Introduction
Yabby occurs west of the Great Dividing Range in New South Wales and over a large area of the Australian continent. It has adapted to many different habitats, from the cold waters of the Snowy Mountains lakes to the hot centre of Australia and is the most abundant and successful of the Australian freshwater crayfishes.
In New South Wales, there are two important groups of freshwater crayfishes. The genus (Euastacus) comprises the spiny crayfishes, most of which live in cool, flowing, rocky mountain streams (the best known are the Murray and the Sydney crayfishes). The yabby belongs to the smooth-shelled genus, (Cherax), of which most species inhabit the still, warm waters of the lowlands. Several related species of (Cherax) are found along the coast, particularly in the north of the State. Although their biology has not been examined, it is probably similar to that of a yabby.
The yabby was recorded as early as 1831 in the north of the State by the explorer Thomas Mitchell and also during Eyre's 1845 expedition into Central Australia. In 1894, the Horn expedition to Central Australia found it 'in abundance' and recorded that it was said 'to be eaten regularly by the wandering tribes of natives who know it as the yabber'. The name has changed only slightly over the years, the accepted spelling now being 'yabby'.
The yabby is very tasty and was welcome on the menu of aborigines as far back as 28 000 years ago; this we know from the remains of shells in riverside middens. It is still a popular country pastime to catch yabbies but over the past decade the yabby has gained the attention of the gourmet and now appears on the menus of the best restaurants. There have been many attempts to culture it however the Sydney market is still supplied mainly from the rivers and lakes of the far west of the State.
Freshwater crayfishes are in the middle of the food chain. They are basically vegetarian but also scavenge decaying plant and animal matter. In turn, they are preyed upon by many native fishes and water birds. The common yabby forms an important part of the diet of white ibis, several cormorants, and warmwater fishes such as the Murray cod and the callop (golden perch or yellowbelly).
Habitat
The smooth-shelled crayfishes occur in lakes, swamps, billabongs, farm dams, irrigation canals and bore drains (mainly still, warm waters) and also in slow, muddy rivers and creeks. The common yabby is especially hardy and can survive years of drought by burrowing, later emerging during wet periods to feed, breed and migrate. In farm dams the density of yabbies can be as high as 5 per square metre and standing stocks of up to 340 kilograms per hectare have been recorded.
It is a peculiar phenomenon of wild yabby populations that numbers can appear to change dramatically over a very short time. A lifeless dry creek or lake will fill with water from floods or heavy rain and suddenly teem with yabbies. The local townsfolk quickly take advantage of this and large catches are taken. Then just as suddenly as they came, the yabbies will vanish - and their disappearance cannot be accounted for by the heavy fishing. This 'boom and bust' phenomenon is not properly understood.
Yabbies have been occasionally blamed for the collapse of dam walls. However, this generally happens only if the walls are less than 2 metres thick, especially if water levels change frequently (for example on rice paddy levees). It is unlikely to happen in the average farm dam with walls over 6 metres thick.
Environmental Requirements
Complete immersion in water is not essential for life for the yabby. If its gills are kept moist (humid air is sufficient), it can absorb oxygen from the air and survive for many days out of water. To breed, however, it must be in water.
The yabby has evolved an ingenious mechanism for surviving drought. As the ground dries up it burrows down following the falling water table, and seals the burrow entrance with an earthen plug. In a small, moist chamber at the bottom, the yabby enters a state resembling suspended animation, its bodily functions (respiration, pulse and digestion) practically ceasing. This mechanism is called aestivation (not hibernation, which is a winter adaptation of warm-blooded animals). The yabby can remain like this for years on end. Burrows well over 5 metres deep have been found.
The yabby is rarely found in clear water. Its natural habitat is usually muddy water, which (although probably not essential to life) may give some protection from predators. Some predators, such as fish, do not depend upon sight alone but can sense pressure changes, tracking their prey even in muddy water; cormorants too can find their prey in muddy waters.
Substrate type is not critically important, although the yabby is commonly found on muddy or silted bottoms with the occasional rock or fallen branch (in contrast to the leaf-littered, rocky or pebbly streams of the spiny crayfishes). Experiments have shown that growth is faster on a natural substrate such as mud or stones, than on an artificial one such as plastic tanks.
Water Quality
Dissolved Oxygen
The yabby can tolerate very low dissolved oxygen (DO) and has been found in ponds where the DO was below 1-% saturation. However, it almost certainly could not grow or breed in such water and would no doubt eventually evacuate. Water should preferably contain over 4 ppm (parts per million) of DO.
Temperature
The yabby can tolerate water temperatures from near freezing to above 35oC.
pH
The water in most good yabby dams is alkaline (pH 7.5 to 10.5). Yabbies are rarely found in acidic waters (pH below 7).
Salinity
A salinity of 12 ppt (parts per thousand) - equal to about 35 per cent seawater - does not seem to affect the yabby, but it will die above 25 ppt. First year yabbies appear to be a little more tolerant than adults.
Hardness
The water must be hard enough (as dissolved calcium) to maintain strength in the shell, although some calcium is recycled when yabbies eat moulted casts.
Chlorine
Young (but not adult) yabbies are reported to have been killed by high concentrations of chlorine in town water. Such water should be 'aged' for a few days to allow the chlorine to 'blow off' (dissipate) before introducing young yabbies.
Heavy Metals
The yabby can concentrate mercury and lead in its body without harm and so may be useful as a biological indicator of environmental pollution. Amounts of these metals already measured in wild specimens are well below the official health limits for human consumption.
Pesticides
The yabby is susceptible to insecticides and herbicides, particularly the organochlorines. It may be that crop-runoff sometimes carries pesticides into a farm dam, making it uninhabitable to crayfish. Petroleum products are known to be highly toxic to crayfish; runoff from roads may kill whole populations in dams.
Reproduction
The sex of a yabby can be determined quite easily. The reproductive or genital papillae of the male crayfish are short projections on the bases of the last pair of walking legs; the female has oval openings on the bases of the third-last pair of legs. It is common (1 in 20) to find individuals with a combination of male and female openings. These 'intersexes' usually prove to be of one sex and can function sexually; they are rarely true hermaphrodites able to produce both eggs and sperm.
The female yabby reaches sexual maturity when about 9 to 10 centimetres long - the male when slightly smaller. (Length is measured from the tip of the rostrum - the spine between the eyes - to the end of tail fan.) Nearly all mature females spawn, but the majority of young recruited to the population are produced by the 2 year olds, as they outnumber the older age groups.
When freshwater crayfish mate, the male deposits a small packet of sperm gel on the female, near the reproductive openings. The female then passes the eggs out through the openings and across the sperm packet, during which process they become fertilised. The eggs are guided to the underside of the tail (kept cupped during egg laying), where they are fastened on to the swimmerettes (the small legs on the abdomen) and carried until they hatch. Juveniles have special hooks on their legs to allow them to cling to the hairs of the female's swimmerettes; they moult several times before leaving the parent.
The female protects the eggs carefully. She elevates her tail and fans the eggs to keep them oxygenated. If the water becomes too warm, she will find a cooler place. However, because the eggs are large, and because of the time and energy she devotes to them, she can afford to produce only a few hundred compared with the hundreds of thousands of relatively minute eggs of the marine lobsters. The newly hatched young are known as 'juveniles'; they resemble the adults and do not pass through the free-living larval stages of lobsters, prawns and many other crustaceans. The juvenile yabby is consequently better equipped for survival than the young of most of the marine crustaceans and probably has a higher survival rate.
Breeding begins in spring when the water temperature reaches 15 to 16oC. The first batch of eggs (100 to 500 eggs per individual, depending upon the size of the female) hatches 8 to 10 weeks later in early summer. As soon as the young have left (a further 3 weeks later), the female is ready to breed again. Because of the higher water temperatures in summer, the second brood takes only 3 to 4 weeks to incubate. Some females will breed three or more times during the breeding season, which, if the temperature remains high enough, can extend into autumn. In the warmer water in the west of the State, the breeding season may continue almost year around.
Diet
The yabby is mainly vegetarian and prefers fresh food but it commonly scavenges in the bottom detritus. The yabby is not averse to attacking and eating its own kind, especially when the prey is smaller, or soft after moulting. Healthy yabby populations are often found where manure (which is basically partly digested grass) is washed into farm dams from surrounding paddocks, or where cattle or sheep drop it directly into the water. Juveniles and young yabbies eat the same food as adults.
Growth
Like all crustaceans, the yabby must periodically moult its shell (an exoskeleton) to grow; this type of growth is not continuous, but occurs in steps. The moulting frequency depends upon day length and temperature, which stimulate the release of moult-inducing hormones. The frequency decreases with age, newly hatched juveniles moulting every few days or so, a 1-year-old two to three times a year, and a 3 or 4-year-old perhaps only once a year. The new shell is very soft for a short time, and the yabby is then vulnerable to attack by other yabbies, fish and other predators. The clean, pink shell of the new moult contrasts vividly with the dirty, algae-coated shell of the pre-moult animal, especially in older animals.
Growth depends mainly upon water temperature, available food and population density (that is, the degree of crowding). Within limits the warmer the water the faster a crayfish grows. This is because, like most cold-blooded animals, yabbies depends upon surroundings for heat and cannot regulate their own body temperature. Within New South Wales, the fastest annual growth is in the low country to the north and west, where the water is warm for most of the year. Growth is not significant at water temperatures below 15oC and appears to be fastest at 23 to 25oC (South Australian studies suggest 28oC).
Although the yabby can tolerate temperatures up to 35oC, growth appears to suffer over about 28oC.
After maturity is reached, the female grows more slowly than the male, apparently because of the greater effort devoted to spawning. A female never grows to the size of an old male, which can weigh 300 grams. The average yabby caught by amateur anglers is 7 to 20 centimetres long and weighs 20 to 80 grams.
In a study in a typical farm dam in the Riverina district of New South Wales, during a long summer, yabbies grew to 40 to 45 grams within 16 months of hatching. A yabby of this weight is about 10 centimetres long and an acceptable size for both local and overseas markets. This growth occurred over two 'growing seasons' - that is, the time of year when the water at the pond bottom is above 15oC (November to March/April in the Riverina).
If a crustacean loses a limb (claw, walking leg or antenna) it will grow back, starting at the next moult. However, unless the lost part is small, total regeneration is not immediate and usually three or four moults are needed to restore the limb completely.
Occasionally, one finds small, round, stone-like concretions in a fish's stomach. These 'stones' are often seen in aquaria containing crayfish, and can also be found in the nests of some water birds and even in Aboriginal middens. They are gastroliths (literally 'stomach stones', sometimes called 'crabs' eyes') and are produced as a pair in the lining of the stomach of a crayfish preparing to moult. Being the hardest parts of the crayfish, they are either refused or are the last to be digested by predator. After the crayfish moults the gastroliths fall into its stomach, where the calcium of which they are composed is resorbed into the blood. In earlier times, gastroliths were used in medicine for their absorbent and antacid properties.
Death and Disease
Causes of death include old age, predation (natural, cannibalism and fishing), injury, starvation and disease.
In a natural population, with limits to space and food, only a few of the 500 to 1000 young produced by each mature female during a breeding season can survive to 2 or 3-years of age to replace the parents. Therefore, a juvenile has only one chance in a thousand of surviving to old age. Studies of wild populations show that mortality is highest (perhaps 95 to 99 per cent) during the first year of life but as a yabby ages and grows its chances of survival to old age increase. Mortality during the second year is somewhat lower (perhaps 50 to 80 per cent) and lower still during the third year. Survival could be kept much higher in tanks or ponds where predators could be removed or controlled, sizes graded, shelter provided, diseases treated, sufficient food and additives supplied and water quality monitored.
Four or five years appear to be the longest a yabby can live in the wild, except under exceptional circumstances like aestivation during drought (see Environmental Requirements). Its natural predators include fishes, birds, insects, man, and other yabbies. Juvenile and young yabbies are consumed in quantity by small fishes such as gudgeons, goldfish and juveniles of other fish and by other yabbies, large and small. Insects such as the voracious water beetles and their larvae (toebiters), backswimmers, and dragonfly nymphs (mudeyes) may also eat them. Larger fish such as the callop and the Murray cod, water birds such as cormorants and white ibis, other yabbies, platypus, water rat, tortoises and man, all prey upon adult yabbies.
Yabbies are commonly found with claws or legs missing, usually from aggressive encounters with other yabbies or escaping from a predator. Loss of limbs may also be caused by an imperfect moult; moulting is a time of great stress and contributes significantly to mortality. Such injuries lower an individual's defence against predation. Competition for a limited food supply in a pond also weakens many individuals, making them more vulnerable to disease or predation.
Few significant diseases have been reported in Australian freshwater crayfishes. Porcelain disease (also 'white tail' or 'white muscle' disease) can occur in the yabby; it is caused by a microscopic, single-celled animal, the microsporidian Thelohania. The disease is easily detected in the late stages, when the underside of the tail turns white and the walking legs often become splayed and rigid. It is invariably fatal and appears to be transmitted by cannibalism of dead or dying crayfish. In wild stocks, the disease does not seem to be important. In farm dams in the Riverina, it is present in 5 to 10 per cent of dams, with perhaps 5% of individuals in an affected population afflicted.
All Australian crayfishes examined so far have proved susceptable to the plague fungus Aphanomyces, which entered Europe last century (apparently carried by plague-resistant crayfish introduced from the United States) and which has since devastated stocks of the native European crayfish. The plague reached Britain in 1986 and had made massive inroads into the native crayfish populations by 1989.
If infected crayfish were to be introduced into Australia, there could be a massive and permanent destruction of our native crayfish stocks.
Small leech-like animals (1 to 10 millimetres long) are invariably found on freshwater crayfishes. These are temnocephalid flatworms and their eggs are laid on the softer undersurfaces of the crayfish, especially under the tail. Microscopic examination of a worm will reveal a suction disc at one end and tentacles at the other. The worms are not true parasites but commensals (literally 'eating at the same table'), because they feed upon particles of food scattered in the water when the crayfish is eating. They are not actively harmful to the crayfish but if there is abundant food (as can occur with excessive food in an aquarium or under aquaculture), they can multiply to such an extent that they or their eggs impede the flow of water through the gills, causing respiratory stress to the crayfish.
The Fishery
The principal waters fished commercially are the Murray River, the Darling River and its Anabranch and associated lakes, other lakes and overflows in the north-west of New South Wales. There is a large amateur fishery in the far western districts, the yabby being taken mainly for the table but interest declines towards the east. Catches in the Riverina are usually highest during February, March and April, decreasing in winter and spring. The yabby is considered to be good fishing bait, especially for Murray cod. During the 1980s, the yabby became more abundant in the lakes of the Snowy Mountains, and is sometimes used as trout bait.
The commercial yabby fishery has been significant since about 1973 and the total commercial catch in New South Wales can be up to 20 tonnes a year. (South Australia and to a lesser extent Victoria, have had local markets and established commercial fisheries for some years).
Before the 1970s the yabby was relatively unknown as a table dish. This soon changed when its export earnings rose dramatically due to the shortage of crayfish in Europe as a result of the crayfish plague. Most of this export was to Sweden, where the freshwater crayfish is considered a delicacy and fetches a high price and where fishing for it has been forbidden for 50 weeks in the year.
In the rush to export, several South Australian processing firms contracted most of the yabby fishermen, guaranteeing them a firm price for their catches.
For the next few years, nearly all of Australia's yabby catch was exported, most of it coming from South Australian waters, particularly Lake Alexandrina near the mouth of the Murray River.
By 1975 several hundred tonnes were being exported each year, but the industry collapsed as suddenly as it had arisen. By 1978 the fishing grounds had failed, with catches falling dramatically from over 100 to less than 10 kilograms/man/day. Among the reasons suggested for the crash were overfishing, competition from increasing numbers of carp, and the natural 'boom and bust' cycle in yabby populations. During the past decade the yabby grounds have been gradually recovering from the exploitation of that time.
There are regulations governing the type and quantity of gear allowed for fishing for yabbies. Certain regions (including trout waters) are often closed to traps or nets. Because such laws can change from time to time, you should consult the local Fisheries Officer for up-to-date information.
It is illegal to release fish or crayfish of any species into natural waterways, unless permission has been obtained from the Office of NSW DPI. This is especially important if the animal is not naturally found in the area (for example, the Western Australian marron). The common yabby, (Cherax destructor), does not occur naturally east of the Great Dividing Range and must not be released there, it can be a very aggressive animal and could compete with and eliminate the resident crayfishes and other aquatic animals.
Preparation and Cooking
Freshwater crayfish provide the base for many delicious recipes, particularly the Creole or Cajun style from Louisiana, USA. The simplest recipe of all is after killing in an ice slurry, plunge them into boiling water for 5 minutes or so (as for prawns), but leave them slightly longer to allow the heat to penetrate the tougher shell. The edible parts of the yabby are the tail meat (about 20 per cent of the total weight), the claw meat, the 'mustard' and the 'coral'. The tail meat forms the bulk of the edible flesh; only in a large yabby is the meat worth extracting from the claw. The 'mustard' (or 'fat') is the soft, orange-brown liver found in the carapace; it has a mustard flavour and connoisseurs relish it when spread on the tail meat. The 'coral' is the developing ovary or egg sac, found in the carapace of the female; it turns red on cooking and is quite tasty alone, or beaten into sauces.
In the Aquarium
The yabby is an entertaining pet in the aquarium and easy to keep. One or two large yabbies, or six to ten medium ones, would suit a 100-litre tank. Give them rocks or cover of some kind in which to hide when moulting. Feed them small amounts of vegetable scraps, chicken pellets, and a little lean meat now and then, removing what is not eaten each day. The yabby can live for months without eating at all, so it is better to feed small quantities at a time rather than risk polluting the water.
Yabbies breed readily. No special food is required for the young, which will find scraps missed by the mother. There should be plenty of cover, such as water plants, where juveniles can escape from the adults.
Aquatic plants should be tied to large stones to stop the yabby uprooting them while burrowing or searching for food; otherwise use artificial plants. Of course, most fish will eat juvenile yabbies. Keep the water level far enough below the top of the tank to prevent yabbies from climbing the air hose and crawling out.
Aquaculture
Until the late 1990's the gourmet ignored Australia's freshwater crayfishes, but stimulated by overseas interest, they are now in demand for haute cuisine.
The uncertain supply from wild stocks has prompted interest in farming the animals, and research has indicated that some of the Cherax crayfishes, such as the yabby, the marron and the Queensland red claw, might be suitable.
References
Johnson, H T (1980) Catching and cooking the yabby. In Fish & Fisheries, NSW State Fisheries. (Trapping methods, net construction, recipes).
Johnson, H T (1986) Australian freshwater crustaceans with potential for culture - some aspects of their biology, with particular reference to the yabby, Proc. 1st Aust. Freshwater Aquaculture Workshop, Narrandera, 1983. Dept of Agriculture, New South Wales.
Lake, P S and Sokol, A (1986). Ecology of the yabby Cherax destructor Clark (Crustacea: Decapoda: Parastacidae) and its potential as a sentinel animal for mercury and lead pollution. AWRC Tech. Paper 87. AGPS, Canberra. Pp. 186. [Detailed summary of yabby biology]
Mills, B (1980. Notes on the aquaculture of yabbies. Proc. 1st Aust. Freshwater Aquaculture Workshop, Narrandera, 1983. Dept of Agriculture, New South Wales.
April 2013
