The effect of ramp slope and surface type on the climbing success of shortfin eel (Anguilla australis) elvers

Instream structures such as dams, weirs and culverts reduce diadromous fish (e.g. freshwater eels) biodiversity and abundance by restricting upstream migrations. Fish passes are used to facilitate fish passage around instream structures, but limited information is available about design aspects that influence the ability of fish to negotiate the structures. We used experimental ramps to evaluate the effects of surface type and slope on the climbing success of shortfin eel (Anguilla australis) elvers (total length <155 mm). Climbing success was examined for three surface types (smooth plastic, sand and gravel, Miradrain) at slopes of 30, 50 and 70 degrees. Elver climbing ability differed among substrates, with the highest success observed on the Miradrain surface (a moulded plastic drainage product). Climbing success decreased with increasing ramp slope. The mean length of elvers successfully negotiating the ramps was longer than that of elvers that failed to climb because only larger elvers could climb the Miradrain surface as ramp slope increased. Results indicated that the optimal ramp-designed passage for climbing species would be lined with Miradrain and set at an angle close to 30 degrees and not exceeding 50 degrees. Elver passage will be detrimentally affected by steep surfaces, which will be exacerbated if suboptimal ramp-surface types are used.