Optimising exclusion screens to control exotic carp in an Australian lowland river

Carp exclusion screens (CES) are used to restrict adult common carp from entering wetlands, thereby minimising their ecological impacts and spawning and recruitment potential, but there is marked variation in current CES design and management. We quantified current CES designs, dimensions and locations within the Murray-Darling Basin, Australia. Directional fyke nets at inlets of six permanently inundated wetlands were used to identify fish using wetlands and therefore potentially vulnerable to CES. Morphometric data from captured fish were then used to design CES that excluded sexually mature carp. The ability of optimised and existing CES designs to exclude large-bodied fishes that used wetlands was then assessed. Fifty-four CES with eight mesh designs and varied dimensions were identified. We recorded 18 species comprising 212 927 fish in the wetland inlets. Two optimised meshes to exclude sexually mature carp were developed: a 44-mm square grid mesh and a 'jail bar' mesh with 31.4-mm gaps. Modelling revealed that up to 92% of carp could be excluded by either optimised mesh design, although few young-of-year carp were caught. Optimised and existing CES designs would also exclude 2-65% of large-bodied native fishes. Optimised CES may allow localised carp control without restricting passage of some key native fishes.