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
conditions.
Unsuitable dams
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.
Suitable dams
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.
Trout survival
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.
Stocking rates
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.
Spawning behaviour
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.
Managing restocking
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