MULLOWAY MOVEMENTS IN THE GEORGES RIVER

 In order to identify key-habitats of sub-adult mulloway, and mulloway movements, fish were implanted with acoustic pingers and tracked using a boat based hydrophone and receiver (manufactured by Sonotornics: www.sonotronics.com). Each pinger emits a series of bleeps on different frequencies that are detected audially, with each pinger having a unique frequency and code combination. This allows individual fish to be identified.

Pingers are implanted surgically by experienced researchers, using procedures similar to that used by vets involving anesthetic at appropriate rates, suturing and infection control (UNSW Animal Care and Ethics Approval # 02/115). Fish generally show good recovery after surgery, with fish held in the hatchery resuming normal swimming behavior within 5 minutes, and feeding within 12 h of the procedure.

Key habitats of juvenile mulloway were discrete deep holes. The preferred holes also appeared to have a good degree of refugia, such as undulating bottom topography, and submerged vertical rock faces. These habitats are ideal for juvenile mulloway as they can provide refuge from strong currents and predators. You can download a bathymetry map showing key habitats for mulloway in a 15km stretch of the Georges River here. The map shows latitude and longitude coordinates so holes are easy to find with a GPS. For those without a GPS, the tributary at the top left of the map is Salt Pan Creek.

We found a large difference between movement of smaller and larger juvenile mulloway. Smaller mulloway (>50 cm) tended to remain with the holes for the majority of the day and night, whilst larger mulloway (50 – 80 cm) moved out of the holes at night. Movement of larger mulloway was significantly greater than small mulloway. This makes sense when the diet changes of juvenile mulloway are considered. Fish smaller than 50 cm are primarily preying on mysid shrimp and school prawns, which are a fairly sedentary prey. Fish greater than 50 cm are preying on baitfish and cephalopods such as squid, which are an active prey that the mulloway actually have to hunt, hence the difference of movement. This also explains the lack of mulloway smaller than 50 cm that are caught by anglers, since the mulloway are moving less and not targeting fish or squid, there is much less chance they are going to swim by a hook and pay any attention to the bait that’s on it.

Longer term movements of mulloway showed a “possible” seasonal migration into warmer offshore waters during May/June. As not many fish were tagged, it is hard to say how many of these fish actually return to the same estuary, however automatic listening posts have been deployed in the Georges River to detect any fish that returning to the estuary. Keep an eye on this site for updates on this……..

 

Hydrophone used to detect signals from impanted mulloway.

 

Acoustic pingers implanted in mulloway for tracking (5c piece for scale).

 

Acoustic pingers being implanted in live, anethetised mulloway by Tom Mullaney. Mulloway respond well to the surgical procedure, with resumption of normal swimming behavior within 5 minutes. Fish are also observed eating with 12 h of the procedure.