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Estimating the abundance and effective population size of Māui dolphins 2020–2021 (PDF, 4,266K)
Summary
Here we report the results from continued genetic monitoring of the Nationally Critical Māui dolphin subspecies (Cephalorhynchus hectori maui) during 2020 and 2021, following the same methods as previously reported for surveys conducted in 2001–2007, 2010–2011 and 2015–2016. Our primary objectives were to estimate the abundance and effective population size of Māui dolphins in 2020–2021 and to document the movements of individuals of this subspecies and migrant Hector’s dolphins (C. h. hectori) using DNA profiles derived from biopsy samples. We also matched the DNA profiles from biopsy samples collected during the 2020–2021 surveys with all other samples collected since 2001, including necropsy samples from beachcast individuals.
Small-boat surveys dedicated to the collection of biopsy samples from Māui dolphins were conducted along the northwest coast of Te Ika-a-Māui / the North Island of Aotearoa New Zealand, from just south of the entrance to the Kaipara Harbour in the north to the Mokau River, Taranaki, in the south during the austral summers of 2020 (11– 27 February) and 2021 (13 February – 15 March). A total of 84 biopsy samples were collected during these surveys from individual dolphins aged 1 year and older (50 in 2020 and 34 in 2021). DNA profiling was undertaken for all samples, including genotyping of up to 25 microsatellite loci (average of 24.94 loci/sample), genetic sex identification and mitochondrial DNA (mtDNA) control region sequencing.
The census abundance (Nc) of Māui dolphins in 2020–2021 was estimated to be 54 individuals aged 1 year or older (1+) (95% confidence interval (CI) = 48–66) within the survey area, using a two-sample, closed-population model. This estimate applies to the number of individuals alive during either sampling year and is comparable to the previous estimates based on the genotype surveys in the same area in 2015–2016 and 2010–2011. An effective population size (Ne) of 35 (95% CI = 21–67) was estimated from the genotypes of the 41 Māui dolphins sampled in 2020–2021 using the one-sample linkage disequilibrium method. This is unchanged from the previous estimate for 2015–2016 but lower than estimates for 2010–2011 and 2001–2007. The smaller size of Ne relative to the capture–recapture Nc estimate is consistent with the expectation that Ne only represents the individuals of the parental population that contributed successfully to the next generation.