Well according to Google, one anchovy school can contain up to 100 million fish, so 1bn marine animals could mean all the whales died or ten schools of anchovie

Well according to Google, one anchovy school can contain up to 100 million fish, so 1bn marine animals could mean all the whales died or ten schools of anchovies. Without further information, I'm unsure what to make of the metric. Of course, microscopic animals are also animals, and important parts of the food chain, but 1billion zooplankton probably would be smaller than the anchovy schools.