Effects of flow regulation on carp (Cyprinus carpio L.) recruitment in the Murray-Darling basin, Australia

Carp biomass density and the numbers of 'sub-adult' (juvenile, plus young-of-the-year) carp were used to indicate net recruitment (hereafter 'recruitment') to carp populations in 'regulated' and 'unregulated' lowland rivers (< 300 m altitude) and 'slope' rivers (300-700 m) of the Murray-Darling Basin, southeastern Australia. Most recruitment occurred at lower-altitude reaches, during October-March (water temperature 95% confidence limits= 16-32 degrees C). Temporal changes in distribution of juvenile carp and sub-adult carp, and high biomass density and slightly larger carp sizes in slope populations, indicated that slope sites were 'sinks' for downstream 'source' populations. The distribution of sub-adult carp also suggested that recruitment differed between regulated and unregulated lowland sites, and unregulated sites had proportionately more sub-adult carp in the total catch. We suggest that while floods might have enhanced recruitment in unregulated rivers, flow regulation provides long-term refuge from mortality associated with high flows. Copyright (c) 2005 John Wiley & Sons, Ltd.