female becoming berried again just 25 days after releasing her previous batch. Egg numbers per female averaged 450 but varied with female size; females as light as 18 g and 7.5 cm long had already become berried, carrying only 30 to 100 eggs at that size.

Feeding, growth and the limits of the trials#

In the diet comparison, the only significant differences in weight gain at the 5 percent level were between soybean meal and meatmeal diets, and between the natural feed treatment and the rest. The best individual animal gained 23.2 g over the trial, and the best replicate averaged 17.4 g, both on the meatmeal diet. Growth was negligible below 16 degrees Celsius, consistent with Frost's report that the species begins to aestivate below this temperature, and with Morrissy's finding of 12 degrees Celsius as a lower limit for growth in Cherax tenuimanus.

Across several shorter growth trials summarised in the paper, growth rates were described as optimal above 20 degrees Celsius, with feed conversion efficiencies of 2.8 to 1 or better achievable on pelleted feed, and a growth rate equivalent to around 8 percent of initial weight per week for animals starting between 10 and 30 g. These figures come from trials that were, by the paper's own account, often only a few weeks long and run mainly by undergraduate students as coursework. That is a reasonable way to generate early leads, but it means the growth and feed conversion numbers should be read as indicative rather than as settled performance benchmarks, a qualification the paper itself effectively signals by calling for further dedicated nutrition study, a gap later addressed in work on nutrition principles for Australian aquaculture.

Survival, shelter and the problem of cannibalism#

Cannibalism is identified as perhaps the single biggest obstacle to successful farming. Stocking density and temperature were not the only drivers; some tanks held what the paper calls rogue individuals that kept attacking tankmates until removed, while otherwise identical populations stayed stable for months. This observation led the authors to suggest that selecting more docile animals for breeding could be worthwhile.

A substrate and shelter experiment, run at a stocking density of 175 juveniles per square metre over 11 weeks with an average starting weight of 0.05 g, found the best growth and survival on a sand bottom fitted with shelters made from beer cans. A follow-up trial using sand as the only substrate recorded average survival of 99.6 percent in tanks with shelter against 77.7 percent without, at a stocking rate of 133 juveniles per square metre over seven weeks at 17 degrees Celsius. For comparison, the paper cites Morrissy's results with Cherax tenuimanus in ponds stocked at up to 15 per square metre, where shelters of rope fibre or PVC pipe produced overall survival of 80.5 percent over four months.

Markets, disease and the marketability yardstick#

On marketability, the paper reports that crustaceans already supplied some of the most sought after and expensive seafood in Australia, with a large share of the country's fish export earnings coming from crustacean exports, chiefly lobsters and prawns, and 21,500 tonnes of prawns caught nationally in 1978-9. Freshwater crayfish, by contrast, reached Sydney and Melbourne markets only in small, variable quantities during summer, fetching between $4 and $5 a kilogram in Sydney and between $2.20 and $6.60 a kilogram in Melbourne during 1979-80. European prices were markedly higher, up to $25 a kilogram in Sweden and France and $10.50 a kilogram in the United Kingdom, though Sweden still needed to import 2,000 tonnes a year to meet demand.

Taste testing by D'Mello found that Cherax destructor tail meat scored milder in crustacean flavour than banana prawn meat on a 1 to 5 scale, though claw meat differed significantly and could be quite strong. Yield testing of 54 specimens from Bourke found an average edible meat proportion of 20.4 percent, compared with 28 to 37 percent in prawns. On disease, the species appeared relatively free of the fungal disease that had devastated European crayfish stocks, though a microsporidian parasite and a fungus affecting polluted water were both flagged as risks, and the animals proved intolerant of chlorinated hydrocarbons, oil products, creosote and heavy metals such as zinc and cadmium. More background on comparable aquaculture feeding work can be found among the other papers in this library.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Animal Genetics and Breeding Unit
• Feedipedia animal feed database: an open-access database of animal feeds
• Poultry Hub Australia: poultry science information

Questions#

— The Livestock Library team