Aggregations of wild fishes are a ubiquitous feature of sea cage farms, with suggestions that these aggregations are caused by habitat complexity provided by the sea cage structure itself and the attraction of wild fishes to 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.

At Bluewater Barramundi farm a DIDSON (Dual frequency Identification Sonar; Sound Metrics Corp., Washington U.S.A.) was hired from NSW Department of Primary Industries and Fisheries and used on one of the field trips. The DIDSON was deployed around cages during periods of feeding and non-feeding periods.

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 schools of sardines which were highly mobile making quantitative analysis difficult.

DIDSON movie 1 shows these schools of small fish around the cage area.