Whales eat much more than we assumed, and it has huge ecological implications

Baleen whales eat a lot more food than previously assumed: three times as much, to be exact, according to new research. The findings are not meant to shame these animals into going on a diet. Rather, they shed light on the key ecological role whales play in the ocean.

Image via Pixabay.

The sheer size and appetite of whales make them important players in the ocean. In particular, they serve as key drivers of nutrient recycling in the ocean. They consume vast amounts of food, releasing important nutrients back into the water following digestion. A new paper refines our understanding of just how much food whales as a group can consume, and take a look at the ecological implications of the decline in whale numbers since the onset of the 20th century.

Big eaters

“While it may just seem like a fun trivia fact, knowing how much whales eat is an important aspect of ecosystem function and management,” Matthew Scott Savoca, a Postdoctoral Research Fellow at Stanford University and corresponding author of the paper, told ZME Science. “If we want to protect whales and make sure they are thriving in modern oceans, then knowing how much food they need to survive and reproduce is critical.”

“There are implicit benefits of having whales on the planet — isn’t it cool to think that we live at a time when we’re alongside the largest animal in the history of life on Earth? Beyond that, whales have direct value as carbon sinks (e.g., sequestering carbon in their bodies and exporting it to the deep sea when they die and sink – which we did not discuss in this study). In addition, whale watching is a multi-billion dollar per year global business that is expanding as whales are recovering.”

Previous estimates of just how much whales eat were built upon data obtained from metabolic models or direct analysis of the stomach contents of whale carcasses. Such data can give us a ballpark figure but, according to the new paper, they are quite inaccurate.

Savoca and his colleagues directly measured the feeding rates of 321 baleen whales across seven species in the Atlantic, Pacific, and Southern oceans. They tracked foraging behavior and estimated prey consumption by tracking the whales using GPS tags. Location data was then combined with sonar measurements of prey density, of the quantity of prey consumed per feeding, and current estimates of how much each species typically eats per feeding event.

Overall, the results suggest that we’ve underestimated how much food baleen whales ingest by a factor of three. On average, these animals consume between 5% and 30% of their body weight per day, depending on species, across all the investigated regions. In total, blue, fin, and humpback whales in the California Current Ecosystem consume over two million tonnes of krill every year per species.

The study also puts into perspective just how massive an impact whaling and other stressors have placed on whales and, by extension, on the ecosystems they inhabit. Prior to the 20th century, the team estimates, whales in the Southern Ocean were consuming around 430 million tonnes of Antarctic krill per year. This figure is twice the total estimated biomass of Antarctic krill today.

Whales, the paper explains, serve an important ecological role as nutrient recyclers, tying into that last tidbit of information. Prior to the 20th century, before whales were hunted in meaningful numbers, these animals consumed a massive amount of biomass, releasing much of the nutrients in their food back into the ocean as waste. This, in turn, allowed for much greater productivity in the ocean (as they made large quantities of nutrients freely-available for krill and other phytoplankton to consume).

“In brief, if whales eat more than we thought, then they also recycle more nutrients (i.e., poop) than we thought. If that is the case then limiting nutrients may have been used more effectively and efficiently in a system that had many more whales,” Savoca said for ZME Science. “It’s not that these whales add more iron (or other nutrients) to the system, they just [move] it from within the bodies of their prey, to in the seawater itself where it could, in theory, fertilize phytoplankton — the base of all open ocean food webs.”

To put things into perspective, the authors estimate that today, baleen whales in the Southern Ocean recycle around 1,200 tons of iron per year; prior to the 20th century, this figure was likely around 12,000 tons of iron per year. In essence, whaling has led to a 90% decrease in the amount of essential nutrients whales can recycle in their ecosystems.

I asked Savoca whether there is any overlap between the decline in baleen whale populations and the detrimental effects of industrial fishing on today’s ocean ecosystems. Should we expect trouble ahead as we’re removing key nutrient recyclers from one side of the equation, and taking more fish out of the sea on the other?

“You are hitting on a major issue,” he admitted. “We have noticed that oceans have become less productive after removing millions of large whales in the 19th and 20th centuries. The same is true of ongoing industrial fishing. The collapse of predatory fish communities have the same detrimental impacts on marine communities as the wholesale decimation of the whales did.”

“I am not against fishing, but we have to do so as sustainably as possible if we want to maintain essential ocean productivity into the future.”

Whales and their extended family — cetaceans — have been experiencing immense pressures ever since the onset of industrial-scale whaling in the early 20th century. Commercial whaling only slowed down in the 1970s, which is a very, very short time ago from an ecological perspective. This has allowed whales and other cetaceans some much-needed respite, but they are still struggling. Over half of all known cetacean species today are inching towards extinction, 13 of which are listed as “Near Threatened”, “Vulnerable”, “Endangered”, or “Critically Endangered” on the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species. Besides the lingering effects of whaling, this family is still struggling under the combined effects of (chemical and noise) pollution, loss of habitat, loss of prey, climate change, and direct collisions with ships.

Research such as this study and many others before it can raise an alarm that not all is well with the whales. But actually doing something about it hinges on us and governments the world over taking the initiative to protect them. Understanding just how important whales are for the health of our oceans and, through that, for our own well-being and prosperity definitely goes a long way towards spurring us into action.

But Savoca’s conclusion to our email discussion left an impression on me. There is great beauty in natural ecosystems that we’re destroying, oftentimes unaware. Beyond the practical implications of conserving whale species, we have a chance to conserve these for our children and all future generations.

“I remember one day in Monterey Bay when we were surrounded by blue whales (likely over a dozen), each about twice the size of the boat we were on. I will also never forget the sound and scale of the ice in Antarctica,” Savoca wrote for ZME Science.

“My life’s work is devoted to making sure people and animals have these (and ideally ever better) ecosystems of awe and plenty well into the future.”

The paper “Baleen whale prey consumption based on high-resolution foraging measurements” has been published in the journal Nature.

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