Summary

Fifty-five freshwater fish species inhabit the rivers of South-Eastern Australia, at least 36 of which migrate at some stage of their life. Adult native fish have been previously observed undertaking large-scale upstream migrations of up to 1,400km and juveniles also perform mass upstream migrations. Dams and weirs often prevent successful migration by blocking pathways to spawning habitats and feeding areas. Since 1913, over 70 fishways have been constructed in NSW to help mitigate these effects.

A cost-effective fishway design that has considerable potential for Australian systems is the Deelder fishlock, which was first constructed on the Muese River, Belgium, in 1958. The Deelder fishlock operates in a similar fashion to a navigation lock for boats and can be best described as a lock incorporating two chambers divided by an internal weir. This paper describes the operation of Australia’s first (and the world’s only currently operating) Deelder fishlock that was constructed at Balranald Weir on the Murrumbidgee River, New South Wales in December 2002. A scientific assessment was done to determine whether the structure could be used at other sites in NSW.

The assessment involved trapping the exit and entrance of the lock over 3 different cycle times (60 min, 120 min and 240 min). A total of 13,448 fish (11 species) and 859 freshwater prawns (one species) passed through the lock. Australian smelt, bony bream and western carp gudgeon contributed 89% of the total catch and the maximum number of fish caught in a single day was 854. Cycle time did not affect the number of fish using the fishlock and sizes of fish sampled from the exit trap ranged from 12 – 540 mm. Movement of fish varied significantly with time of day. The greatest movement occurred between midday and 4pm whilst few fish migrated at dawn or at night. The Deelder lock provided passage for a wide range of species and size classes and is recommended for further installation at other sites where existing fishways are not suitable.