Yabby Farming - Exploring the Potential of Aquaculture

About

The popular freshwater crayfish known as the yabby (Cherax destructor) is one among many freshwater crayfish species found in Australia. It should not be confused with the saltwater yabby, ghost shrimp, or pink nipper, which is commonly used as fish bait. The yabby is commonly found in New South Wales, particularly west of the Great Dividing Range, and is known for its adaptability to various habitats throughout Australia. It is the most abundant and successful freshwater crayfish species in the country.

In New South Wales, there are two significant groups of freshwater crayfishes. The spiny crayfishes, which belong to the genus (Euastacus), are predominantly found in cool, rocky mountain streams. Examples of these crayfishes include the Murray and Sydney crayfishes. On the other hand, the yabby belongs to the smooth-shelled genus (Cherax), and most species within this genus inhabit the still, warm waters of the lowlands. Several related species of (Cherax) can be found along the coast of New South Wales, particularly in the northern regions. While there is limited information available about their biology, it is likely similar to that of the yabby.

The yabby was first documented in the early 19th century by explorers such as Thomas Mitchell and Eyre. Over the years, the name remained relatively unchanged, with the accepted spelling being 'yabby'. Its popularity as a food source can be traced back thousands of years, as evidenced by the remains of yabby shells found in riverside middens. Today, catching yabbies remains a popular pastime, and they have even made their way onto the menus of top-class restaurants. Although attempts to culture yabbies have been made, the Sydney market continues to rely on the rivers and lakes of the far west of the State for supply.

Freshwater crayfishes play a crucial role in the food chain. While they primarily feed on vegetation, they also scavenge decaying plant and animal matter. As prey, they are targeted by various native fishes and water birds. The common yabby, in particular, is an important food source for white ibis, cormorants, and warmwater fishes like the Murray cod and callop.

Habitat

Smooth-shelled crayfishes can be found in a variety of habitats, including lakes, swamps, billabongs, farm dams, irrigation canals, bore drains, and slow muddy rivers and creeks. The common yabby is highly adaptable and can survive extended periods of drought by burrowing and emerging during wet periods for feeding, breeding, and migration. In farm dams, the density of yabbies can reach up to five per square meter, with standing stocks of up to 340 kilograms per hectare being recorded.

One peculiar phenomenon observed in wild yabby populations is their sudden fluctuation in numbers over a short period. A dry creek or lake can quickly become teeming with yabbies after heavy rain or floods, resulting in large catches by locals. However, just as suddenly, the yabbies can disappear, and this phenomenon cannot be solely attributed to heavy fishing. The "boom and bust" nature of their population dynamics remains poorly understood.

Yabbies have been occasionally associated with the collapse of dam walls, typically occurring in walls less than 2 meters thick, especially when water levels fluctuate frequently, as seen in rice paddy levees. It is unlikely to happen in the average farm dam with walls over 6 meters thick.

Environmental Adaptations

Contrary to popular belief, yabbies do not require complete immersion in water to survive. As long as their gills are moist (which can be achieved through humid air), they can absorb oxygen and live for several days out of water. However, for breeding, they must be in water.

The yabby has developed a remarkable mechanism for surviving drought. As the ground dries up, the yabby burrows deeper, following the decreasing water table, and seals the entrance of its burrow with soil. In a small, moist chamber at the bottom, the yabby enters a state similar to suspended animation, where its bodily functions nearly cease, including respiration, pulse, and digestion. This mechanism is known as aestivation, and it allows the yabby to remain in this state for extended periods, sometimes lasting years. There have been burrows discovered that exceed depths of 5 meters.

The presence of yabbies in clear water is a rare occurrence. Typically, these crustaceans can be found in muddy water, which provides them with some level of protection against predators, although it may not be essential for their survival. Certain predators, such as fish, are not solely reliant on sight and can detect changes in water pressure to track their prey even in murky conditions. Cormorants also have the ability to locate their prey in muddy waters.

The type of substrate where yabbies are found is not of paramount importance, although they are commonly discovered in muddy or silted bottoms with occasional rocks or fallen branches. This is in contrast to spiny crayfishes, which are typically found in streams with leaf-littered, rocky, or pebbly environments. Experiments have shown that yabbies experience faster growth when in their natural habitat with substrates like mud or stones, as opposed to artificial ones like plastic tanks.

WATER QUALITY

Dissolved Oxygen

Yabbies have a remarkable tolerance for low levels of dissolved oxygen (DO) and have been observed in ponds where the DO was below 1-% saturation. However, it is highly unlikely that they would be able to thrive or reproduce in such conditions and would eventually leave. It is preferable for the water to contain over 4 ppm (parts per million) of dissolved oxygen.

Temperature

Yabbies have the ability to withstand water temperatures ranging from near freezing to above 35oC.

pH

In most favorable yabby habitats, the water tends to be alkaline with a pH level between 7.5 and 10.5. Yabbies are seldom found in bodies of water with acidic conditions (pH below 7).

Salinity

Yabbies do not seem to be affected by a salinity level of 12 ppt (parts per thousand), which is equivalent to around 35 percent seawater. However, a salinity level above 25 ppt would lead to their demise. First-year yabbies appear to be slightly more tolerant of salinity than adults.

Hardness

The water must contain sufficient dissolved calcium to ensure the strength of the yabby's shell. However, some calcium is recycled when yabbies consume moulted casts.

Chlorine

While young yabbies have been reported to be affected by high concentrations of chlorine in town water, adult yabbies do not face the same risk. If using town water, it is recommended to allow it to "age" for a few days to allow the chlorine to dissipate before introducing young yabbies.

Heavy Metals

Yabbies can accumulate mercury and lead in their bodies without experiencing harm. This makes them potentially useful as indicators of environmental pollution. The amounts of these metals found in wild specimens are well below the official health limits for human consumption.

Pesticides

Yabbies are vulnerable to insecticides and herbicides, especially those containing organochlorines. There is a possibility that pesticides from crop runoff can make farm dams uninhabitable for crayfish. Additionally, petroleum products are highly toxic to crayfish, and runoff from roads can result in the death of entire populations in dams.

Reproduction

Determining the sex of a yabby is relatively straightforward. Male crayfish have short projections called reproductive or genital papillae on the bases of their last pair of walking legs, while females have oval openings on the bases of their third-last pair of legs. It is quite common (1 in 20) to find individuals with a combination of both male and female openings. These "intersexes" typically belong to one sex and are capable of functioning sexually, but true hermaphrodites that can produce both eggs and sperm are rare.

Female yabbies reach sexual maturity at around 9 to 10 centimeters in length, while males reach maturity at slightly smaller sizes. Length is measured from the tip of the rostrum (the spine between the eyes) to the end of the tail fan. Almost all mature females are capable of spawning, but the majority of young that join the population are produced by 2-year-old yabbies, as they outnumber the older age groups.

During mating, the male yabby deposits small packets of sperm gel near the female's reproductive openings. The female then passes the eggs out through these openings, fertilizing them as they come into contact with the sperm. The eggs are attached to the underside of the female's tail, which she keeps cupped during egg-laying, where they are fastened to the swimmerettes (small legs on the abdomen) and carried until they hatch. Juvenile yabbies have specialized hooks on their legs, allowing them to cling to the hairs of the female's swimmerettes. They undergo several molts before leaving the parent.

The female diligently protects the eggs, using her tail to elevate and fan them in order to provide oxygen. If the water becomes too warm, she will seek out a cooler place. Due to the size of the eggs and the energy the female invests in their care, she can only produce a few hundred compared to the hundreds of thousands of relatively small eggs found in marine lobsters. The newly hatched young are referred to as "juveniles" and closely resemble the adults. Unlike many other crustaceans, they do not go through free-living larval stages. As a result, juvenile yabbies are better equipped for survival and likely have a higher rate of survival than the young of most marine crustaceans.

Breeding commences in springtime once the water temperature reaches a favorable 15 to 16 degrees Celsius. In early summer, after a span of 8 to 10 weeks, the initial clutch of eggs is hatched, ranging between 100 to 500 eggs per individual and contingent upon the size of the female. Once the young have ventured out, which occurs about 3 weeks later, the female is immediately prepared for another breeding session. Due to the warmer temperatures in the summer season, the second brood only requires 3 to 4 weeks of incubation. Some females may even reproduce three or more times during the breeding season, and if the temperature consistently remains high enough, this season can stretch into the autumn months. In the western part of the State, where the water is warmer, the breeding season may persist almost all year round.

Diet

The yabby primarily adheres to a vegetarian diet and favors fresh sustenance, although it will scavenge among the sediment on the bottom. It is not averse to preying on its own species, particularly when the target is smaller or weakened after molting. Flourishing yabby populations are often discovered where manure, which is essentially partly digested grass, is washed into farm dams from surrounding paddocks. Alternatively, they may also thrive in areas where cattle or sheep directly discharge manure into the water. Juveniles and young yabbies consume the same nourishment as adults.

Growth

Similar to other crustaceans, the yabby must shed its exoskeleton periodically in order to grow. This type of growth is not a continuous process, but instead occurs in distinct stages. The frequency of molting is influenced by the length of daylight and temperature, which stimulate the release of hormones that induce molting. The frequency of molting decreases with age; the newly hatched juveniles molt every few days, a one-year-old molts two to three times a year, and a three or four-year-old may only molt once a year. During the short period when the new shell is tender, the yabby is susceptible to predation from other yabbies, fish, and other predators. The fresh, pink shell of the molt contrasts vividly against the dirty, algae-coated shell of the yabby before it molts, especially in older individuals.

The growth rate of the yabby is mainly dependent on water temperature, available food, and population density, which refers to the level of crowding. Generally, warmer water facilitates faster growth for crayfish. Since yabbies, like most cold-blooded creatures, rely on their environment for heat and cannot regulate their own body temperature, the fastest growth occurs in the low-lying areas to the north and west of New South Wales where the water is warm for most of the year. Growth is not significant when water temperatures drop below 15°C, and studies conducted in South Australia suggest that the optimal conditions for growth occur at 23 to 25°C.

Although the yabby can tolerate temperatures up to 35°C, growth seems to be hindered above approximately 28°C.

Once maturity is attained, females experience slower growth compared to males, likely due to the added energy expended during spawning. As a result, a female yabby never reaches the size of an older male, which can weigh up to 300 grams. On average, yabbies caught by recreational anglers measure between 7 to 20 centimeters in length and weigh between 20 to 80 grams.

In a study conducted in a typical farm dam in the Riverina district of New South Wales, yabbies grew to a weight of 40 to 45 grams within 16 months of hatching during a prolonged summer. At this weight, a yabby would be roughly 10 centimeters long, making it suitable for both local and international markets. This growth occurred over two "growing seasons," which corresponded to the period when the water temperature at the pond bottom remained above 15°C (November to March/April in the Riverina).

If a crustacean loses a limb, such as a claw, walking leg, or antenna, it has the ability to regenerate it, starting at the subsequent molt. However, unless the lost limb is relatively small, complete regeneration is not immediate, typically requiring three to four molts to fully restore the limb.

Occasionally, small, round, stone-like masses are found in the stomach of a fish. These "stones" are often observed in aquariums containing crayfish and can also be found in the nests of certain water birds and even in Aboriginal middens. Known as gastroliths, these stomach stones are formed in pairs within the lining of the crayfish's stomach in preparation for molting. Because they are the hardest parts of the crayfish, they are either rejected or are the last to be digested by predators. Following the molt, the gastroliths fall into the crayfish's stomach, where the calcium they are composed of is absorbed into the bloodstream. In the past, gastroliths were utilized for their absorbent and antacid properties in traditional medicine.

Death and illness

Reasons for mortality include advanced age, the natural food chain (including cannibalism and fishing), physical harm, lack of sustenance, and disease.

In a natural population, where resources and living space are limited, only a small fraction of the 500 to 1000 offspring produced by each mature female during the breeding season can survive to the age of 2 or 3 and take the place of their parents. Thus, a young individual has a mere one in a thousand chance of reaching old age. Research on wild populations reveals that mortality rates are highest (perhaps as high as 95 to 99 percent) during the first year of life. However, as a yabby ages and grows, its chances of surviving to old age significantly increase. Mortality rates are somewhat lower (around 50 to 80 percent) during the second year and even lower during the third year. By removing or controlling predators, segregating sizes, providing shelter, treating diseases, ensuring adequate food and supplements, and monitoring water quality, survival rates could be significantly improved in tanks or ponds.

Under normal conditions, the wild lifespan of a yabby appears to be limited to four or five years, excluding extraordinary circumstances such as dormancy during droughts (see Environmental Requirements). Its natural predators consist of fish, birds, insects, humans, and other yabbies. Juvenile and young yabbies are consumed in large quantities by small fish like gudgeons, goldfish, and young individuals of other species, as well as by both large and small yabbies. Insects such as the ravenous water beetles and their larvae (toebiters), backswimmers, and dragonfly nymphs (mudeyes) may also feed on them. Adult yabbies fall prey to larger fish like the callop and the Murray cod, as well as water birds like cormorants and white ibis, other yabbies, platypuses, water rats, tortoises, and humans.

It is common to find yabbies with missing claws or legs, usually as a result of confrontations with other yabbies or their attempt to escape predators. The loss of limbs can also occur during the molting process, which is a period of great stress and contributes significantly to mortality. These injuries weaken an individual's ability to defend against predation. Additionally, competition for limited food resources in ponds weakens many individuals, rendering them more susceptible to disease or predation.

Reports of significant diseases in Australian freshwater crayfishes are rare. One such ailment, known as porcelain disease (also referred to as 'white tail' or 'white muscle' disease'), can affect yabbies. It is caused by a microscopic, single-celled organism called Thelohania. The disease becomes apparent in its advanced stages when the underside of the tail turns white and the walking legs become rigid and splayed. Unfortunately, it is invariably fatal and seems to be transmitted through cannibalism of deceased or dying crayfish. In wild populations, this disease does not seem to be a major concern. However, in farm dams in the Riverina region, it affects approximately 5 to 10 percent of dams and about 5 percent of individuals in an affected population.

List of prime yabby habitats in New South Wales

Region

Town

Bodies of Water

Far West

Menindee

Darling River and Anabranch; Menindee Lakes; lakes, creeks,

billabongs

South-west

Balranald

Murray River and backwaters; creeks, billabongs, swamps

Riverina

Narrandera

Lake Urana, Mulwala Canal; Urangeline, Urana and Yanco Creeks;

farm dams

Central

Condobolin

Lakes Cowal, Ballyrogan (Brewster); farm dams;

Willanra Creek area

North Central

Bourke

Warrego, Paroo Darling, Culgoa Rivers;

Bore Drains:

North-east:

In the north-east region of Armidale, there are various bodies of water such as creeks, farm dams, billabongs, and bore drains.

North Coast:

Moving to the North Coast, specifically from Newcastle to Ballina, you will find a diverse range of water sources including creeks, farm dams, rivers, and swamps.

Southern:

In the southern area of Albury, Tumut, and Eden, the main bodies of water are Lake Mulwala and farm dams, with a few lakes scattered throughout.

It's worth mentioning that the North Coast is home to several species of edible yabbies.

All crayfish species in Australia that have been examined so far have shown vulnerability to the Aphanomyces plague fungus. This fungus was brought into Europe in the last century, most likely by plague-resistant crayfish imported from the United States. Since then, it has wreaked havoc on the native European crayfish populations. The plague reached Britain in 1986 and by 1989, it had significantly decimated the indigenous crayfish populations.

If infected crayfish were to be introduced to Australia, it could result in a massive and irreversible destruction of our own native crayfish stocks.

Freshwater crayfishes are typically inhabited by small leech-like creatures measuring between 1 to 10 millimeters in length. These are known as temnocephalid flatworms, and their eggs are laid on the softer undersurfaces of the crayfish, especially beneath the tail. Upon microscopic examination, these worms reveal a suction disc at one end and tentacles at the other. They are not true parasites but rather commensals, feeding on food particles left behind in the water while the crayfish is eating. Although they do not actively harm the crayfish, in cases where there is abundant food, such as in aquariums or aquaculture setups, they can multiply to significant numbers. This can obstruct the flow of water through the crayfish's gills, leading to respiratory distress.

The Fishing Industry:

The main bodies of water targeted for commercial fishing are the Murray River, the Darling River and its Anabranch, and associated lakes. Additionally, other lakes and overflows in the north-west of New South Wales are also fished. In the far western districts, there is a considerable amateur fishery mainly focused on yabby fishing for consumption. However, interest gradually declines further east. The Riverina region experiences the highest catches during February, March, and April, with numbers decreasing in winter and spring. Murray cod fishing particularly benefits from using yabbies as bait. During the 1980s, the yabby population in the lakes of the Snowy Mountains increased significantly, resulting in their use as trout bait on occasion.

The commercial yabby fishery has been significant since approximately 1973, with New South Wales contributing up to 20 tonnes of the total commercial catch per year. South Australia and, to a lesser extent, Victoria have had established local markets and commercial fisheries for several years.

Before the 1970s, the yabby was relatively unknown as a culinary delicacy. However, its export earnings dramatically rose due to the crayfish shortage in Europe caused by the crayfish plague. Sweden became a major importer of the yabby, considering it a delicacy and willing to pay a high price for it. In Sweden, fishing for crayfish is prohibited for 50 weeks of the year. In response to the export demand, multiple South Australian processing firms contracted the majority of yabby fishermen, ensuring them a steady price for their catches.

For a few years, the majority of Australia's yabby catch was exported, mainly sourced from South Australian waters, particularly Lake Alexandrina near the mouth of the Murray River.

However, the industry's boom suddenly collapsed around 1978. Fishing grounds failed, resulting in a drastic drop in catches from over 100 kilograms per person per day to less than 10 kilograms. Overfishing, competition from an increasing number of carp, and the natural boom and bust cycle of yabby populations were all factors attributed to this crash. Thankfully, over the past decade, the yabby population has gradually started to recover from the effects of that period of exploitation.

There are regulations in place concerning the type and quantity of fishing gear allowed for catching yabbies. Certain regions, particularly trout waters, may have restrictions on the use of traps or nets. Since these laws can change periodically, it is advisable to consult the local Fisheries Officer for the most up-to-date information.

Releasing any type of fish or crayfish into natural waterways is illegal, unless you have obtained permission from the Office of NSW DPI. This rule is especially important if the animal is not naturally found in the area, like the Western Australian marron. The common yabby, known as Cherax destructor, is not native to the eastern side of the Great Dividing Range and should not be released there. These yabbies can be highly aggressive and pose a threat to resident crayfish and other aquatic animals as they could potentially dominate and eliminate them.

Preparation and Cooking

Freshwater crayfish serve as a fantastic base for various delectable recipes, particularly the Creole or Cajun style from Louisiana, USA. The simplest recipe involves killing the crayfish in an ice slurry and then immersing them in boiling water for approximately 5 minutes (similar to prawns). However, it is advisable to leave them in slightly longer to allow the heat to penetrate the tougher shell. The edible parts of the yabby include the tail meat (which constitutes about 20% of the total weight), the claw meat, the 'mustard,' and the 'coral.' The majority of the edible flesh comes from the tail meat. Extracting meat from the claw is only worthwhile if dealing with a large yabby. The 'mustard,' also referred to as 'fat,' is a soft, orange-brown liver found in the carapace. It provides a mustard flavor and is enjoyed by connoisseurs when spread on the tail meat. The 'coral' is the developing ovary or egg sac found in the carapace of the female yabby. When cooked, it turns red and can be delicious on its own or when incorporated into sauces.

In the Aquarium

The yabby makes for an entertaining and easily kept pet in an aquarium. A 100-litre tank can accommodate one or two large yabbies or six to ten medium-sized ones. It is important to provide them with rocks or some form of cover for hiding during moulting. Feed them small quantities of vegetable scraps, chicken pellets, and occasional amounts of lean meat, ensuring any uneaten food is removed daily. Yabbies can go for months without eating, so it is better to offer small portions at a time to avoid water pollution.

Yabbies breed readily, and the young do not require special food as they will find any scraps missed by their mother. Adequate cover, such as water plants, should be provided for juveniles to escape from adult yabbies.

To prevent yabbies from uprooting aquatic plants while burrowing or searching for food, it is advisable to tie the plants to large stones or use artificial plants. Juvenile yabbies are often eaten by most fish species, so it is essential to maintain the water level below the top of the tank to prevent them from climbing the air hose and escaping.

Aquaculture

Until the late 1990s, Australian freshwater crayfish were largely overlooked by gourmet enthusiasts. However, spurred by international interest, they are now highly sought after for haute cuisine.

Due to fluctuating supply from wild stocks, there has been a growing interest in farming these crayfish. Research indicates that certain Cherax species, such as the yabby, marron, and Queensland red claw, may be suitable candidates for aquaculture.

For the latest figures on aquaculture production, refer to the NSW DPI aquaculture production report available at this link.

FURTHER INFORMATION

For more information, please contact the NSW DPI Port Stephens Fisheries Centre at 02 4916 3900.

REFERENCES

Johnson, H T (1980). A comprehensive guide to catching and cooking the yabby. In Fish & Fisheries, NSW State Fisheries. (Includes trapping techniques, net construction, and recipes).

Johnson, H T (1986). Australian freshwater crustaceans with the potential for aquaculture - exploring their biology, with specific emphasis on the yabby. Proceedings of the 1st Australian Freshwater Aquaculture Workshop, Narrandera, 1983. Department of Agriculture, New South Wales.

Lake, P S and Sokol, A (1986). Ecology of the yabby Cherax destructor Clark (Crustacea: Decapoda: Parastacidae) and its viability as an indicator species for mercury and lead pollution. AWRC Tech. Paper 87. AGPS, Canberra. Pp. 186. [In-depth summary of yabby biology]

Mills, B (1980). Insights into the aquaculture of yabbies. Proceedings of the 1st Australian Freshwater Aquaculture Workshop, Narrandera, 1983. Department of Agriculture, New South Wales. [A collection of miscellaneous facts and anecdotes]