Distribution and establishment of the alien Australian redclaw crayfish, Cherax quadricarinatus, in the Zambezi Basin

Crayfish are invasive polytrophic keystone species, which are phylogenetically unique on the African continent. The Australian redclaw crayfish Cherax quadricarinatus is invasive in southern African freshwater systems including the Zambezi River Basin. Surveys conducted across the Zambezi Basin (Botswana, Zimbabwe, Namibia and Zambia) between 2017 and 2019 showed that C. quadricarinatus is broadly distributed across the Upper and Middle Zambezi and is rapidly spreading through natural and human-mediated means across several ecoregions. The probability of capture (P-capture), catch per unit effort (CPUE) and population characteristics of C. quadricarinatus from the recent Barotse floodplain invasion were compared with older invasions from Lake Kariba and Kafue River. The P-capture and CPUE of C. quadricarinatus in the recently invaded region of the Barotse floodplain were similar to those of the older invasions. Mass and carapace length of C. quadricarinatus from the Barotse floodplain were significantly lower than those of C. quadricarinatus from the older invasions. Sex ratios differed significantly between the three invasive populations. The Barotse floodplain population had a disproportionate investment in females (65.3%) and intersex individuals (8.4%). No crayfish were detected in the Zambezian Headwaters or the Okavango Floodplains ecoregions, but current spread rates are 49 km yr(-1) downstream and 12 km yr(-1) upstream. Investment in population management and the prevention of spread will have high conservation value across the invaded regions in order to restrict crayfish ecological impacts via direct predation and competition. In areas where crayfish are in high abundance, existing fisheries are affected through damage to nets, leading to increased net abandonment and ghost gear pollution in invaded regions. The emphasis should be on developing cohesive transboundary biosecurity policies in southern Africa to limit further spread that will threaten the integrity of freshwater ecosystems. However, long-term monitoring is needed to gauge invasion risk to sensitive areas such as the Okavango Floodplains ecoregion and determine field-based ecological impacts.