Abstract:
Little is known of the movements and seasonal occurrence of humpback whales
(Megaptera novaeangliae) of South Africa and the Antarctic, populations once brought
to near extinction by historic commercial whaling. We investigated the seasonal
occurrence and diel-vocalizing pattern of humpback whale songs off the west coast of
South Africa (migration route and opportunistic feeding ground) and the Maud Rise,
Antarctica (feeding ground), using passive acoustic monitoring data collected between
early 2014 and early 2017. Data were collected using acoustic autonomous recorders
deployed 200-300 m below the sea surface in waters 855, 1,118 and 4,400 m deep.
Acoustic data were manually analyzed for humpback whale vocalizations. While non-song
calls were never identified, humpback whale songs were detected from June through
December in South African waters, with a peak in percentage of acoustic occurrence
around September/October in the austral spring. In Antarctic waters, songs were
detected from March through May and in July (with a peak occurrence in April) where
acoustic occurrence of humpback whales was negatively correlated to distance to the sea
ice extent. Humpback whales were more vocally active at night than in the day at all
recording sites. Detection range modelling indicates that humpback whale vocalizations
could be detected as far as 18 and 45 km from recorders in South African and Antarctic
waters, respectively. This study provides a multi-year description of the offshore acoustic
occurrence of humpback whales off the west coast of South Africa and Maud Rise,
Antarctica, regions that should continue to be monitored to understand these
recovering populations.
