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

Abstract

Southern Hemisphere humpback whales, Megaptera novaeangliae, migrate annually from summer Antarctic feeding grounds to winter breeding grounds in coastal tropical waters, including South Africa. Given changing climatic conditions and population recovery from whaling, there may be changes in migrating whales' distribution, numbers, densities, and migration characteristics (speed, bearing, and distance offshore) over the last few decades. This research aimed to calculate a relative abundance estimate and a population rate of increase of humpback whales migrating past Cape Vidal, KwaZulu-Natal, South Africa, part of the so-called C1 sub-stock of the species. Shore-based surveys were performed from two 70 m-high independent platforms located 22 m apart in 2018 and 2019 (from 27th June to 7th August each year) to compare with previous data collected in the same area between 1988 - 1991 and 2002. Observed whale groups (within timed daily observation effort) were tracked by position-fixing on surfacing bouts using survey theodolites. The numbers of whales observed (or projected at tracked speeds) to cross the midline of the survey area within the observation period each day were tallied in each of three distance bins and adjusted to account for daily sighting effort. The proportions missed by distance offshore and sighting conditions were calculated using the mark recapture-based estimates of missed groups between independent platforms. The counts allowed daily sighting rates to be aggregated across the season to result in annual relative abundances. Relative abundances of 11,098 (2018) and 13,485 (2019) individuals were estimated, and an annual increase rate of 7 - 8% over the 31 years from 1988 was estimated. These results represent a slowing of the population’s rate of increase, as previous studies indicated an annual increase rate of 11% between 1988 to 2002. Such a decrease in the rate could suggest that the population is approaching the pre-exploitation numbers on their summer Antarctic feeding grounds. However, anthropogenic factors could influence various environmental factors driving humpback whale ecology and impact the species’ recovery. Recommendations on further studies at Cape Vidal on humpback whales are provided.