Using radio telemetry to evaluate the depths inhabited by Murray cod (Maccullochella peelii peelii)

Radio telemetry is widely used in studies of freshwater fishes, but the vertical position of fish in riverine environments is rarely reported. The present study tested the application of radio transmitters fitted with depth sensors to determine the vertical position of Murray cod in the lower Ovens River in south-eastern Australia. As the scale of depths in rivers is usually limited (<10 m in the present study), there is a greater need to assess measurement error. The study first involved trials to define depth measurement errors, and a mean relative bias of 9% (range 1.5-14.8%) towards greater depth was recorded. These data were then used to correct the depths recorded from tagged fish. Although data from this preliminary study are somewhat limited, results from the tagged fish showed that by day they all occupied the lower 15% of the water column, indicating that Murray cod exhibit demersal behaviour, using bottom rather than mid-water habitats. Although the present study highlights the importance of tag trials in determining errors, it also indicates the potential application of this technique to understanding the depth-integrated habitat preferences of Murray cod and other species.