Estimating population changes in humpback whales Megaptera novaeangliae migrating past Cape Vidal, South Africa

Abstract

Shore-based surveys of humpback whales Megaptera novaeangliae were performed from Cape Vidal, on the east coast of South Africa (iSimangaliso Marine Protected Area, Indian Ocean), from two independent platforms between 27 June and 7 August in 2018 and 2019, to estimate the relative abundance and growth rate of the C1 breeding substock of the species. Observed whale groups were tracked by analogue survey theodolites, and observed numbers were adjusted to account for daily sighting effort and the proportions of groups missed by observers. Daily sighting frequency was aggregated across the season to result in annual relative abundance estimates of 10 499 (2018) and 11 009 (2019) individuals, with peak frequencies from 28 July to 3 August in both years. When compared with previous estimates from the same study area, we estimated an average annual increase rate of 7.4% to 8.8% over 31 years from 1988. These results indicate a slowing of the rate of increase from previous estimates, which could suggest that the population is approaching pre-exploitation numbers or that yet unidentified threats are negatively impacting the growth rate. Continued monitoring of the recovering humpback whale stocks is critical to identify any possible effects of Southern Ocean ecosystem changes on the stock health of these whales.