Kategorie: News
40 years of data show a clear trend
For four decades, the Whale Unit at the University of Pretoria has been monitoring southern right whales (Eubalaena australis) off the South African coast - with clear results: Today, female whales are in about 23 percent poorer physical condition than they were in the 1980s.
The three-year calving cycle once typical among southern right whales has lengthened to an average of over seven years. The study, published in the journal *Scientific Reports*, is considered one of the longest continuous population surveys of a single marine mammal worldwide.
An investment that no longer pays off
“They still seem very large to us humans; it just means they’re a little less robust - let’s say, a little slimmer,” explains Dr. Els Vermeulen, research director of the Whale Unit at the Mammal Research Institute, to Eyewitness News. The mothers’ physical condition is crucial because they barely eat along the South African coast: During the approximately three-month nursing period, they rely exclusively on previously stored energy reserves to nourish their calves, which drink up to 600 liters of milk daily and grow about three centimeters per day. A nursing female can lose up to 25 percent of her body condition during this time.
“If a female has to start this process with a 23 percent reduction in body condition, it shows just how great the energy expenditure is and how severely weakened she already is as a result,” said Vermeulen. If a female does not receive enough food, she postpones her next pregnancy until she has rebuilt sufficient reserves - thus turning a recovery period of just under a year into two, three, or even four years.
Declining Availability of Krill
The research team suspects that the main cause is a declining availability of Antarctic krill (Euphausia superba), the primary food source for right whales in the Southern Ocean. A complementary study by Matthew Germishuizen, Marcello Vichi, and Els Vermeulen, published in 2024 in *Frontiers in Marine Science*, supports this suspicion with environmental data: Over the past 40 years, the pack ice boundary has retreated significantly southward, sea ice concentration has decreased by 15 to 30 percent, while primary production metrics have more than doubled. These are conditions that are unfavorable for krill reproduction in the known feeding grounds of right whales.
As a result, the whales are essentially left with two options, as Vermeulen summarizes: “Either they swim farther in search of good food, or they stay closer to shore for lower-quality food” - for example, in the subtropical convergence zone south of South Africa, where the Atlantic and Southern Oceans meet. Although food is available there, it has lower nutritional value than the preferred krill in the whales’ traditional feeding grounds further south.
In addition to climate-related changes, krill is also under pressure from another source: industrial krill fishing. As we reported in DiveInside back in April, the company Aker Qrill alone now controls over 60 percent of the global catch quota for Antarctic krill - a resource that serves as the foundation of the entire Antarctic food web and is considered a significant natural CO2 sink. While the current right whale study itself attributes the decline primarily to climatic factors, the increasing commercial exploitation is likely to place additional strain on the already pressured population.
An Early Warning System for the Ocean
Southern right whales reproduce slowly compared to other baleen whales, which, according to Vermeulen, makes them particularly susceptible to environmental changes - they could therefore serve as a kind of “indicator species” for the state of the Southern Ocean. Changes in reproduction and body condition could therefore also indicate broader consequences of climate change for krill and the marine ecosystem as a whole.
Vermeulen therefore calls for greater awareness of the impacts occurring far from the coast in distant ocean regions. In addition, researchers are investigating whether decades of whale data could also help better justify the establishment of marine protected areas beyond national territorial waters, since the animals traverse large portions of the high seas on their journey back to Antarctica.
In the short term, the current calving season does not yet give cause for immediate concern: So far, around 300 calves have been counted, a level similar to that in 2022 and 2024; the official count will follow in the next two weeks. However, what will happen next remains unclear: A strong El Niño event building in the Pacific could further affect krill availability during the coming Antarctic summer - with as-yet-unknown consequences for the already weakened animals.
Further information / Sources:
Eyewitness News
University of Pretoria
Scientific Reports (Nature)
Nature Africa, WWF
Taucher.Net / DiveInside