This article does not mention krill oil as a reason, though. Naturally, it suspects climate change, but another factor is an interesting possibility -- a rebound in whale populations after the end of commercial whaling. [I don't recall commercial whaling being ended - was it?]
Krill Are Disappearing from Antarctic Waters Whales, seals and penguins could be hurting as this tiny creature--fundamental to the food web--declines, Scientific American, by Andrea Thompson, August 29, 2016