Underwater sonar reveals the hidden secrets of NSW rivers

In an Australian first, scientists from the NSW Department of Primary Industries are using high-resolution underwater sonar to measure fish migrations and other behaviour.

The dual-frequency identification sonar, known as Didson, allows the movements of fish to be monitored through the transmission of high quality images to a laptop or video screen, without catching or handling them first.

This enables experts to collect new information on fish migration in the state’s rivers, allowing for better management of our freshwater fish populations without interrupting their natural behaviour.

The technology was developed in the mid-1990s by the US Navy for divers to survey mines attached to the hull of ships, but this is the first time underwater sonar will be used in Australia for scientific research.

So far, the unit has allowed the remote observation of fish in areas where scientists have had difficulty observing them in the past, particularly in muddy or dirty waters.

It is common knowledge that dams, weirs and floodgates block fish migrations. What this piece of equipment does is allow scientists to see how fish react when they are confronted with such barriers to migration.

If researchers are able to predict what a fish will do when it reaches a migration barrier, they’ll be able to develop better methods to reduce these effects, such as constructing suitable ‘fishways’ - channels around a barrier that a fish can use to complete migrations.

DPI scientists have also used the Didson to see how fish react to natural structures such as snags, and artificial objects such as traps.

The data revealed that while some fish were caught in traps many more avoided it altogether, or in some cases managed to escape.

The Didson also has many potential uses for studying how fish use habitats. Trials revealed a great deal of valuable information about the behaviour of fish in snag habitats.

Juvenile fish were captured on film hovering in stationary positions within the snags, while larger fish moved through and around in small groups or individually.

The final test for the unit will be a calibration trial at the Narrandera Fisheries Centre to see how accurately it can count fish and measure their sizes.

The information collected from all of these trials will help determine when, and how often fish migrate. More importantly, studies of fish behaviour can now be done without actually catching and handling the fish.

This information will allow much better management of fish populations to ensure completion of their migrations, which is important for dispersal and spawning.

DIDSON FACTS SHEET.

• Freshwater researchers from the NSW Department of Primary Industries are collaborating with staff from the Murray Darling Basin Commission to study fish migrations using a high-resolution underwater sonar.

• The dual-frequency identification sonar, or DIDSON, produces high quality video images, which allow the movements of fish to be monitored, without tagging, up to 30 metres away. Other optical systems such as cameras can provide similar information but cannot be used in turbid or muddy waters, such as around weirs or floodgates.

• The DIDSON overcomes this problem by producing sonar beams (sound waves transmitted through the water to detect vibrations). These beams then transmit high quality images to a laptop or video screen. The unit can also count and measure fish automatically.

• The US Navy began developing the technology in the mid-1990s for divers to survey mines attached to the hulls of ships. It has since been used for a wide range of applications, including search and rescue operations.

• Although the unit has been tested overseas, NSW DPI scientists are the first to trial the unit for fish migration studies in Australia.

• So far, the DIDSON has allowed for the remote observation of fish in areas where scientists have previously had difficulty collecting data. In particular, the size, direction, number and, in some cases, species of fish can now be easily determined from images produced by the sonar.

• If researchers are able to predict what a fish will do when confronted with a migration barrier, they will be able to develop more efficient methods to reduce these effects, such as constructing suitable “fishways.”

• Initially, DPI scientists collected data on fish migration rates with and without a fish trap. While many fish were caught in the trap, the DIDSON revealed that many more were able to avoid it, and some fish were even able to escape the traps. When the trap was removed, small Murray cod and predatory birds entered the fishway to feed on other fish. Scientists had long suspected this occurred, but until now it has been difficult to prove because the technology was not available to directly observe fish.

• The final field test for the unit will be a collaborative study with Primary Industries and Resources South Australia to monitor fish passage through the barrages at the Murray mouth in SA.

• In ponds at the Narrandera Fisheries Centres, DPI scientists will then determine the accuracy of the unit’s counting and measuring. If successful, a DIDSON could be installed in any river channel to provide constant monitoring of fish migrations.

• The information will lead to better management of fish populations to ensure completion of their migrations, which are important for dispersal and spawning and therefore in maintaining and increasing fish numbers.