Abstract:
Southern Hemisphere humpback whales (Megaptera novaeangliae) generally undertake
annual migrations from polar summer feeding grounds to winter calving and nursery grounds
in subtropical and tropical coastal waters. Evidence for such migrations arises from seasonality
of historic whaling catches by latitude, Discovery and natural mark returns, and results of
satellite tagging studies. Feeding is generally believed to be limited to the southern polar
region, where Antarctic krill (Euphausia superba) has been identified as the primary prey item.
Non-migrations and / or suspended migrations to the polar feeding grounds have previously
been reported from a summer presence of whales in the Benguela System, where feeding
on euphausiids (E. lucens), hyperiid amphipods (Themisto gaudichaudii), mantis shrimp
(Pterygosquilla armata capensis) and clupeid fish has been described. Three recent research
cruises (in October/November 2011, October/November 2014 and October/November 2015)
identified large tightly-spaced groups (20 to 200 individuals) of feeding humpback whales
aggregated over at least a one-month period across a 220 nautical mile region of the southern
Benguela System. Feeding behaviour was identified by lunges, strong milling and repetitive
and consecutive diving behaviours, associated bird and seal feeding, defecations and the
pungent ªfishyº smell of whale blows. Although no dedicated prey sampling could be carried
out within the tightly spaced feeding aggregations, observations of E. lucens in the region of groups and the full stomach contents of mantis shrimp from both a co-occurring predatory fish
species (Thyrsites atun) and one entangled humpback whale mortality suggest these may be
the primary prey items of at least some of the feeding aggregations. Reasons for this recent
novel behaviour pattern remain speculative, but may relate to increasing summer humpback
whale abundance in the region. These novel, predictable, inter-annual, low latitude feeding
events provide considerable potential for further investigation of Southern Hemisphere humpback
feeding behaviours in these relatively accessible low-latitude waters.
