Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela

Abstract

Seasonal feeding behaviour of humpback whales (Megaptera novaeangliae) has been observed in the coastal waters of the Southern Benguela where the species has been observed forming super-groups during the austral spring in recent years since 2011. Super-groups are unprecedented densely-packed aggregations of between 20 and 200 individuals in low-latitude waters and their occurrences indicate possible changes in feeding behaviour of the species. We accessed published data on super-groups occurrence in the study area in 2011, 2014 and 2015, and investigated oceanographic drivers that support prey availability in this region. We found that enhanced primary production is a necessary but not sufficient condition for super-groups to occur. Positive chlorophyll anomalies occurring one month prior to the super-group occurrences were identified, but only a concurrent significantly reduced water volume export from the region throughout October were conducive to the aggregations in the specific years. Hydrodynamic model results attributed the anomalous decreased volume export to the strength and orientation of the Goodhope Jet and associated eddy activity. The combination of random enhanced primary production typical of the region and emerging anomalous conditions of reduced water export in October since 2011 resulted in favourable food availability leading to the unique humpback whale aggregations. The novelty of this grouping behaviour is indicative of the lack of such oceanographic conditions in the past. Given the recency of the events, it is difficult to attribute this reduction in ocean transport to climatic regime shifts, and the origin should be likely investigated in the distant water mass interaction with the greater Agulhas system rather than in local intensifications of the upwelling conditions. A positive trend in the humpback whale population abundance points to the need to monitor the exposure of the species to the changing climate conditions.

A review of post-whaling abundance, trends, changes in distribution and migration patterns, and supplementary feeding of Southern Hemisphere humpback whales

“…To better understand this behaviour, data from Findlay et al (2017) was used in association with oceanographic data of the Southern Benguela System. A combination of relatively high chlorophyll-a concentration in the month before the observation of the whales, in association with the decrease in the water export from the area throughout October, seems to support prey availability in the area, composing a scenario favourable to the formation of the supergroups (Dey et al, 2021).…”

Whales and climate

“…Climate change is modifying the conditions of the critical habitats of the humpback whales (Askin et al, 2008;De Weerdt and Ramos, 2019;Dey et al, 2021). The increase in SST has triggered several responses in humpback whales, such as changes in the timing of their migration, and variations in the length of stay in their feeding and breeding areas, humpback whales have also been observed feeding in more coastal areas and forming aggregations from tens to hundreds of individuals to feed Santora et al, 2020).…”

“…To better understand this behaviour, data from Findlay et al (2017) was used in association with oceanographic data of the Southern Benguela System. A combination of relatively high chlorophyll-a concentration in the month before the observation of the whales, in association with the decrease in the water export from the area throughout October, seems to support prey availability in the area, composing a scenario favourable to the formation of the supergroups (Dey et al, 2021).…”

“…Humpback "super-groups" from the same E1 breeding stock have been documented feeding off Southeast Australia (Pirotta et al, 2021). Super-group formation has been linked to phytoplankton blooms 1 month prior as well as reduced outward transport favouring an increase in humpback whale prey in coastal waters, as found in Southern Benguela (Dey et al, 2021). E1 humpbacks are known to supplement their feeding anywhere between Eden, NSW, all the way down through the Bass Strait, Tasmania, and New Zealand, as shown through satellite tracking of the same population .…”

Decreases in encounter rate of endangered Northeast Pacific humpback whales in Southern Costa Rica: Possible changes in migration pattern due to warming events

“…Climate change is modifying the conditions of the critical habitats of the humpback whales (Askin et al, 2008;De Weerdt and Ramos, 2019;Dey et al, 2021). The increase in SST has triggered several responses in humpback whales, such as changes in the timing of their migration, and variations in the length of stay in their feeding and breeding areas, humpback whales have also been observed feeding in more coastal areas and forming aggregations from tens to hundreds of individuals to feed (Ramp et al, 2015;Findlay et al, 2017;Avila et al, 2020;Santora et al, 2020).…”