Sorting natives from the carp

IN a bid to remove introduced carp from inland waterways NSW Department of Primary Industry fisheries researchers have been deploying a device called a Carp Seperation Cage (CSC). The cage is basically a trap that takes advantage of carp jumping behaviour by isolating them in a holding cage from which they can then be removed.

Carp attempt to migrate to breeding sites when water temperatures increase. If a carp cage is located in their migration pathway the fish will approach the cage and swim into it through a funnel and become trapped. This is where the jumping behaviour comes in. When confronted by a barrier within a river, carp will attempt to jump over it so by placing a wire barrier in the carp cage, their jumping instinct is triggered and the carp are then trapped as they attempt to swim further upstream.

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Native fish can swim under the barrier and baffle component as it cycles through its lifting procedure. The number of carp escaping out of the cage with the native fish is minimised through the instalment of a baffle and timing of cage lifting. Carp are “generalists” in terms of food and habitat requirements and will thrive in modified waterways, which often results in an environment more suited to carp than to native species. CSCs are one effective tool for reducing carp numbers but fisheries scientists say integrating carp removal with river rehabilitation is the most effective method. Strategic aquatic habitat rehabilitation in rivers, which includes restoring fish passage and retaining instream habitat, is imperative to establishing healthy native fish populations and giving them a fighting chance against carp. See how a Carp Seperation Cage works below.

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Diagram: NSW DPI
Figure 1: Details of the Williams’ carp separation cage (from Stuart et al, 2006).
A) shows the operating position to catch and separate jumping carp (black fish symbols) and non-jumping Australian native fish (grey fish symbols)
B) shows the raised position, where
1 is the false lifting floor
2 the cone trap
3 native fish exit gate
4 the non-return slide.

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