Estimating the abundance and effective population size of Māui dolphins 2015-2016
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2016Download the publication
Summary
Here we report on initial results from the continued genetic monitoring of the Māui dolphin subspecies (Cephalorhynchus hectori maui) in a study carried out over 2 years from 2015 to 2016, following methods reported previously for surveys conducted in 2010–11 (Oremus et al. 2012; Hamner et al. 2014b) and from 2001 to 2007 (Baker et al., 2013). Our primary objectives were to estimate the abundance and effective population size of Māui dolphins in 2015–16, as well as to document movements of individuals, including migrant Hector’s dolphins (C. h. maui), using DNA profiles derived from biopsy-dart samples. We also matched DNA profiles from biopsy samples collected during the 2015–16 surveys with those from previous surveys in 2010–11 and in 2001–07, as well as with necropsy samples obtained from beachcast individuals. The integration of initial results from 2015–16 with previous results provides records of identification by DNA profiles of individuals, both living and dead, extending across 16 years.
Small-boat surveys dedicated to the collection of biopsy samples of Māui dolphins were conducted from just south of the entrance to the Kaipara Harbour in the north to the Mokau River, Taranaki in the south during austral summers from 12 February to 1 March in 2015 and from 10 February to 5 March in 2016. Details of the annual surveys are included in Appendices 1 and 2 of this report. A total of 92 biopsy samples were collected during these surveys from individual dolphins of age one year and older (48 in 2015 and 44 in 2016). DNA profiles were completed for each sample, including genotyping of up to 25 microsatellite loci (average of 23.8 loci/sample), genetic sex identification and mitochondrial (mt)DNA control region sequencing.
For the 2015–16 surveys, the census abundance of Māui dolphins, excluding the two Hector’s dolphins, was estimated to be 63 individuals of age 1 year or older (95% CL = 57, 75), using a two-sample, closed-population model. This estimate is comparable to, but slightly larger than the previous estimate of N = 55 (95% CL = 48, 69) based on the genotype surveys in 2010–11. An effective population size of Ne = 34 (95% CL = 24, 51) was estimated from the genotypes of the 49 Māui dolphins sampled in 2015–16, using the one-sample, linkage disequilibrium method. This estimate has declined compared with estimates for 2001–07 and 2010–11, although the confidence limits of the previous estimates were relatively large and overlap with those of the current estimate. The smaller size of Ne relative to the capture-recapture estimate of census abundance is consistent with the expectation that Ne only represents the breeding individuals of the parental population. The apparent decline is consistent with the expectation that changes in Ne will lag behind a decline in the census population in the previous generation.
Including the 2015–16 surveys with the previous records (Hamner et al., 2014b), there have now been seven Hector’s dolphins sampled alive or dead on the west coast of the North Island (including Wellington Harbour). Three of these, two females and one male, were sampled alive among social aggregations of Māui dolphins. Despite the intermingling of the two subspecies, there is of yet no evidence of interbreeding between the Hector’s and Māui dolphins (i.e. all subspecies identification has been consistent with a diagnostic difference in mtDNA and assignable differentiation of microsatellite genotypes).
Our results highlight the importance of individual identification and genetic monitoring using biopsy samples and DNA profiling. The ‘register’ of DNA profiles, now extending across 16 years, is providing new information on the life history parameters of Māui dolphins, their local movement, census abundance and effective population size, as well as the long-distance dispersal of Hector’s dolphins into the range of the Māui dolphin.