The experiment was conducted in pen
enclosures within a 0.2-ha earthen pond at
the Freshwater Fisheries and Aquaculture
Research Centre, Walkamin in Northern
Australia (l7.l0S, 145.5"E) over 162 d from
22 June to 1 December (i.e., winter-late
spring). .
Pens were fabricated from 9-mm extruded
plastic mesh. Each pen was 4 m X 4 m
X 1.8 m high with no top or bottom. Pens
were secured to the pond floor by burying
the bottom margin of the pen approximately
300 mm into the pond soil. The four corners
of the pens were secured to steel poles,
placed inside the comers and driven deeply
into the pond bottom. PVC pipe (90-mm)
was attached to the top inside margin of
each pen to prevent crayfish escape.
The pond was prepared with applications
of dolomite at the rate of 1,000 kgha, diammonium
phosphate at 250 kgha, and
mulching hay at 1,000 kgha. Additional
applications of fertilizers were used
throughout the experiment to maintain a
plankton bloom. Water was maintained at a
constant depth of between 1.3 m and 1.8 m
for all pens. New water was added only to
match losses due to evaporation and seepage.
Dissolved oxygen, pH, secchi depth,
and maximum and minimum temperatures
at the pond bottom were measured twice
per week at 0830.
Each pen was furnished with a single
100-mm diameter PVC airlift pump (Jones
and Curtis 1994) to provide aeration and
water circulation. Air was injected at 0.435
kPa through a perforated 12-mm polythene
pipe at a depth of 1 m within the 100-mm