Contribution of invasive carp (Cyprinus carpio) to fish biomass in rivers of the Murray-Darling Basin, Australia

Following the introduction of the 'Boolarra' strain to Australia in the mid 1960's, Common carp (Cyprinus carpio) rapidly spread in waterways across the Murray-Darling Basin (MDB) in southern-eastern Australia. A previous study of the population expansion based upon data up to the early 2000s suggested the carp population may have reached, and was fluctuating around, the ecosystem carrying capacity. It has been widely stated in the scientific literature and media that carp comprise up to 90% of fish biomass in MDB rivers; however, this figure is largely based on sparse data collected in the 1990s and the accuracy of this estimate is uncertain. Here, we analyse three decades (1994-2023) of electrofishing data to estimate the median proportion of total fish biomass contributed by carp across > 1,400 sites in lowland rivers (< 700 m elevation) of the New South Wales (NSW) portion of the MDB. The proportion of sites with fish biomass dominated (> 90%) by carp has declined from approximate to 33% of sites in the mid-1990s to approximate to 20% of sites. The proportion of biomass comprised by carp varied depending on the scale of analysis. Across the NSW MDB, carp accounted for a median of 57% (95% CI: 42.5-70.4) of fish biomass at the site level. At the river catchment scale estimates ranged from approximate to 45-66%. In contrast, the median proportion of carp within key rivers ranged from approximate to 46-80%, suggesting that carp can dominate total fish biomass in some rivers for short time-periods. The proportion of biomass contributed by carp at the NSW MDB scale was remarkably stable across decades, suggesting that the carrying capacity of carp may have been reached. Our revised estimates of the contribution of carp to overall fish biomass can facilitate informed debate around the dominance of carp in MDB rivers and provide important information for carp management in the future.