Rainbow trout fry have been introduced to farm
dams throughout the South-West of Western Australia for over 25
years. In most stocked dams these fry have grown and survived
successfully enough to provide worthwhile fishing. However, trout
need very specific water and catchment conditions to grow
properly, and many smaller farm dams do not provide these
Small excavated tank dams, usually 1000m2 (1/4 acre) or less, are generally too small for worthwhile numbers of trout. Because trout feed by sight, very muddy dams are also unsuitable, as are shallow or very clear dams which provide no protection from predatory birds. Similarly turkey nest dams and swept bank or ring dams with limited catchments are even less suitable.
The best type of dam for growing trout is the creek-line or gully dam characteristic of the more undulating, higher rainfall areas of the South-West. Dams worth growing trout in may also be found further east in the drier, cleared, agricultural area near the Albany Highway, but the rainfall, catchment area and volume of these dams needs to be large enough to maintain a high water level.
High salinity levels do not usually affect trout adversely. Both rainbow and brown trout, being members of the northern hemisphere salmon family (Salmonidae), can tolerate salinity up to the level of sea water, given time to acclimatise, and often grow quite rapidly in these conditions.
Catchments are crucial
Trout are carnivorous, and eat all manner of water and land dwelling insects, crustacea and other small animals. The catchment of the gully dam has a very important influence on the production of trout by determining their food supply. Catchments of native forest or scrub do not usually make for productive fish farms. The high quality drinking water yielded by these catchments is not only generally low in salinity, it is also low in the nutrient content needed as a food base by the small aquatic animals that trout eat. Catchments that are well-grassed and fertilised for grazing pasture are usually the best for growing trout. Cattle should have a limited access lane to the dam for drinking and not be allowed to "pot-hole" the shallow margins and banks, particularly the dam wall.
New gully dams are very productive when the water covers grassed or scrub covered ground. The organic matter, in rotting away, provides the nutrient basis for aquatic food animals to multiply. This effect only lasts for one or two years, after which trout production in forest catchment dams declines very markedly.
Stocking and water fertility
The fertility of the water for producing food animals very largely determines the number of trout to stock and the size they will attain. The point to remember is that trout have enormously flexible growth rates. In one or two rich South-West dams trout nave reached 2kg at the end of their first year of life, and 3kg by two years of age. At the other extreme, in infertile or overstocked dams, fish growth may be stunted, and the trout may not reach fishing size of 30 cm in length in two years.. These trout will also have poor body condition and eating qualities.
The survival of young trout is influenced by two factors. If redfin perch are present, it is a waste of time to stock a dam with trout fry. Redfin multiply to large numbers, eat out the food supply and stunt in size as a consequence, and prey heavily on trout fry. Cormorants (shags) can prey heavily on young fish, too, if the dam water is bare of cover for fish. Natural aquatic weed-beds will provide some cover, but a number of underwater "islands" of logs and large rocks provide far better refuge. Place these where they will least annoy anglers! standing dead trees in the water or near the dam provide roosting spots for cormorants and should be removed or felled.
For a first time stocking of a pasture catchment gully dam try a planting of 1000 rainbow trout fry per hectare (two plus acres). After 12 months the number and sizes of trout available will allow you to judge the future stocking rate. This rate can then be adjusted to produce a large number of smaller fish, or fewer, larger fish.
To get the best fishing or production out of your stocked trout, it is important to understand the effect that spawning condition can have on your stock, and the effect this may have on the tactics you use to re-stock your dam. In Western Australia trout spawn in June. To do this they need a running stream with a gravel bed. The large yolky eggs ( about 2 000 per kg of fish) are laid in holes (redds) dug by the female in the gravel and then covered by her. The eggs hatch and young fish emerge within two months. If you have a good winter creek flowing into the dam males and females, which mature at two years of age, will "run" up the creek searching for suitable gravel. If no gravel is found upstream they will then move ("escape") downstream through the spillway. If the fish can't spawn and remain in the dam, the eggs of the female are reabsorbed, but a new batch is produced in the following year. Energy requirements for producing eggs and searching for spawning grounds decrease the "condition" ( fatness) and slow the growth rate of fish.
In most dams the trout don't spawn so the stock of fish can be controlled easily by fishing and periodically restocking. Generally, after the fist planting of trout fry, fishing should start on, and take advantage of, the good numbers of takeable one year-old fish. Declining numbers of these fish will remain up to four years of age ( rainbows don't seem to survive longer in Western Australia even if not fished). Older fish in the dam will prey to some extent on newly introduced fry so restocking every two or three years is usually best, but fishing pressure may require stocking every year. If a number of dams are available stocks can be "out of phase" so that, by, rotation, fishing on 1-2 year old fish can be continually available.
Other restocking options
Although most trout farmers find it most rewarding to fish and restock as described above, in dams where trout do spawn, their spawning behaviour gives you a number of other options for restocking your dam. The first option, particularly important for gully dams which overflow strongly, is to screen the spillway to prevent escape downstream. Chicken wire should not be used because it blocks badly with leaves and weed. The screen should be made of vertical bars, welded to a frame with 2.5 cm (1 inch) between bars, inclined at 45 towards the top in the downstream direction. The second option is to stock 'triploid' fish. Female triploids are unfertile, though the males ripen each year. "Natural" spawning and restocking by artificially creating the right stream conditions is a third alternative, although few farmers bother with this process because of the time and uncertainty involved. To allow natural spawning, parts of the winter creek need to be widened. If natural water-rounded gravel (anything from pea to football size) is present this will allow the gravel to settle into an area suitable for the female to dig her redds. You can also import suitable gravel for a spawning bed. If natural spawning is successful the results are likely to be uncontrolled, probably annual overstocking of the dam. A fourth choice is to trap spawners by placing a fish trap on the creek, "strip" them of spawn, then return the fish to the dam. Trapping is time-consuming, and the stripping and raising young fry requires considerable expertise. The necessary hatchery facilities for raising young fish from the eggs are also expensive.
This page was last updated on November 15, 2002