The models of physical circulation within Conn Creek indicate the tidal flushing times in Conn Creek are rapid. Tidal exchange removes 60% of the water within Conn Creek within 12h during spring tides and within 24h on neap tides. This is the most important mechanism for the removal of suspended and dissolved wastes from Conn Creek.
To estimate the time it takes for biota to absorb dissolved nutrients such as ammonia, which may be higher around fish farms because of fish excretion, AIMS scientists conducted experiments in which a small amount of ammonia or nitrate labelled with a stable isotope of nitrogen was incubated with natural plankton communities. At the conclusion of the experiment the sample was filtered and the amount of15N incorporated into particulate material (phytoplankton) was measured with a mass spectrometer.
The results indicated that the pool of dissolved nitrogen within Conn Creek is turning over quite slowly: 33h in the case of15NH4, and 157h for15NO3, but these rates are similar to those in other North Queensland mangrove creeks. The long turnover times of both these forms of dissolved N suggest that the growth rate of phytoplankton within Conn Creek was not limited by the supply of dissolved forms of nitrogen i.e. that the assimilative capacity of the water column for dissolved nitrogen is saturated.
Particulate wastes sedimenting to the benthos
AIMS scientists used sediment respiration rates as an indicator of organic carbon input to the sediments, including that from waste feeds and faeces of aquacultured fish. Organic C is the main fuel for microbes living in the sediments, and the combined oxygen consumption of the benthos is an index of their activity.
There was no significant difference in mean respiration rates between the wet and dry season, neap and spring tides, or between locations. These results indicate that there is no build-up of organic material underneath the cages, probably because they are rapidly scoured by the strong currents in this creek system.
Particulate wastes and wild fish
Aggregations of wild fishes are a ubiquitous feature of sea cage farms, because fish are attracted by the habitat complexity provided by the sea cage structure itself and by the waste foods. The role of wild fishes around tropical sea cage farms is not well known but could represent a significant loss term for wastes. Accordingly, AIMS scientists attempted to determine whether this was the case at Bluewater Barramundi farm.
A DIDSON (Dual frequency Identification Sonar;
http://www.soundmetrics.com) was deployed around cages during periods of feeding and non-feeding periods. Using this instrumentation we were able to image wild fish communities in the immediate vicinity of the cages irrespective of the turbidity of the water.
The DIDSON was deployed around cages during non-feeding periods, and prior to and during feeding. Though there was evidence of aggregations of small fishes around the cages, we did not witness any convincing difference in fish abundance or behaviour between feeding and non-feeding periods. Most of the fish observed around the cages were highly mobile schools of sardines, but there was also a resident community of rabbit fish that lived in between the predator nets and the main net structure.