Fish Facts

Fish Facts: Electronic fish tagging

THE development of electronic tags has revolutionised the study of large pelagic fishes, particularly sharks. So called “smart” electronic tags have allowed scientists examine in great detail many aspects of the behaviour of these large and highly mobile fishes which was previously impossible to study.

State-of-the-art tag technology these days has several different formats. One type of more basic tag that has proven very useful for studying fish with relatively small home ranges are the electronic acoustic (or pinger) tags. These come in a wide variety of sizes, but all send out regular electronic signals that are picked up by receivers (also called listening stations) which are placed in strategic locations throughout the fishes habitat. Whenever the tagged fish swims within range of the receiver, the unique signal emitted by each tag is detected and the time of detection is logged. All the researcher has to do is travel to each listening station on a regular basis and download the data from the receiver to determine which fish swam past that location, and when.

From the data quite detailed information can be obtained on the fine scale movements of tagged fish. These tags can also be tracked using a hydrophone if the location of an individual fish needs to be determined at any given time. High end pinger tags can also be equipped with temperature and pressure sensors to transmit additional information, if the researchers budget extends that far.

Pinger tags have been widely used to determine the post release survival and home ranges of a variety of fish ranging from black bream in southern Australian estuaries to offshore sportfish such as Giant Trevally (Caranx ignoblis) on coral reefs. In one study in the Hawaiian Islands, adult GTs were found to roam over entire reefs, but seldom travelled between reefs. Similar results were also obtained for gold spot trevally (Carangoides fulvoguttatus) tagged by CSIRO scientists at Ningaloo Reef in WA. Acoustic tags have also been used on smaller sharks, including black tip reef sharks (Carcharhinus melanopterus).

Pinger tags were used to discover that blacktip reef sharks have quite complicated population structures on coral reefs with most sharks having very small home ranges, and predictable daily foraging patterns.

When it comes to the larger pelagic sharks and gamefish such as marlin, even fancier (and more expensive) electronic tags called pop-up satellite archival tags are being used to provide unprecedented and fascinating insights into the life history of these oceanic wanderers. Pop up tags can log pressure (depth), temperature, light levels and location for many months up to a year, depending on battery life, and also communicate the position of the fish to certain satellites whenever the tagged fish is on the surface. After a predetermined time period, they are programmed to detatch from the tagged fish and return to the surface where they then upload their data via a satellite link direct to the researcher’s computer.

For those species of fish which only surface for short periods (making it difficult to get a satellite fix), dedicated GPS tags have also been developed and used for species such as broadbill swordfish. As an example of the sort of information these tags can provide, recent studies of 2.2. to 2.6 metre long dusky sharks (Carcharhinus obscurus) tagged with conventional pop up satellite tags in Spencer Gulf (South Australia) during the summer months found these sharks all migrated westward and across the Great Australian Bight during autumn to offshore shelf waters off Western Australia. The minimum distances travelled by these medium sized sharks (dusky sharks grow to around 3.6 metres and over 300 kg) ranged from 1760 to 2736 km, with the sharks swimming at depths ranging from the surface to 355 m.

Other interesting satellite tag information for the larger sharks includes: a track of a 2.2 metre tiger shark that swam from Ningaloo Reef in WA to Indonesia and back over a period of around one year. Several whale sharks tagged at Ningaloo have also moved north to Indonesian waters. CSIRO scientists concluded from this that the Ningaloo Marine Park green zones could only “protect” these sharks for short periods.

A 3.5 metre white shark that was tagged off the Neptune Islands in South Australia which travelled west and north to Exmouth in WA over winter, then returned to the Neptune Islands (a distance of over 5000 km) over a period of six months.

White sharks tagged off eastern Australia do not seem to migrate west through Bass Strait. However, one white shark tagged off Port Lincoln in South Australia swam over 6000 km over winter to near Rockhampton in Queensland while another was recaptured in New Zealand. It seems possible that the large whites follow migrating humpback whales up Australia’s east and west coasts, where they presumably try to feed on newborn whales in the whales’ calving areas during the winter months.

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