From 1910 to 1970, humans killed an estimated 1.5 million baleen whales in the freezing waters surrounding Antarctica. They were hunted for their fat, their baleen – the filtering fringe they have in place of their teeth – and their meat. One would assume that from the standpoint of krill – the tiny shrimp-like creatures that whales feast on – this would be a godsend. But new research published Nov. 4 in Nature from a collaboration led by Stanford University’s Goldbogen Lab suggests the opposite: that the decline of baleen whales in the Southern Ocean has led to a decline in krill.
This paradoxical result shows how the precipitous decline of large marine mammals has had a negative impact on the health and productivity of ocean ecosystems, according to the researchers.
“Fifty years after we stopped whaling, we’re still learning what impact that had. The system isn’t the same,” said Matthew Savoca, postdoctoral researcher at Stanford’s Hopkins Marine Station’s Goldbogen Laboratory and lead author. of the item. “We are looking for ways to use this information to restore ocean ecosystems and bring whales back. And I hope it will have benefits for everything from biodiversity conservation to fishing yield to carbon storage. .”
The researchers came to their disturbing conclusion after asking a very basic question: how much do whales eat?
Modernizing whale research
Large whales are inherently difficult to study because they cannot be studied in captivity. Thus, previous estimates of whale consumption have generally been limited either to studies of dead whales or to 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 swallowing a large amount of water and filtering it through the fringed baleen of their mouths. until only their prey remains. They used several high-tech tagging devices that attach to the whales typically for around five to 20 hours, recording their movements, acceleration, sound and, light permitting, video. Drones, operated by the Duke Marine Robotics and Remote Sensing Laboratory, measured the length of individual tagged whales, which helps researchers estimate their swallow size. Working with NOAA’s Environmental Research Division and the University of California, Santa Cruz, the researchers also used an underwater device called an echo sounder – which Savoca likens to “a sophisticated fish finder” – which uses sound waves at many different levels. frequencies to measure the amount of prey around.
“All of this put together really gives us this incredible view,” said Shirel Kahane-Rapport, a graduate student in the Goldbogen lab and co-author of the paper. “From each 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 blue and humpback whales feed on krill off the coast. California are eating two to three times more than previously thought. However, fish that feed humpback whales could 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 larger, the anatomical machinery that allows them to eat also becomes relatively larger,” said Jeremy Goldbogen, co-director of Hopkins Marine Station and associate professor of biology in the School of Humanities and Sciences, who is lead author of the paper. “They’ve developed these systems that allow them to eat machines. This disproportionately larger sip size allows them to enjoy plentiful food, like krill.”
The researchers made their consumption estimates based on their data on prey density, sip size, and cleft frequency, as recorded by the beacons. Going from hours of data to general estimates – and applying them to whales around the world – required careful calculations.
“We’ve come up with a very complex process, and we’re trying to do our best to retain as much uncertainty as possible along the way,” said Max Czapanskiy, a graduate student in the Goldbogen lab and co-author of the paper. “No one else has data like this. It’s a huge step forward, but at the same time it’s a difficult system to study and there’s still a lot of uncertainty.”
With these new consumption estimates, the researchers calculated that the abundance of krill in the Southern Ocean at the start of the 20th century had to be around five times greater than it is today to feed the whale population before the whaling. This implies a complex role for whales in their ecosystems where the decline or recovery of their populations is strongly linked to the productivity and overall functioning of the ecosystem.
“Hopefully, work like this can really get people to consider the impacts of human activities on the whole ecosystem, as we continue to continuously affect their environment,” Kahane-Rapport said.
Mobile processing plants
The Southern Ocean is one of the most productive ecosystems on the planet, thanks in large part to the abundance of microscopic algae, called phytoplankton. Phytoplankton is a vital food source for krill, small fish and crustaceans – which are, in turn, eaten by larger animals including whales, birds and other fish. But whales also help maintain phytoplankton. By eating krill and then defecating, the whales release the iron locked up in the krill into the water, making this iron available to phytoplankton, which needs it to survive.
“Without phytoplankton, you’ll never get all the animals and everything we care about so much,” Czapanskiy said. “When the whales were very numerous, they had this incredible role in strengthening the ecosystem.”
“Think of these large whales as mobile krill processing factories,” Savoca added. “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 million of these krill processing plants from the size of 737 that moved around the Southern Ocean eating, pooping and fertilizing.”
The many twists and turns in these results demonstrate the potential impact of asking simple questions. By trying to determine how much food whales eat, this work 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 take out the big whales there’s actually less productivity and potentially less krill and fish is amazing,” Goldbogen said. “It’s a reminder that these ecosystems are complex, very complex, and we need to do more to fully understand them.”
Additional Stanford co-authors of this research include graduate students William Gough and James Fahlbusch; postdoctoral researcher Paolo Segre and Elliott Hazen, adjunct professor at Hopkins Marine Station. Other co-authors are from Cascadia Research Collective, Duke University Marine Lab, Oregon State University, University of Copenhagen in Denmark, University of Southern Denmark, Aarhus University in Denmark, Nelson Mandela University in South Africa, National Oceanic and Atmospheric Administration ( NOAA) /Stellwagen Bank National Marine Sanctuary, Smithsonian National Museum of Natural History, Burke Museum of Natural History and Culture, University of California, Santa Cruz and NOAA Southwest Fisheries Science Center. Goldbogen is also a member of Stanford Bio-X and affiliated with the Stanford Woods Institute for the Environment.
This research was funded by the National Science Foundation, Office of Naval Research Young Investigator Program, Defense University Research Instrumentation Program, National Geographic Society, Percy Sladen Memorial Trust, PADI Foundation, Society for Marine Mammalogy, Torben og 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.
