The bio-invasion and population dynamics of Cherax quadricarinatus in Zambian waters

Abstract

Since the 1990s, the exotic Australian redclaw crayfish, Cherax quadricarinatus, has been spreading in the Zambezi River basin in Zambia. The population structure of crayfish was determined at locations along the north shore of Lake Kariba 'near Siavonga' and the Kafue River. Crayfish were sampled from the sites using the catch of fishers at the sites on a monthly basis. Population structure was analysed by length-frequency plots using total length or carapace length measurements. Estimates of population growth rates were estimated by cohort analysis. Data reveal that population structures of crayfish in both areas are robust with multiple year classes present. Estimates of the Von Bertalanffy growth parameters of crayfish were L_∞ = 196.15 mm, ranging from 168 to 215.25 mm total length (TL), k = 0.238, ranging from 0.14 to 0.43 y⁻¹, and mean t_o = −0.465 y, ranging from −0.71 to 2.107. Gravid females were trapped primarily during the austral spring and early summer. There is evidence of growth overfishing at heavily fished sites, but no evidence of recruitment overfishing, suggesting difficulty of extirpation once populations become established. The presence or absence of crayfish at sites throughout Zambia was determined by use of an online survey administered to staff from the Fisheries Department at the Ministry of Fisheries and Livestock, Zambia, assigned to various districts around the country, and to local officials at towns along the Zambezi, Kafue and Kwando Rivers in areas where crayfish may have extended their range. Data were verified by focus group interviews of fishers. Crayfish in Zambia have been spreading unabated throughout western and southern regions of the country. The crayfish have appeared in six previously unreported locations in the Zambezi River basin including the Kabompo River in the Northwestern Province and the Chingola Reservoir in the Copperbelt Provinces, and in the Kwando sub-watershed of the Zambezi Basin in Western Province that borders with Angola and has a seasonal aquatic connection to the critical Okavango Delta Wildlife Refuge in Botswana.