From 1910 to 1970, humans killed an estimated 1.5 million baleen whales in the frigid water encircling Antarctica. From the perspective of krill—the tiny shrimp-like creatures the whales feast on—this would seem to be a boon. But new research published November 3 in Nature suggests the opposite: that the decline of baleen whales in the Southern Ocean has led to a decline of krill.
This paradoxical result is a sign of just how much the precipitous decline of the large marine mammals has negatively impacted the health and productivity of ocean ecosystems, the researchers say.
“Fifty years after we stopped hunting whales, we're still learning what impact that had. The system is not the same,” said Matthew Savoca, a postdoctoral scholar at Stanford’s Hopkins Marine Station and lead author of the paper. “We're looking into ways of using this information to restore ocean ecosystems and bring whales back. And hopefully, that will have benefits on everything from biodiversity conservation to fisheries yield to carbon storage.”
Coauthor Ari Friedlaender, professor of ocean sciences at UC Santa Cruz, led the Antarctic field projects over the past 10 years that yielded key data for this novel study.
“This study not only increases our fundamental knowledge of the ecology of baleen whales, but also gives a more holistic view on their function in marine ecosystems around the world,” he said. “This information is critical because many of the places where these whales live are under direct and indirect threats from human activities. The Antarctic Peninsula is experiencing rapid warming that is occurring as these whales begin to slowly repatriate this ecosystem after commercial whaling. Our ability to predict and understand the impacts of climate change and commercial krill fishing are intimately tied to whales and their recovery in a way that we certainly didn't understand and appreciate fully.”
The researchers came to their troubling conclusion after asking a very fundamental question: How much do whales eat?
High-tech tags
Large whales are inherently difficult to study because they can’t be studied in captivity. Previous estimates of how much whales consume were generally limited to either studies of dead whales or metabolic extrapolations based on much smaller animals.
For this study, the researchers looked at blue, fin, humpback, and minke whales—all whales that feed by gulping a large amount of water and filtering it through their mouths’ fringed baleen plates until only their prey remains. They employed high-tech tagging devices developed by Friedlaender’s Biotelemetry and Behavioral Ecology lab at UC Santa Cruz and the Goldbogen lab led by senior author Jeremy Goldbogen, co-director of Hopkins Marine Station. The tags attach to the whales by suction cup and stay on for up to 24 hours, recording their movements, acceleration, sound, and, if light allows, video.
Drones, operated by the Duke Marine Robotics and Remote Sensing Laboratory, measured the length of individual tagged whales, which helped the researchers estimate the size of each whale’s gulp. In collaboration with the Environmental Research Division at NOAA and UC Santa Cruz, the researchers also ran an underwater device called an echo sounder, which Savoca likens to “a fancy fish finder” and which uses sound waves at several different frequencies to measure how much prey is around.
“All of that put together really gives us this amazing view,” said coauthor Shirel Kahane-Rapport, a graduate student in the Goldbogen lab. “From each one, you can learn a lot about whales, but the combination takes the research to another level.”
Analysis of the data they captured revealed that whales in the Southern Ocean eat about twice as much krill as previous estimates suggested, and that krill-feeding blue and humpback whales off the coast of California eat two to three times as much as previously thought. Fish-feeding humpback whales, however, might eat the previously estimated amount or even less. This range seems to reflect the energy density of the food—whales need to eat more krill to get the same energy as they would from a smaller amount of fish.
“As large baleen whales get bigger, the anatomical machinery that allows them to eat also gets relatively bigger,” Goldbogen said. “They have evolved these systems that allow them to be eating machines. That disproportionately bigger gulp size allows them to take advantage of abundant food, like krill.”
The researchers made their estimates of consumption based on their data about prey density, gulp size, and lunge frequency, as recorded by the tags. With these new consumption estimates, the researchers calculated that the early 20th-century abundance of krill in the Southern Ocean had to be about five times what it is now in order to feed the pre-whaling whale population. This implies a complex role for whales in their ecosystems where the decline or recovery of their populations is strongly tied to overall ecosystem productivity and functioning.
“Hopefully, work like this can really get people to consider the ecosystem-wide repercussions of human activities, because we are still continually affecting their environment,” said Kahane-Rapport.
Processing krill
The Southern Ocean is among the most productive ecosystems on Earth, largely due to the abundance of microscopic algae, called phytoplankton. Phytoplankton are a vital food source for krill, small fish, and crustaceans—which are, in turn, consumed by larger animals, including whales, birds, and other fish. But whales also help sustain phytoplankton. Through eating krill and then defecating, whales release iron locked within krill back into the water, making that iron available to phytoplankton, which need it to survive.
“Think of these large whales as mobile krill processing plants,” Savoca said. “Each fin whale or blue whale is the size of a commercial airliner. So in the first half of the 20th century, before whaling, there were an additional one million of these 737-sized krill processing plants moving around the Southern Ocean eating, pooping, and fertilizing.”
The many twists and turns of these findings demonstrate the potential impact of asking simple questions. By trying to pin down how much whales eat, this work has cast doubt on what people thought whales needed to survive, and how the activities of whales and humans affect ocean ecosystems.
“Just this idea that if you remove large whales, there's actually less productivity and potentially less krill and fish is amazing,” said Goldbogen. “It’s a reminder that these ecosystems are complex, highly intricate, and we need to do more to fully understand them.”
Additional coauthors of the paper include scientists from Cascadia Research Collective, Duke University Marine Lab, Oregon State University, University of Copenhagen in Denmark, Nelson Mandela University in South Africa, National Oceanic and Atmospheric Administration (NOAA), Smithsonian National Museum of Natural History, and NOAA Southwest Fisheries Science Center.
This research was funded by the National Science Foundation, the Office of Naval Research Young Investigator Program, the Defense University Research Instrumentation Program, the National Geographic Society, the Percy Sladen Memorial Trust, the PADI Foundation, the Society for Marine Mammalogy, Torbenog Alice Frimodts Fond, the Volgenau Foundation, the International Fund for Animal Welfare, and MAC3 Impact Philanthropies which is part of the Stanford One Ocean Initiative.
