A $650,000 price tag for a single tuna might seem outrageous. But that’s what a sushi restaurant owner paid for a monster-sized tuna at an auction in Japan this past January.

Sushi’s popularity has led tuna to become an expensive commodity. Its popularity has another downside, too. “The Pacific bluefin tuna population has dropped by more than 97 percent due to decades of overfishing,” says Jamie Gibbon, a tuna conservation expert at the nonprofit Pew Charitable Trusts.

Most bluefin are caught before they have a chance to reproduce, making it hard for the population to bounce back. If fishers and sushi lovers want tuna to survive over the long run, stricter limits need to be set on how many tuna can be caught each year, says Gibbon.