Density effects on juvenile shortfinned eel (Anguilla australis) cover preferences in replicate channels

The effects of density on day-time cover preferences of juvenile (100–199 mm total length) shortfinned eels (Anguilla australis Richardson) were tested in replicate channels provided with natural (cobbles, macrophytes, woody debris) and artificial (shade, plastic pipes) cover during summer. The densities used were 10, 20, 30, 40, 50, 60, 80, 100, and 200 eels channel–1 (each channel = 2.4 m2), with each density tested in triplicate, or better. Eels used cobbles and macrophytes almost exclusively, with cobbles being the most preferred and their use increased with increasing density; however, at 200 eels channel–1, escape behaviour was evident during the night. In tests at 100 eels channel–1, with only woody debris, plastic pipes, and shade sections available, the eels occupied woody debris, pipes, and shade in decreasing order. In further tests at 100 eels channel–1, with cobbles, macrophytes and woody debris included, but with plastic pipes partially buried in the substrate and shade sections lowered to within 20 mm of the bottom, still only cobbles and macrophytes were used. Finally, in tests of 10, 50, and 100 eels channel–1, with cover in each channel consisting of cobbles, macrophytes, woody debris, fallen sod (clumps of natural stream bank with riparian grasses), and modified shade (i.e., lowered to within 20 mm of the bottom with macrophyte rootlets attached to underside), eels preferred cobbles, shade, and fallen sod in decreasing order at all three densities. Overall, the findings of this study do not lend strong support to the hypothesis that juvenile eels use a greater variety of cover with increasing population density.