Yabby Frequently Asked Questions - Trapping and Yabby Handling
What type of trap and bait do most farmers use?
Most farmers use one of several variations of commercially available folding traps of much the same design- we'll keep out of the argument about which make is best. Traps are rather expensive ($30-40 each), being hand-made, but are robust for a long life with rough use. Bait is dog cubes, sometimes with pieces of pilchards (mulies). We use fine-meshed, double ring marron drop nets and chicken pellets, with a little blood and bone, as bait for our research catches.
Why can't I use a Swan River prawn drag net
This is the traditional gear used by marron poachers in wheatbelt farm dams; they can be in and out quickly. This method is not used by the yabby farming industry. Of course, it needs two operators. It stirs up the bottom sediment in the dam greatly (see gill washing below). Efficiency-wise, it is unselective, catching a higher proportion of unwanted juveniles than traps (see catchability below). Drag netting catches and damages soft, about to ecdyse, ecdysing and just ecdysed yabbies which then have little chance of surviving. Drag netting, then, is a bad harvesting method during the growing season, particularly when dams are low.
My catches vary a lot from month to month and dam to dam; why?
The proportion of yabbies in a dam that you'll catch on any occasion (called catchability) depends on many factors: the number of yabbies present, type of bait, number of traps, water temperature, oxygen level, light (water turbidity), recent feeding, moult stage, sex, spawning stage of females and the size of yabbies.
How many traps should I use in a dam and where?
About one trap every 10 metres of bank at a maximum, which is 15 in a full (springtime) "2000 cubic yard" dam and 10 at low water in autumn. Most farmers use fewer than these numbers of traps in a dam. More yabbies tend to be caught in the shallower, gently sloping water at the "front" (open end) of a dam. During summer, as the water level recedes, traps should be set close to the bank to avoid the oxygen depleted deeper water. Since catchability increases with higher water temperature and oxygen levels, trapping during late afternoon is more effective than in the early morning.
How do I separate out the marketable sizes from the undersize on the dam bank?
A quick method is to use a grader, which you can make. A grader has round, parallel bars, with the gap between the bars set at the carapace width corresponding to the selected Yabby weight, say 30 grams. Since the weight of yabbies for a given carapace width varies, a grader does not give precise ("knife-edge") selection; some of the yabbies retained will be just under 30 grams and some that fall through will be just over 30 grams. If yabbies lose legs or are dehydrated they'll weigh less, too. To make sure that the grader retains only yabbies that are over a selected weight add a millimetre to the following "true" gap values:
| weight (grams) |
grader gap (millimetre) |
|---|---|
| 20 | 18.2 |
| 30 | 21.2 |
| 40 | 23.5 |
| 50 | 25.5 |
Most of the underweight yabbies will fall through the grader bars easily, but those close to the selection weigh need to be put on their backs in a gap. To make a grader, you need a plastic tote bin- with the bottom cut out, some 25 mm dia. PVC pipe for the gap bars and some threaded rod to put at right angles through the PVC bars, every 200 mm, and the sides of the bin for gap spacing and rigidity.
For packing for market, processors prefer to put yabbies into grades more precisely- by weighing each yabbie on an electronic balance (a small, battery-powered one costs $200-300).
What does gill-washing mean and why should I bother?
When you haul a trap it disturbs the bottom, stirring up the black, oxygen deficient (anaerobic) mud, so the yabbies in the trap are pulled through this bacterial-laden water. As the trap is pulled out of the water, the yabbies in it stops pumping water through their gill chambers and close the chambers at the bottom to stop the water running out. As a result, their gills are surrounded by bacterial-laden water. If you now keep the yabbies out of water for some time, they become infected internally by the bacteria. Over the next few days the bacteria multiply in the yabbies, no matter how clean their subsequent tank holding conditions (purging tanks) may be, and then the yabbies die.
The remedy is to have a small tank on the bank with some clean dam water from an undisturbed part of the dam and put the yabbies into it, immediately the trap is hauled. The yabbies then open their gill chambers and pump clean water over their gills. This essential practice is known as "gill-washing".
How can I recognize harmful mud conditions?
When dams are low, pick a clearish dam and near the bank look at the bottom mud, closeup. You'll see that the very top layer of mud is light brown, but if you scratch it, underneath it's black. If you can grab a handful of the black stuff, it smells of rotten eggs or hydrogen sulphide; exposed to air, the black colour will change to brown. The brown top layer is well aerated with oxygen, or aerobic, and the black stuff is anaerobic. Anaerobic sediments harbour Vibrio and Aeromonas bacteria which are harmful to fish and crayfish. As dam levels fall over summer, deeper mud over the clay bed down the batters comes closer to the receding shoreline and trap positions.
How do I store catches until I've got enough for a trip to sell them?
For the unlicensed farmer who delivers to a depot or processing shed, the industry has developed a "floating dam sock" to store yabbies between deliveries. A sock is a flat bag of shade cloth, floated by a rectangular frame of PVC pipe at a dam surface. Oxygen is high at the surface and, with a loosely moored sock, some wind-driven movement of the bag circulates water through it. Yabbies in the sock will purge their guts. You should never hold yabbies in sugar, wheat or other close woven mesh bags in a dam, particularly lying on the bottom, even in shallow water. Never store or transport yabbies in water in a bucket. If all you've got is bags, it's better to store yabbies out of water; keep the bags damp and out of the sun and protect them from any drying wind (hot or cold).
Experienced farmers and harvestors use foam boxes or eskies, often with ice packs in hot weather, to move yabbies. Water or air temperatures over 36oC kill yabbies and they are stressed increasingly by temperatures over 28oC; large rapid changes in temperature, up or down, can kill them ,too. As importantly, they must be kept in a moist atmosphere, when out of water. Crustacea don't have wax in their shells like insects and can dry out rapidly by evaporation in a dry breeze. Even more importantly, their gills must not be allowed to dry out. For crayfish to be able to breath in air, some water must be kept around their gills. Gills that dry out are permanently damaged and damaged yabbies suffocate when they are put back into water, since water contains only a small amount of oxygen (compared with air).
What about holding and purging tanks?
If you intend to "direct market" yabbies yourself, then you'll need to build a packing shed with holding-purging tanks (and have a Yabby farming licence). The tanks should be well-shaded from sunlight by a tin roof, not shade cloth. Sunlight encourages growth of algae ("green water") and yabbies are stressed by strong light. A major problem is water supply. If you have a supply of bore water which does not contain iron (but it can be, beneficially, somewhat brackish), use it; simply dump the tank water between weekly batches of yabbies and refill with fresh bore water. You don't need to pump (circulate) the water through filters, but the tanks should be well aerated, by an air blower (large volume, low pressure), under crates on stands holding the yabbies off the bottom. Bore water is deoxygenated, so aerate the tanks for a while before introducing yabbies. Bore water is also sterile (free of bacteria).
What if water is scarce?
If you have no borewater (and scheme water is too dear to dump) then you'll need to reuse water by cleaning it. The main cleaning that needs to be done is on the nitrogen excretion product from yabbies, called ammonia. Ammonia is very toxic in very small concentrations. A reuse system circulates the Yabby tank water through a physical filter which removes organic waste particles and an aerobic biological filter ("biofilter"), containing good bacteria which "remove" toxic dissolved ammonia- they oxidize it to less toxic nitrite and then nitrate. Some types of biofilters can also act as physical filters, eg large sandbed filters. Instead of fine sand (which tends to clog), more efficient biofilters have the necessarily, very large surface area for bacteria to grow on ("bacterial substrate") as synthetic rings, balls, etc., over which there is a thin, trickling flow of the recirculating water for better aeration. Some guidelines for these water reuse systems are:
- Don't use one if you have good bore water. The simpler the reuse system, the better (and less costly). There are a number of very expensive, specialized add-ons: carbon filters, "clino" filters, UV sterilizers, micron filters. Exclude all natural sunlight, otherwise you'll be battling green water, due to algal growth on the accumulating dissolved phosphate and nitrate. Even the best reuse systems need the addition of, at least, 10% fresh water ("makeup water") a day, to replace lost water and dilute nitrate, etc.. Biofilters (the bacterial colonies) take some time to start up (up to a month). You can't turn a biofilter system off (worse, dry it out) without crashing the bacteria; power and pump failures are problems. The bacterial system works best with a constant loading of excreted ammonia from the yabbies and reacts poorly to load shocks (tanks alternately full and then empty of yabbies). A reuse system, if poorly designed, can have stagnant pockets of water where harmful anaerobic bacteria build up and bleed into the rest of the system. Keep all metal fittings out of a reuse system; copper and zinc (brass) dissolve in the water (worst in acid water) and are very toxic to crustacea. This rule also applies to algae control blocks (copper sulphate used in stock drinking troughs), if you have green water in your tanks.
- Do not use any domestic insecticides anywhere in the shed (fly sprays, cockroach baits, etc); insecticides are extremely toxic in tiny amounts to crustacea as well as insects; sprays drift.
How do I transport yabbies to market?
Purged yabbies (and marron) are usually packed as a particular weight range or grade category in a polystyrene (foam) carton with layering material and a cold pack. Successful shipment overseas is now routine for experienced exporters, though some improvements in materials and cost savings may be possible. Crayfish lose weight in air, so some allowance must be made in packing; an investigation has been carried out for marron and the report also mentions other aspects of transport (Fisheries Research Report No. 99, 1992. " An investigation of weight loss of marron (Cherax tenuimanus) in air during live transport to market. By Noel Morrissy, Peter Walker, Cliff Fellows and Warren Moore.).
I want to keep records of my catches, dam conditions, etc ; any advice?
The golden rule is to write everything down at the time and not rely on memory. Use a notebook on the dams and, later, transfer the notes to a PC spread-sheet, or to a ruled ledger book with a double page for each numbered dam. Column headings could be: date, number of traps set, times of trap setting and hauling, total catch removed in kilograms, catch for each weight grade category, Secchi depth (turbidity), water colour (muddy or green), water level (see below), observations (on newly ecdysed yabbies, female spawners, bugs in the water,..).
Since dams differ in size, Yabby production needs to be related to dam size to compare dams. The easiest measurement dam size is the nearly square surface area of water; from a front corner, measure the width of water, in metres, as a straight line across the front of the dam and, then at right angles, the length of water down a side, averaging over bends in the shoreline. You only need to measure the maximum surface area once, when the dam is just full; the minimum surface area, at low water, can be measured each year in late autumn.
Water level can be read from a pole driven into the clay bed, as the height of the pole above the water surface; - if you measure the corresponding maximum depth of the dam at the centre once, this depth of the dam at any time can be calculated from the water level easily read on the post.
Records kept over a number of years can be analysed for differences between dams, good or bad trends with time and the effects of management practices.
Do the yabbies get any natural food in dams?
The first winter rains each year provide future food for yabbies in the next growing season. The runoff carries crop stubble, pasture debris and sheep droppings into a dam. This organic matter from outside the dam ("allochthonous" material) can often be seen for a time floating in the corners of a dam. Eventually it sinks and is gradually decomposed by bacteria to form "detritus", a protein-rich feed for crayfish. Organic matter is also produced in a dam ("autochthonous" material), from sunlight and nutrients by plant growth, usually as surface "blooms" of minute single cells of suspended green algae. Various small animals, many microscopic, feed on the algae and detritus, providing natural food chains for yabbies.
On these natural feeds, the richer farm dams can maintain a total "biomass" (living weight) of crayfish of up to 1500 kilograms per hectare of surface area of water. However, most dams average only 300-400 kilograms per hectare, without additional hand feeding.
How much and what should I feed?
Farmers have been using readily obtainable, plant material, mostly lupins. About 20 kilograms of lupins per month is the amount usually fed, cast by hand in the shallow water while walking around the dam. As a general rule in aquaculture, it is best to feed little and often; large "one-shot" feeding tend to cause oxygen depletion as the uneaten feed rots.
Can I stock fish in a Yabby dam?
Farming two, or more, species in the same water is called polyculture. The principle of polyculture is that the species should eat different foods and not eat each other (predation) or otherwise harm one another, e.g. through disease. If these favourable relationships exist, more production can be harvested from a body of water than if only one of the species is farmed there. The problem in WA is finding a suitable combination of species. Many fish can't see to feed in muddy water. Some farmers are stocking eastern states Murray-Darling fish (Murray cod, golden perch or callop, and silver perch) in their larger dams. At the moment, fingerlings need to be purchased from a disease-free eastern hatchery, with a permit to import them obtained from the Department of Fisheries. These fish eat small yabbies (which may not be bad where the Yabby stock tends to be crowded and stunted), but they can grow to large sizes and eat big yabbies, as well. How successfully these fish can be farmed in wheatbelt farm dams, alone or with yabbies, needs to be researched.
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