Validation of mark-recapture population estimates for invasive common carp, Cyprinus carpo in Lake Cresent, Tasmania

Summary A mark-recapture study based on the Petersen method was implemented in 1998 to estimate the abundance of the invasive common carp, Cyprinus carpio L., in Lake Crescent, Tasmania. Multiple gear types were employed to minimise capture bias, with multiple capture and recapture events providing an opportunity to compute and compare Petersen and Schnabel estimates. A single Petersen estimate on recapture data and two Schnabel estimates - one each on mark (forward-Schnabel estimate) and recapture (reverse-Schnabel estimate) data - were conducted. An independent long-term double tag study facilitated estimation of the annual natural mortality. Subsequent fish-down of the population suggests that, in all likelihood, the carp have been eradicated from the lake, providing an unprecedented opportunity to verify the forward population estimates carried out in 1998. Results suggest that all three estimates were close to the true population size, with the reverse-Schnabel estimate being the most accurate and within 1% of the true population in this relatively large lake (~2365 ha). Greater accuracy of the reverse-Schnabel approach can be attributed to either minimised fish behavioural (i.e. gear susceptibility or avoidance) or computational bias associated with the forward- Schnabel and Petersen approaches, respectively. While the original estimates served as a guide in eradication of carp from the lake, the ultimate validation provides a reliable framework for abundance estimation of this invasive fish in relatively large water bodies elsewhere.