Can a Fish Tell If a Scuba Diver Is Armed?

That's what Valerio Sbragaglia, an ecologist at the Berlin Leibniz Institute of Freshwater Ecology, and recreational spearfisher wanted to find out. He chased more than 1,300 fish off the coasts of France and Spain between May and October 2016, sometimes carrying a speargun and sometimes not. By measuring their flight initiation distance (FID), Sbragaglia discovered that fish flee from speargun-wielding divers at a much greater distance than from those without.

The ecologist and his colleagues came up with two possible explanations: Either spearfishers had killed the boldest fish, leaving only their more skittish counterparts, or the fish most frequently targeted have learned to recognize a diver bearing a weapon.

Bigger fish are more likely to be targeted, and those that have lived longer have grown wiser, too. "In those years, the fish learns how to cope with the spearfisher," Sbragaglia says. "If you survive, you know very well what a speargun means." His explanation would account for the reason why fish living in marine protected areas are less fearful of divers -- they've had far less exposure to spearfishers.

Sbragaglia thinks measuring FID is a better way to judge the effect of human activity on marine fish populations, rather than using gross metrics like harvest biomass, which only accounts for how many fish are killed.