Summary

Carp is a notorious pest fish that colonised most lowland areas of the Murray-Darling Basin in the mid-late 1990s. Its spread into upland areas (above 400 m altitude) has been less dramatic and not as thoroughly documented. In order to determine the current distribution and abundance of carp within the upper Murray River, an electrofishing survey was conducted at ten sites between November 2006 and January 2007. Carp were the most common species sampled. They were collected at all sites below 310 m altitude, and were observed at one site where the altitude was 410 m. They were not found at the three sites above 470 m. These data are consistent with the recently reported observation of carp approximately 10 km downstream of Tom Groggin Station, which equates to an altitude of approximately 450 m. A majority of the carp collected were very large individuals, indicating that carp recruitment has been poor for a number of years.

Carp are a highly mobile species, migrating up and down rivers throughout the year. Given that carp can disperse great distances, that there are no artificial barriers to fish passage (i.e., weirs) along the length of the upper Murray River, and that they have been present in the Upper Murray catchment for at least 27 years, the current distribution of carp in the upper Murray catchment must be limited by too steep a stream gradient or some other factor occurring near an altitude of 450 m. Apart from possibly colonising the Khancoban pondage, it is unlikely that carp will continue to spread much further upstream than they already have.

There is limited evidence to suggest that carp are having a major impact on the aquatic ecosystem of the upper Murray River. Given these limited impacts, the costly exercise of implementing a carp control program in the upper Murray catchment is a lower priority than developing nation-wide control alternatives. However, some control options currently exist, or are being investigated, that may be suitable for implementation in the upper Murray catchment. Two physical control options are currently available.

The ‘Judas fish’ approach, where radio tagged males are used to locate spawning aggregations, has been successfully used in Tasmania and can maximise the success of physical removal activities. Williams’ Carp Separation Cage technology is currently the most cost-effective physical control option, but requires the construction of a new ‘leaky weir’ and vertical-slot fishway in the lower reaches of the upper Murray River (near the upper limit of Lake Hume).

Two potentially viable biological control options, Koi Herpes Virus and Daughterless Carp Gene Technology, are currently under investigation by the Invasive Animal CRC. But, the results of these investigations are unlikely to be available for several years. However, given the isolation of the upper Murray catchment from the lowland reaches by the presence of Hume Weir, the upper Murray may provide a suitable field trial location for either of these control options.