Summary

Observing fish in their natural environment is often difficult, especially in turbid water or at night. Traditional sampling methods generally require scientists to interact with the fish in some way to obtain biological information. Disturbing or handling fish may alter their behaviour, so it is difficult to know how ‘natural’ such observations are, especially for short-term studies.

Recently developed sonar systems are currently being assessed in North America and their non-invasive application to fish migration studies is extremely promising. One such device, the Dual-Frequency Identification Sonar (DIDSON), uses sound-distorting lenses to create high quality video images. When operating in high frequency mode, the DIDSON creates images from sound beams that can show the outline, shape and even the fin details of target fish. In addition, the unit’s software can count and measure fish automatically. With such features, this technology should allow the direct observation of fish behaviours such as spawning, feeding and migration even in very low visibility conditions. Therefore, the DIDSON has enormous potential to improve scientific understanding of fish migrations and behaviour in Australian aquatic systems.

A recently completed study demonstrated that the DIDSON is a powerful tool for observing freshwater fish. When used with traditional trapping equipment, the DIDSON consistently provided additional data on fish behaviour that could not be otherwise determined. For example, in fishways along the Murray River, the DIDSON demonstrated that many more fish were approaching and entering the fishways than were actually trapped. In many cases, these fish were actively avoiding traps. The DIDSON also provided useful observations of non-migratory activity by fish and previously unknown information about other aquatic animals. In particular, predatory birds and fish were observed to use fishways primarily to actively hunt prey. Given these many advantages over other sampling tools, the capabilities of this technology should be further explored. Once sufficiently developed, the DIDSON could enhance existing research programs and also provide a new sampling tool for future projects.