Despite Recovery, Humpback Whales in the South Atlantic Ocean Face Threats

Population’s impressive rebound could falter due to climate change, decline in krill

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Despite Recovery, Humpback Whales in the South Atlantic Ocean Face Threats
Humpback whale
L. Candisani Courtesy Instituto Aqualie

The population of humpback whales in the western South Atlantic Ocean is healthier than previously thought, according to research that incorporated new and more accurate data than were previously available. Unfortunately, this growing population could soon face additional difficulty due to climate change and changes in prey abundance in regions off the eastern coast of South America.

A research team led by Alexandre Zerbini, Ph.D., of the NOAA Fisheries Alaska Fisheries Science Center’s Marine Mammal Laboratory and the Foundation for Marine Ecology and Telemetry Research, assessed recovery of the western South Atlantic (WSA) humpback whales Megaptera novaeangliae, which were driven to near extinction by exploitation in the 1800s and 1900s.

The newly updated population assessment is more optimistic than previous estimates, finding that the WSA humpback population has recovered to approximately 93 percent of its pre-exploitation level. This represents a substantial increase from 2006, when the International Whaling Commission (IWC) estimated the population to be at 47 percent of its pre-whaling size.

The Zerbini team produced more accurate estimates of the recovery and current status of this population using information from the whales’ breeding grounds, trends in abundance, historical catch, genetics, and life history data.

The study methods, while similar to the IWC’s, produced more accurate results because they integrated more complete data on premodern and modern whaling catches, whales maimed by hunters and presumed dead, and the deaths of young whales caused by hunting of their mothers. The Zerbini research also incorporated new information on humpback genetics and life history.

Decline and recovery of population

Whales were hunted for centuries over vast areas of the ocean, and many species nearly disappeared. The WSA humpback population was hit hardest during the early 1900s when commercial whaling expanded to the higher latitudes near South Georgia Island. Between 1830 and the mid-1950s, the number of humpbacks there fell from about 27,000 to just 450.

After the IWC implemented an international moratorium on commercial whaling in the mid-1980s, the population of WSA humpbacks rebounded, with current abundance estimated at around 24,900 whales. This number is more optimistic than previous estimates, with models suggesting the population could recover to pre-exploitation levels within the next 10 years.

Migration and feeding

The recovery has important fishery management and ecological implications, namely a potentially large impact on Antarctic krill (Euphausia superba), which are the primary prey for whales and many other marine species and an important Antarctic fishery in their own right.

Each year, WSA humpback whales migrate from breeding areas off the coast of Brazil toward sub-Antarctic waters in the South Atlantic Ocean, and spend summer and early fall feeding around South Georgia and the South Sandwich Islands, and the Scotia Sea—areas with some of the world’s highest densities of Antarctic krill.

Scientists estimate that WSA humpbacks remove between 2.5 percent and 4.3 percent of total krill biomass in the area during summer feeding, a number likely to increase as whale populations recover. As humpback populations grow, there is also the possibility of competition with other krill predators, including Antarctic fur seals, penguins, and other whales.

An uncertain future

Recent studies have shown that krill abundance may be declining and that krill are shifting poleward due to climate change. The distribution of krill has moved about 450 kilometers southward over the past 90 years, with a substantial contraction from the waters in the area around South Georgia, and average krill biomass in the South Atlantic has decreased by nearly 60 percent during that time.

Although these changes do not seem to have affected the WSA humpback recovery, they may in the future. Recent research has predicted that baleen whales—which include blue, fin, humpback, Antarctic minke, and southern right whales—will be negatively affected by climate change because of reduced krill availability. For example, by the year 2100, baleen whale abundance is predicted to decline, and whales feeding in mid-latitudes (40 to 60 degrees south)—including WSA humpbacks—are more likely to be affected as krill move further toward the South Pole.

“It is important to continue monitoring abundance and potential shifts in distribution to understand how krill and their predators will respond to climate-driven effects in the WSA Ocean ecosystems and whether these effects will impact their populations,” says Zerbini.

Although the near-term future for WSA humpback whales appears bright, scientists should continue to monitor whales and krill populations to help maintain the long-term health of their habitats, some of which are already responding to climate change.

Jim Palardy directs research projects for The Pew Charitable Trusts’ conservation science initiative.

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