I remember watching a documentary once and learned that whales need to eat tonnes (literally) of krill per day to survive. I don’t understand why this is better than eating small amounts of large fishes instead.
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Because there’s half a billion tons of krill in the ocean and krill is ubiquitous and is replenished far faster than larger fish.
Another item that no one mentioned so far – bioaccumulation is a large issue for sea life since pretty much everything is predatory. It’s actually healthier to eat krill than salmon or tuna due to the amount of toxins that accumulate in larger animals.
This is why orcas have a huge discrepancy in female vs male lifespans – that blue and humpback whales don’t. The females of orcas offload a lot of toxins by giving birth – and the first born tends to not survive. So male orcas live to 30 and females live to 60-80 or more. Of course it depends on their diet and it’s worse/ better for some pods vs others.
While it’s true that whales need to eat a lot – they can also fast for months at a time. They are very efficient swimmers and have a lot of reserves. This is why they will travel to the far reaches of the planet for very specific events that let them feast. Like herring migration to Alaska.
Baleen whales – blue, bowhead, gray, and others – do not have teeth. They have filters for the krill you mention.
Other whales – sperm, beluga, narwhal, etc – have teeth. They don’t filter-feed on krill. They eat larger fish.
It’s more advantageous in terms of thermodynamics, i.e. energy expenditure vs energy gain, to eat the stuff that there’s more of, that’s easier to find, that’s lower on the food chain.
Now you might reasonably ask, why not go even lower on the food chain and eat, say, plants? Answer is, plenty of animals do that. When there’s competition for a food source, you have one fewer ecological niche to fill so as to avoid this costly competition, ergo, eating animals. And in addition digesting plant matter is a massive energy expenditure.
It’s like eating rice. Would you eat 1 big rice?
Small creatures are easier to grow in mass quantities and are more resilient to any one thing hurting their population. Any environment that’s shared by predator and prey has countless prey animals for every single predator. A grassy field will have fifty breeding pairs of mice for every single owl that hunts them. Food animals eat constantly and breed constantly and can feed themselves within days. Predators eat single meals and raise small families until their babies can hunt on their own.
A one-ton whale will take a year to raise a calf to breeding age and have it bear an offspring. If that whale gets attacked by a shark or a Japanese ~~whaling~~ “science” boat, that ton of whale meat stops reproducing and disappears. But one ton of krill will produce trillions and trillions of eggs in a year and scatter them all over the ocean, millions of which will grow up for a few days and start making eggs of their own before they get eaten.
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