Yabby Frequently Asked Questions - Farm Dam Characteristics
When I've swum in a dam during summer, why is the water warm at the surface but much colder lower down?
Heat in sunlight penetrating water is progressively absorbed down from the surface. In deep, still, clear water, the water temperature decreases gradually with depth; some metres down the temperature decreases rather more rapidly, most rapidly through a layer called the thermocline, and then changes little to the bottom. In contrast, in muddy farm dams, most of the sunlight heat is absorbed, or reflected (albido), very close to the surface by the suspended clay particles and the thermocline is usually less than a metre down from the surface.
Why doesn't the hot surface water mix with the cold bottom water?
Hotter water is lighter (less dense) than colder water so the water layers on either side of the thermocline mix only very little and heat is transferred only slowly by diffusion and not by convection- circulation; this so-called temperature stratification during the daytime is very stable and hard to mix.
When does the water mix?
Mixing occurs regularly during the cold, windy winter and to a lesser extent in the warmer, calmer spring and autumn. During daytime in summer only the winds of the occasional cyclonic storms are strong enough to mix the water layers; the stronger the wind, the deeper the mixing, so the deepest bottom water, down to 3-4 metres in the centre of a dam, mixes very infrequently. Mixing can also occur before sunrise if the night is cool enough to bring the surface water down to the same temperature and density as the deeper water; some wind will then mix the dam. A rare cold, summer night can cool the surface water to a lower temperature than the deeper water; the density layers are then unstable and the surface topples over without much wind help- the dam "overturns". The problem with natural reaeration of bottom water by early morning mixing with surface water is that algae at the surface may have removed most of the oxygen there overnight.
How much of this colder water, cut off from the surface, is there?
About two thirds of a dam volume is under the thermocline; of course, there is more cold, stagnant water in a muddier or deeper dam. More importantly, you need to think of the large area of the bed of the dam overlain by this stagnant water, since yabbies crawl on the bottom.
What does this temperature stratification mean for yabbies?
The temperature of the deeper water in dams is not very favourable for yabbies. The water under the thermocline is usually about 14oC at the start of hotter spring weather and warms to only 19oC by the end of summer. Above the thermocline, the water heats and cools daily in the range 20oC to over 30oC (they can tolerate temperatures up to 36oC). Yabbies grow most rapidly in the range 24oC to, a maximum at, 28oC and below 20oC growth slows down to a halt below 15-16oC. These facts mean that there is only a relatively small area of the bed of a dam, under shallow water around its edges, which is likely to have the most favourable temperatures for growth of yabbies during summer.
Is the stagnancy of the deeper water bad ,too?
The oxygen concentration of the mixed surface water above the thermocline is usually close to 100% during the daytime; it may even be "supersaturated" (>100%) right at the surface due to algae, even in a muddy dam. However, the oxygen declines rapidly through the thermocline to well below 50%, and frequently close to zero, in the deeper water.
What causes the low oxygen in deeper water?
It's a combination of the removal of oxygen by rotting organic matter in the water and, more so, on the bed of the dam and lack of circulation of this water to the surface where natural reaeration could occur.
What do the yabbies do?
We know that marron in these dams are forced into shallower water during summer. Although yabbies are more tolerant of low oxygen, they probably favour the warmer, well oxygenated, shallow water, too. The bad effects of this crowding onto a smaller area of the dam bed are reduced survival and growth, since these production factors are very much influenced by density (crowding) of crayfish.
What can I do to aerate my dams?
A lot of people have thought about this problem and we are researching it. There are two related aspects. Firstly, if you spend money on an aerator and its maintenance, the cost has to be offset, at least, by increased production of yabbies, for your Yabby farming to stay in the black. Probably, mechanical aeration is not worthwhile unless Yabby farming is made much more profitable by it. Secondly, the amount of energy needed to mix a dam is quite high, so it seems that a powerful and, therefore, costly aerator is needed.
What types of aerators have been considered?
In one of our shallow, purpose built marron ponds (area, 1000 m2 ), a paddlewheel aerator (~ $1000) is run briefly three times a day on mains power; this aeration is necessary to offset the oxygen demand of a high rate of daily feeding and allows crayfish production at about five times that of a farm dam. Farm dams don't have mains power, so use of wind and solar power are obvious possibilities. The problem with windmills supplying power directly (no battery storage) is lack of wind when you need the power to mix dams during summer. We are currently researching the industry suggestion of a single solar panel unit ($400) operating a small aquarium pump to circulate surface water to mid-level in a dam. One of our suggestions has been for a farmer to visit dams once a week during midafternoon and mix each in turn for about 20 minutes or so with a big outboard motor or a paddlewheel on a tractor power take-off. This method needs to be trialled.
Is there any way of improving the natural aeration of dams?
Although muddy dams are an advantage for Yabby farming (see below), many dams are too muddy with clay. The Department of Fisheries of the thermocline and, therefore, the area of favourable dam-bed could be increased by reducing the muddiness, somewhat. We know how to do this in theory (by liming, see below) but the technique needs to be researched on dams for fine tuning. It has other benefits, too.
Why do yabbies do so well in muddy dams?
Crayfish have a great instinctive fear of bird predators (e.g., shags or cormorants). During daylight hours crayfish in clear water are very vulnerable to shags and are inactive in hiding places. However, the muddy water in most dams provides 24 hour nighttime light conditions on the bed of a dam, so shags can't see to hunt there and crayfish are active during the daytime. Crayfish don't need to see to feed; they can do it by touch, smell and taste. Underwater darkness in muddy dams is the reason why yabbies can be harvested so readily with baited traps during the daytime.
Why are some dams muddier than others?
Clay turbidity, as muddy water is called, is due to small particles of clay suspended in the water. These "colloidal" particles have electrical (negative) charges so they repel each other (like the same poles of two magnets) and don't settle out by clumping together. Some clays are finer than others; red montmorillinite gives much more turbid dams than white kaolin. However, the major factor controlling clay turbidity is salinity; the more saline the water, the clearer the dam water will be; the suspended clay particles are neutralized by the positively charged salt "ions" dissolved in the water. Basically, this is why dams towards the salinized bottoms of valleys in the wheatbelt tend to be clearer.
Why are some dams muddy in winter and clearer in summer?
The salinity of all dams increases over summer as the water evaporates as a gas (water vapour), leaving behind the dissolved salts (stock drinking and seepage removes both water and salts). Increasing salinity drops out (flocculates) the clay turbidity. When the dams overflow in winter, salts are diluted and flushed out by lower salinity inflow so the dam salinity drops- to the inflow salinity if the dam overflows for long enough- and the dam goes turbid again.
If I wanted to keep a record of the clay turbidity of my Yabby dams, how would I measure it?
A simple, but scientific, measurer is called a Secchi disc. Cut out a 30 cm diameter tin disc, mark it out in eight sectors of the circle and paint alternate sectors with flat white and black paint; screw the disc at its centre to one end of a pole, a metre or so long, marked at one centimetre intervals. The Secchi depth, as a measure of turbidity, is the water depth where the disc just disappears from view as you lower it or just reappears as you raise it (take the average of the two readings). Monthly recording is often enough. Very muddy red clay dams may have a Secchi depth of 5 cm (the Department of Fisheries of penetration of most of the sunlight).
What is salinity?
Salinity is the quantity of all the salts dissolved in water. The major salt in the southwest is table salt (sodium chloride = NaCl). In the dissolved state this salt splits (dissociates) into "ions", the positively charged sodium (Na+) cation and the negatively charged chloride anion (Cl-). Other salts give the cations, potassium (K+), magnesium (Mg+2) and calcium (Ca+2) and the anions, bicarbonate (HCO3-) and sulphate (SO4-). The southwest waters are rather unusual amongst world freshwaters in having a very high proportion of sodium and chloride due to their accumulation in the wheatbelt groundwater from seawater spray carried inland by rain. The other salts are low because of the ancient, leached- out southwest catchments.
I've got a water analysis report which is hard to understand; what does it all mean?
Salinity may be reported as total dissolved salts (TDS) or total soluble salts (TSS) and will be given as a concentration- a total weight of all the dissolved salts present per unit volume of water. The old British units of concentration were "grains per gallon" (gpg); the metric units are milligrams per litre (mg/L) which can also be called parts per million (ppm); to convert gpg to mg/L, multiply by 14.25. These units are used for low freshwater salinities, which in the southwest range from 100 mg/L to 500 mg/L for uncleared (non-salinized) catchments; most farm dams are in the range 300-1500 mg/L. You can start to taste salt in water at 2000-3000 mg/L (brackish) and seawater is about 33000 mg/L. Higher salinities are given as grams per litre (g/L) or parts per thousand (ppt) or with the symbol S%o (1000 mg/L = 1 g/L = 1 ppt). All the salts in water are difficult and expensive to measure individually. However, salinity can be measured quickly with an electrical conductivity meter- the more salts, the more current is passed. The conductivity (a "K" value, subscripted by a standard measuring temperature-usually 25oC) is then multiplied by a factor to get the salinity concentration; usually the freshwater factor is 5.5-6.2 for K given as mmhos/m (0.55-O.62 for K units given as umhos/cm).
How much salinity can yabbies take?
Yabbies can survive up to about half seawater (17000 mg/L), but as a group they become unsociable over 12000 mg/L and growth slows over 6000 mg/L. Research is needed on Yabby eggs and hatched young, which may be less tolerant. Salinity tolerance is not really an issue because yabbies do well, and you can trap them, in muddy dams. Muddy dams will usually go clear over about 1000 mg/L salinity.
Do I need to be concerned about any particular salt ions in my dam water?
One element known to influence freshwater crayfish production in WA is calcium. Most southwest soils are notoriously deficient in calcium. Since crayfish use a lot of calcium to make their hard shells, they are good at extracting calcium from low levels in water. When changing their shells to grow, crayfish conserve calcium as the hard white buttons or gastroliths stored in their heads. However, processors occasionally notice deliveries of lighter/softer shelled yabbies from particular farms and dams. Some dams have very low levels of dissolved calcium and may develop even lower levels because a large proportion of the dam's calcium is removed in harvested yabbies. Also, overseas studies show that alkaline waters (pH >> 7.0), richer in calcium, have more productive natural food chains. Waters with 0-10 mg/L of calcium are regarded as calcium poor; with 10-20 mg/L as medium range; and with 20-30 mg/L or more as rich. Calcium is not a threshold survival factor; crayfish occur naturally in the purest southwest stream water and elsewhere at salinities of less than 100 mg/L with calcium at 1-2 mg/L (though, don't try them in distilled or rain water). However, calcium concentration may be a limiting growth factor. Two past studies on marron in wheatbelt farm dams both showed a positive relationship between calcium concentration ( poor to rich) and crayfish production or biomass over a range of dams. Whether, or not, this a direct relationship needs to be confirmed by more research.
How do I know whether I've got low calcium?
Calcium in water is expensive to get measured through a commercial laboratory. We may be able to come up with a cheaper, easier way for farmers to rate their dams soon, probably in cooperation with the Ag. Dept. which is familiar with the calcium problem for crop and pasture. On a regional, if not locality, basis, the Ag. Dept can give you an idea of how your farm rates for soil calcium.
For differences between dams on your farm, you can get a good indication from the fairly close relationship between salinity and calcium from measurements of water conductivity in springtime. Very turbid dams, well up the valley slope with salinity as mg/L in the low hundreds, also usually have low calcium. If you see many noticeably lighter/softer shelled yabbies trapped from particular dams, these may indicate a calcium problem, particularly if your processor notices the same incidence amongst many batches. However, some caution is needed in becoming unduly concerned; crayfish are normally softer shelled over the gill covers just after ecdysis in moult stage early C, when they are most catchable in traps.
How do I increase the calcium in a dam?
Regular liming is a well-recognized aquaculture practice for dirt fish ponds elsewhere in the world and there is a good deal of information on its benefits. However, while recognising that we need to take the chronic calcium deficiency of soils in the southwest into account for marron and Yabby farming, more local investigation is needed and we hope to report the results of a detailed study for yabbies in a year or so. In the meantime, adding powdered agricultural limestone to a dam, if you suspect a problem, won't hurt the yabbies. However, adding calcium will decrease clay turbidity, so stick to very turbid dams- which are the most likely to be very low in calcium. To raise the calcium concentration of a "2000 cubic yard" dam by 10 mg/L, you'll need to add at least 50 kgs of powdered agricultural limestone.
Some dams have green water over summer and go clear after autumn-why?
The green water or "algal bloom" is another form of water turbidity called algal turbidity; it is due to microscopic, single-celled, floating plants, called "uni-cellular algae" or "phytoplankton". As with all plants, algae needs strong sunlight (plus nutrients- phosphate and nitrate- and carbon dioxide) to grow. In clay turbid (muddy) dams the algae are shaded out and grow unnoticeably in lesser numbers right at the surface. Green water is characteristic of salinized dams, cleared of clay turbidity, near valley bottoms. These lower dams with larger catchments are often characterized by higher inputs of organic matter which leach to supply nutrients for algal growth. Nutrients are largely stored in the dam bottom sediments during winter- so they don't get flushed out of the dam- and they are released from the sediments during summer when the bottom water becomes deoxygenated. Algal blooms die off and these dams clear, when the daylight decreases in autumn; the rotting mass of a large dead bloom causes very poor oxygen conditions for a while.
What do algal blooms do?
Green algal blooms produce very large changes in oxygen concentration in the surface water over 24 hours. Plants produce oxygen by photosynthesis in excess of its use for their respiration (breathing, like us) during the daylight hours, but at night they just respire. So during mid-afternoon dam surface water becomes supersaturated with oxygen from an algal bloom, but overnight the oxygen falls to a very low level by sunrise. Some algae are present in even the most turbid dams. At very high nutrient levels (polluted dams), green algae give way to blue-green algae which produce toxins. Some blue-green algae can move up and down in the water with light change; at sunrise they can be seen momentarily as a dense, dark matt at the surface, which rapidly disappears as the sun comes up. You've probably read about all this in relation to Peel-Harvey and the suburban lakes in Perth, not to mention the Darling River going blue-green in early 1993. Nutrient enrichment is also called "eutrophication".
Should I try to kill off an algal bloom in a Yabby dam?
Trying to kill an algal bloom (green water) is really "treating the symptom rather than the cause." The cause is too much nutrient, or fertiliser in the water which is usually due to pollution from a summer flood (see below). As noted above, killing off the algal bloom will drastically reduce oxygen levels for a time and the nutrients (stored in the bottom mud) will soon be recycled to produce another bloom.
What about using algicides?
Using chemicals to kill off algae needs some caution, as noted above. One that should never be used is Copper Sulphate (blue crystals "bluestone"). Copper Sulphate is found in "algae blocks", used to treat stock water troughs. Copper is extremely toxic to yabbies and other crustacea and is non-biodegradable (i.e. accumulates in the environment). House drinking water dams can be treated with a chemical called simazine which in itself is not toxic to yabbies at the dose recommended byDepartment of Agriculture (1-2mg per litre).
How well do yabbies cope with low oxygen levels?
Yabbies can survive for much longer times at lower oxygen levels than many other crayfish, such as marron. At zero oxygen they can change their metabolism to a form which doesn't need oxygen, although this can't go on indefinitely and they must be inactive. They can survive and prosper relatively well in richer dams where the winter clay turbidity is replaced by algal blooms at the start of summer; marron will die off in these very rich dams. One observation by farmers, which fits our current Yabby dam theory but needs to be confirmed by actual research, is that the, usually overabundant, spring-summer spawning in dams with year round clay turbidity is less successful in algal bloom dams because the eggs and hatched young attached under females' tails are less tolerant of low oxygen. Fewer less crowded juveniles will grow through to marketable weights more quickly. However, it needs to be remembered that while yabbies can survive at extremely low, if not zero, oxygen levels, they can't be active (trapped), feed, moult, mate and spawn under such extreme dam conditions.
There seems to be many different types of water conditions in dams; is there any pattern which is related to Yabby production?
There are large physical and chemical differences between farm dams and in the amount of crayfish they produce. We could measure many factors- and there are a lot- over a large number of Yabby dams for several years and hope that at the end there is some pattern. The more scientific method we are using in our experimental research program on yabbies over the next few years is to test and improve a theory developed some years ago from marron research in dams in the Kojonup area (Fisheries Research Bulletin No. 24, 1980. " Production of marron in Western Australian wheatbelt farm dams". By N. M. Morrissy). This theory related marron production to dam conditions, but obviously needs to be extended into more enriched dams for yabbies. Some important factors need to be considered separately- clay type, rainfall, evaporation, air temperatures, catchment calcium- as regional variations. Basically, our theory takes a series of dams on the slope of a typical wheatbelt valley and models the range of dam conditions and Yabby production.
Are there extreme types of dams where yabbies do very poorly?
One extreme type of dam is the highly organically polluted dam on the valley floor, which is quite saline and, therefore, clear of clay turbidity (exposed to shag predation) in autumn and spring and has a toxic blue green algal bloom in summer. The other extreme type of dam is the very muddy, low salinity (and low calcium) dam well up a valley hill slope, with a small catchment, perhaps roaded, and little organic input.
What about those summer floods, which turn sheep off drinking dam water?
One wildcard is the summer storm which can hit and miss anywhere. However, lower dams with larger catchments and inflows along gully lines seem to be more at risk from the organic pollution. These floods knock out successful marron dams. Yabbies appear to fare better. Our Yabby study dam at Pingelly received an enormous inflow from the March 19, 1993 downpour. Although the dam immediately went clear (dark with dissolved organic matter which flocculates clay particles) and trap catchability was virtually zero in April, the Yabby stock survived; clay turbidity and catchability were back to normal in September 1993, though spawning was depressed in the springtime.
What can I do about pollution of my dams by summer floods?
For many years, farmers have placed a small "silt-trap" dam above a main dam that they especially want to keep clean. Some farmers have placed a low chicken wire fence across the front of the dam to hold back the larger debris in a flood runoff. More recently, a prominent Yabby farmer has trialed a "fish net" which can be drawn across a dam to remove floating debris from a flood. As a last resort, farmers siphon drain a polluted dam and dragline out the black organic bottom ooze before winter.
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