Genotype-based estimates of local abundance and effective population size for Hector's dolphins

Abstract

Conservation and management decisions for wild populations are often based on demographic estimates of abundance (N̂), and less frequently, on genetic estimates of effective population size (N̂e). We used genotype-based methods to estimate both parameters from the same set of biopsy samples, while also providing information on the geographic closure of a local population of Hector's dolphins in Cloudy Bay, New Zealand. The assumption of closure in Cloudy Bay was supported by the lack of genetic differentiation between the two survey years and the absence of any genetically detectable migrants. Using recapture analysis based on genotype identifications, we estimated the abundance of individuals age 1+ (N̂1+) to be 269 (95% CL=233−319, CV=0.12). This was similar to, but more precise than, N̂=230 (95% CL=130−407, CV=0.30) from the more traditional analysis using contemporaneously collected photo-identifications. The N̂e of the parental generation was 191 (95% CL=23−362), and the resulting N̂e/N̂1+ of 0.71 was in reasonable agreement with species of similar life history characteristics. Although N̂e was below the recommended threshold (500, recently increased to ≥1000) thought to be necessary to preserve long-term evolutionary potential in perpetuity, genetic connectivity with neighboring populations on a generational time scale is likely to mitigate the negative effects of low local Ne. Our work demonstrates the breadth of management-relevant information (e.g., N, Ne, sex ratio, genetic diversity, and connectivity to neighboring populations) that can be obtained from a genotype-based analysis, and how conservation implications can change when demographic and genetic population size are considered along with connectivity.