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management of clay turbidity using flocculants in ring tanks must therefore consider that such activity
may stimulate problematic algal blooms in a system where new water may only be introduced on a
seasonal and opportunistic basis.
Average ring tank water temperatures were typically seasonal with summer temperatures reaching
27.4ºC and winter temperatures reaching 13ºC. Water temperatures were below 20ºC for 6 months
from May to October in 2001. Optimum temperature for silver perch culture has been reported in the
range of 23 to 28°C with rapid growth when temperatures exceed 20°C (Rowland and Bryant, 1995).
Reduced growth does occur at temperatures below 20°C but no substantial growth has been recorded
lower than 13°C (Barlow and Bock 1981). At optimum temperatures it would be expected for perch to
reach market size in 12 - 24 months.
Differences in both surface and ring tank floor measures of DO and temperature indicate that
stratification of the ring tank was greatest in the afternoons during summer. Stratification occurs when
sunlight heats the surface layers of the water body making them less dense. These upper layers of
warmer water serve to trap cooler more dense water underneath. This effect is most evident in the
afternoon when surface water is heated by the sun but at night these surface layers cool and mix with
deeper waters reducing the degree of stratification. Thermal stratification also prevents mixing
between highly oxygenated surface layers and deeper water resulting is marked differences in DO
levels. The largest difference between the average surface and bottom DO in this study was 5.96 mg/L
for afternoon readings in January 2003. While average DO readings in deeper water were
comparatively lower than surface waters the stratification was not severe enough to constitute an
immediate threat to the aquaculture operation. In severely stratified water bodies the deeper waters are
completely depleted of oxygen. During storm events stratification can be broken and these deeper
oxygen depleted waters become mixed with the shallow oxygenated surface layers. Often referred to
as a ‘turnover’ event, the resultant reduction in the water bodies surface oxygen levels can result in
significant, if not total, stock losses.
The low concentration of DO at the bottom of the ring tank indicates that either there is some mixing
occurring within the storage or the BOD in deeper waters was at the time insufficient to fully deplete
available oxygen. The large surface area (4 ha) of the ring tank is likely to facilitate some level of
regular mixing of water layers but it is unlikely to result in complete mixing of the entire storage on a
continuous basis. It is likely that as the aquaculture activity continues, the accumulation of organic
wastes in the ring tanks sediments will deplete available oxygen in deeper waters and increase the
risks associated with a turnover event. Therefore, any determination of the aquaculture potential of a
ring tank like that at Loch Eaton must consider the long term impacts of management regimes on the
sediments. One management option is rotation of the area used for culture in line with fallowing
principles used in sea-cage culture. Moveable production systems such as cages and floating raceways
would enable fallowing but do not serve to break up the stratification itself. Mechanical
destratification by means of upwelling or downwelling units should be considered when stratification
becomes severe.
Riverine pumping during periods of high flow are an essential means of securing water supplies for
many cotton farmers and other irrigators in Queensland and New South Wales. This type of water
harvesting is often opportunistic and is dictated by seasonal rainfall and river flow conditions.
Pumping this water into the aquaculture ring tank in its raw form clearly had a negative impact on ring
tank water quality to the extent that the conditions at times were unsuitable for intensive aquaculture.
Using an alternative storage or modifying the existing pumping infrastructure, will enable low quality
water to be first be directed into other storages, rather than passing through the aquaculture storage.
This would reduce the impact of flood harvesting on water quality, enable controlled exchanges
through the aquaculture storage and reduce the cost of treating clay turbidity by reducing the total
exchange of water through the aquaculture ring tank. With respect to the Loch Eaton site, relocation of
the aquaculture facility to the adjacent ring tank would achieve these outcomes.
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