Identification of 'hot-spots' of carp reproduction in the Murray-Darling Basin
Both in Australia and internationally, common carp are a significant freshwater pest in many catchments where they have been introduced. Despite many assessments of potential control methods, few options have been identified for the control of carp across the huge areas they have invaded. The identification of key locations that are important to carp may provide suitable targets for focussed control as part of an integrated pest management strategy. The identification of important carp breeding sites, or ‘hotspots’, may provide these targets. After hatching, carp have a short period where they can be washed out from where they nursery areas during floods. Netting rivers that are in flood using special larval fish nets is an inexpensive way to find where these baby carp are coming from. Over the last three carp breeding seasons, we have netted numerous sites within rivers of the Murray-Darling Basin that were in flood. The information collected shows carp do not breed widely within river systems and that most of the young carp come from a small number of locations within a catchment. Of around 150 sites sampled across 15 catchments, young carp were only caught at 18 locations, with large numbers of carp caught from only seven of these. Each of these seven locations was downstream from a large low-lying wetland which had been flooded, suggesting that these type of habitats are often acting as carp hotspots. However, there are several potential weaknesses of the larval sampling strategy. To confirm the results, electrofishing data collected throughout NSW since 1994 and from the Murray Darling Basin since 2004 was used to map where the very young carp (< 75 mm), carp less than one year old (<150 mm) and adult carp are throughout the basin. This data confirmed that the hotspots identified by the inexpensive larval sampling strategy were in fact carp reproduction hotspots, and also identified hotspots in catchments that had not been sampled during the current project due to lack of flooding associated with the drought. In total, only 18 carp hotspots were identified in the Murray-Darling Basin. In addition, nine other areas have habitat features suggesting that they may act as carp hotspots when flooded. However, seven of these 27 hotspots are much more important than the rest, producing very high abundances of juvenile carp. These include important wetlands like the Macquarie Marshes, Namoi wetlands, Gwydir wetlands and Barmah-Millewa Forest. Finding these carp breeding ‘hotspots’ is a major step forward in developing an integrated pest management strategy for carp in the Murray-Darling Basin, as a small number of carp breeding areas can be targeted, rather than trying to implement carp control activities spread over thousands of kilometres of river.