Tracking Snapper in Port Phillip Bay

Your fishing licence fees have recently funded a major research project on the movement behaviour of snapper. In November 2011, scientists from the Victorian Fisheries Authority began tagging snapper in Port Phillip Bay with electronic transmitters, called "acoustic tags". These tags, which are surgically implanted into the fish's stomach cavity, emit unique acoustic signals that are detected by arrays of strategically located monitors called 'listening stations' moored on the seabed. The listening stations detect the presence of the transmitter tags within a detection radius of 300-400 m and log data on the dates, times, and unique tag identification number of each fish.

By December 2013 approximately 150 snapper had been tagged in Port Phillip Bay across a size range of 22-87 cm total length. The movements of these fish were monitored until January 2015. Inevitably some fish were recaptured, some died or were eaten by something bigger shortly after release, and some simply didn't get detected by listening stations, but overall,information was obtained from 85 adult (fish > 35 cm) and 57 juvenile (fish < 35 cm) snapper. These fish recorded a staggering 700,000+ detections across the array of listening stations in Port Phillip Bay and across the Port Phillip Heads. Researchers have now completed analysis of all this information and the detailed results can be found in the project report:

The study has increased our understanding of:

1. Movement of snapper between Port Phillip Bay and coastal waters,
2. Movement of snapper within Port Phillip Bay, and their patterns of residency in the main spawning and fishing grounds (i.e. Carrum Bight to Hobsons Bay),
3. Relationships between movement patterns and bay water temperatures,
4. Habitat use, focussing on natural and artificial reefs, and sediment habitats.

Underwater listening stations identify individual fish as they move within 400 m of them.

This is the first time snapper have been studied with acoustic tags in Victoria. It has opened up a window to their movement behaviour with some fascinating results, including the first unequivocal demonstrations of individual adult snapper making repeat migrations between the bay and coastal waters over multiple years.

Acoustic tagging: a) surgical platform for tagging snapper showing continuous flow of anaesthetic, b) acoustic tag being inserted c) floating recovery creel, d) recovered snapper in creel.

Overall the results have shown that snapper have a highly systematic pattern of annual migration into and out of the bay that is cued to seasonal water temperature patterns that optimise the survival of their offspring. The adults aggregate during November in the Carrum Bight to Hobsons Bay region in the plume of the Yarra River, and within a large eddy current. This eddy retains the eggs and larvae in this ideal feeding environment. Both adult and juvenile snapper showed remarkable navigation skills, being able to repeatedly relocate specific location such as artificial reefs within the bay, and for adults, finding their way to and from the bay, often arriving only days apart each year.

When  do adult snapper arrive to Port Phillip Bay during each year's  spawning migrations?

• The earliest tagged snapper arrival occurred on September 2, 2012, but overall most fish arrived in October each year.

When do adult snapper leave Port Phillip Bay after each year's spawning  season?

• Two periods of snapper departures were discovered, the main period was December-January, and a smaller second period occurred in April-May.

How do these migration patterns relate to the bay's water temperature?

• Most snapper arrived though Port Phillip Heads when the water temperature in the bay was between 13-16°C,
• The peak period of detections on the main fishing grounds occurred in November, when the water temperature was between 16-18 °C,
• The migrations are cued to ensure that the egg and larval stages develop in optimal temperature conditions of 18-22°C,
• As temperature reaches 19°C adult snapper disperse away from the main Carrum Bight spawning region and many leave the bay, those that remain appear to go further south and likely stay deeper off Mornington.

Do 'individual' adult snapper come and go each year?

• Yes, many snapper come and go each year and they show consistent times of arrival and departure (often within days between years - see the links to example movement maps)
• Repeat migrants also generally used the same areas of the bay in different years.

How do they behave once in the bay?

• During October-December adult snapper move around a lot. They rarely spend any more than one day near an individual listening station, and mostly spent less than a few hours at any one time,
• Adult snapper commonly move distances of up 10 km in a 24 hour period, some fish traversed the bay from north to south in 24 hours, covering a distance of 40-50 km,
• Adult snapper regular used artificial reefs, particularly in November, and repeatedly moved to and from specific locations, showing exceptional navigational capabilities.

What did the Pinkies do?

• Pinkies showed much longer periods of residency at specific locations, often staying putt for weeks to months at a time,
• Pinkies tagged on artificial reefs typically used these habitats over spring/summer but moved to natural reefs in the north of the bay (Mordialloc-Hobsons Bay) in autumn/winter,
• Pinkies tagged on natural reefs almost exclusively used natural reefs,
• Some Pinkies moved large distances from the Carrum Bight to Geelong Arm, typically following the shallow reef areas to the north and west,
• Pinkies that were tagged on artificial reefs in summer and moved to naturel reef in autumn/winter often homed back to artificial reefs were they were tagged the following spring,
• Only two pinkies appeared to leave the bay over the 400 day period of their tag life,
• Overall pinkies showed a high dependency on shallow natural and artificial reef habitat.

Did you know?

Some of the tagged snapper are still out there, if you find an acoustic tag (left) inside a snapper, or an external yellow tag (right), please measure the fish, note the location and data, keep the tag and call 52580288

Port Phillip Bay showing locations of listening stations (red dots). Click for full-size version (opens in new tab).