Interchange of Southern Hemisphere humpback whales across the South Atlantic Ocean

Abstract

The cosmopolitan distribution of humpback whales (Megaptera novaeangliae) is largely driven by migrations between winter low-latitude breeding grounds and summer high-latitude feeding grounds. Southern Hemisphere humpback whales faced intensive exploitation during the whaling eras and recently show evidence of population recovery. Gene flow and shared song indicate overlap between the western (A) and eastern (B1, B2) Breeding Stocks in the South Atlantic and Indian Oceans (C1). Here, we investigated photo-identification evidence of population interchange using images of individuals photographed during boat-based tourism and research in Brazil and South Africa from 1989 to 2022. Fluke images were uploaded to Happywhale, a global digital database for marine mammal identification. Six whales were recaptured between countries from 2002 to 2021 with resighting intervals ranging from 0.76 to 12.92 years. Four whales originally photographed off Abrolhos Bank, Brazil were photographed off the Western Cape, South Africa (feeding grounds for B2). Two whales originally photographed off the Western Cape were photographed off Brazil, one traveling to the Eastern Cape in the Southwestern Indian Ocean (a migration corridor for C1) before migrating westward to Brazil. These findings photographically confirm interchange of humpback whales across the South Atlantic and Indian Oceans and the importance of international collaboration to understand population boundaries.

Figure 1

Maps of sighting locations and fluke images of six Southern Hemisphere humpback whales (Megaptera novaeangliae) photographed across the South Atlantic Ocean and Southwestern Indian Ocean (with the indication of the respective Breeding Stock or substock, A, B1, B2, and C1). The top left inset map shows whale sightings along the Atlantic coast of Brazil and the top right inset maps depicts sightings in the Western and Eastern Capes of South Africa (top) and zoomed in view of sightings in and outside of False Bay. Fluke images of each identified whale in the hexagons are numbered with sightings in Brazil colored gray and in South Africa colored black. The white oval in the Southern Ocean shows the Atlantic sector (Area III) of the Antarctica feeding grounds visited by humpback whales from Africa. Maps were created in QGIS version 3.28.3 (www.qgis.org).

Figure 2

Bar charts showing the number of individual encounters of Southern Hemisphere humpback whales photographed by month and by year off the Atlantic coast of Brazil from 1989 to 2022 and off the Western and Eastern Capes of South Africa from 2002 to 2022. The horizontal gray bars in the yearly bar charts show the timespan between photo-ID recaptures of humpback whales in each region. The red line shows the cumulative frequency or ‘discovery curve’ of identified individuals. The graph was made in R statistical software using the package ggplot2.