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Baleen whales eat up to 30 percent of their total body mass in krill (tiny shrimplike creatures) per day. So in one day, a humpback whale weighing 30,000 kg could eat 9,000 kg, while a blue whale weighing 150,000 kg could ingest a whopping 45,000 kg. Over the last century, baleen whale populations have declined, and contrary to some scientists' expectations, so have krill populations. Matthew S. Savoca and colleagues resolve this apparent discrepancy by pointing out that baleen whales cycle iron in the ocean, helping support phytoplankton populations, which, in turn, sustain krill populations.