Southern right whale numbers off SA have plummeted again, and scientists say consistent low counts point towards large-scale ecosystem changes in the Southern Ocean.
The 41st annual aerial survey between Muizenberg and Nature’s Valley, in the Western Cape, conducted between September 27 and 29, found just 165 whales.
This is 17.5% lower than the 200 counted a year ago, and compares with more than 1,000 whales seen in the 2018 survey.
Staff from the whale unit in the mammal research institute at the University of Pretoria (UP) conducted the count from a helicopter that spent 13 hours in the air over three days.
“In total, 136 females and calves (68 pairs) of southern right whales were counted and photographed, as well as 29 adult whales without a calf (so-called ‘unaccompanied adults’),” said a statement from the whale unit on Friday.
“[This brings} the total to 165 southern right whales. Most female-calf pairs were observed in De Hoop Nature Reserve and Walker Bay [off Hermanus].”
The number is the second lowest in 32 years, beating only 2016 when 55 pairs were counted.
Researchers led by Els Vermeulen said it was a further sign that female southern right whales “continue to limit their residency time in the South African breeding ground, with possible negative effects on the chances of calf survival”.
The number of unaccompanied adults – a group of males, resting females and receptive females – remained “extremely low”, as it had since 2009, “indicating that non-calving right whales are still not migrating to the South African coast as they used to do before 2009”.
The whale unit said successful calving in southern right whales – which migrate 3,000km from the sub-Antarctic southern ocean to SA’s warmer, calmer waters to mate and calve – depended on the energy reserves in their blubber, which are directly influenced by feeding success.
“It is therefore believed that a decrease in their feeding success lies at the heart of these anomalous trends,” said the unit, adding that similar findings were being reported in South America and Australia.
“As our research continues, this hypothesis is being confirmed by our scientific data, which indicate strong correlations between the southern right whale prevalence along our shores with climate conditions in the Southern Ocean and fluctuations in food availability.
“In fact, new data reveals that the South African southern right whales have drastically changed their feeding locations in the past two decades, suggesting that their previously productive feeding grounds have changed over time.
“These findings point towards large-scale ecosystem changes in the Southern Ocean, likely impacting several different oceanic top predators.”
The unit said its findings indicated that while shifts in foraging locations may be an attempt to keep up with an evolving ocean, “the changes may not be sufficient to ensure an adequate body condition is obtained, negatively impacting on the success of their calving and migration”.
The UP researchers are now using drone images to investigate the whales’ nutritional condition and analysing stress hormones in blubber samples.
The aerial survey involved photographing the pattern of callouses on the whales’ heads. The photos will be compared to more than 2,300 recognisable adults from the previous 40 surveys, with the help of a computer-assisted recognition system.
“Through such analyses, we will be able to determine which females calved this year, how long it took them to produce a new calf, their individual distribution and movement patterns and, with considerable accuracy, assess their overall reproductive success,” said the unit.
“The analyses will allow us to investigate further possible causes and consequences of the concerning decrease in sightings along our shores in recent years.”