Key words: Introduced fish, Pest fish, Carp, Carp Cage, Native Fish Strategy
Fishways facilitate movement of both native and non-native fish and thus provide species such as carp with a significant opportunity to migrate and disperse upstream. Common Carp (Cyprinus carpio, henceforth referred to as Carp) are highly migratory and often dominate the biomass utilising fishways. A carp separation cage (CSC) is a specially designed trap, usually installed on infrastructure such as a fishway, that takes advantage of the jumping behaviour of migrating Carp by drafting them into a holding cage for later removal, while allowing native fish to continue swimming upstream.
Broad aim: The aim of this project was to develop low cost technology to automatically separate adult carp from native fish, and to identify the season, time(s) of day, environmental cues and biomass of adult and juvenile Carp migrating in fishways to better target the use of the technology.
Methods: Two versions of the CSC were trialled within a straight section of a channel near a fishway at Torrumbarry Weir on the middle reaches of the Murray River about 1630 km from the Murray mouth. A design was first trialled requiring manual operation, with on-site weir keepers checking for fish every 24 h. Fish were removed daily. A second automated design was later trialed incorporating a mechanical counterweight system to automatically crowd and release non-jumping fish via a lifting false floor and native fish exit gate. A cage was placed at the exit of the fishway to trap, count, and measure all fish that exited the crowding system.
Findings: The prototype CSC demonstrated that large numbers of Carp can be removed with minimal catch of native fish. However, the need to manually release any trapped native fish limited the application of the technology, especially in remote areas. Subsequent versions of the initial design resulted in several improvements, including:
- being able to operate on the exit of any fishway type (Denil, vertical-slot, lock);
- an increase in the biomass of Carp and native fish that can be held;
- trapped fish can be held in lower water velocity conditions;
- native fish are exited into the weir pool rather than into the fishway;
- the cage is now more transferable among exits or different fishways; and
- access and removal of Carp is more efficient.
Lessons learned and future directions: This study highlighted the opportunity to utilise fishways to remove Carp. The CSC should be targeted to periods of strong carp movement. Spring is a critical time for native fish movement and utilising the technology outside the spring period will maximise catches of Carp but minimise disruption to native fish movements.
For both monitoring and Carp removal purposes it is essential that trap construction, fishway trapping and data collection are standardised across the many locks and weirs of the Murray River.
A commercial trial of the CSC in the fishway at Lock 1 (Blanchetown) has been underway for some years. From 2007-2011, 300 tonnes of Carp have been harvested.
The CSC has also been modified to suit Carp separation at wetlands.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy and undertaken by Ivor Stuart, Alan Williams, John McKenzie & Terry Holt from the Arthur Rylah Institute and Goulburn Murray Water.
Contact: Arthur Rylah Institute, 23 Brown St, Heidelberg, Victoria, Australia, +61 3 9450 8600.