The Krill Paradox: A good example of how ecosystems work

We all know that whales consume krill. A lot of krill.

But did you know that whales maintained a high abundance of krill? (1) What? you might wonder. But that is  true. That is ecology for you. Want to know how? Then read on ..

Krill abundance was predicted to rise after the end of commercial whaling in the Southern Ocean due to the release of predatory pressure from 2 million whales that were killed between 1915 and 1970, but contrary to expectations, there has been a substantial decline in abundance of krill since the end of whaling!

When whales eat lots of krill and when whales decrease in numbers  due to this consumption, how is this possible? The krill numbers  should have been actually increasing. But researchers have found that the opposite has happened in reality. As the whales disappeared, so too did the krill. Hmmm! This is where ecological cycles help understand this paradox. 

From 1910 to 1970, humans killed an estimated 1.5 million baleen whales in the frigid water encircling Antarctica. They were hunted for their blubber, baleen – the filtering fringe they have in place of teeth – and meat. One might assume that from the perspective of krill – the tiny shrimp-like creatures the whales feast on – this would be a boon. But new research published on November 3, 2021, 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 of krill (2,3).

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.

So the researchers asked the Q,  'How much do whales eat?'

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 several high-tech tagging devices that attach to whales typically for about five to 20 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 helps the researchers estimate the size of their gulp.

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. 

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.

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.

Without phytoplankton, you’re never going to get all the animals and everything that we care so much about. When whales were very numerous, they had this incredible role in bolstering the ecosystem.

Large whales are mobile krill processing plants. 

By trying to pin down how much whales eat, this work has cast doubt upon what people thought whales needed to survive, and how the activities of whales and humans affect ocean ecosystems.

As it turns out, whales eat a lot more than we originally thought. When whales eat, they poop - when they poop, Phytoplankton - floating plants - absorb that poop. And  do you know what feeds on Phytoplankton? The Krill!

So, as it turns out, whales and krill keep each other alive, and have a somewhat-different relationship than most predators and their prey. This is the Krill Paradox.

The strange eco balance of the krill paradox 

Credit: Stop Killing Whales

This idea that if you remove large whales, there’s actually less productivity and potentially less krill and fish is amazing. It’s a reminder that these ecosystems are complex, highly intricate, and we need to do more to fully understand them to protect them and us.

Footnotes:

1. https://www.researchgate.net/publication/278196939_Whales_maintaine...

2. https://scitechdaily.com/solving-the-krill-paradox-researchers-find...

3. https://news.stanford.edu/2021/11/03/researchers-find-whales-eat-ex...