Richard Robinson

When humans fish, we tend to go after the big fish first—the meaty, confident predators that are easy to catch. Tuna, swordfish, crayfish, snapper. As those become rarer, we move on to the next-biggest fish, and finally on to the grazers, like kina. This phenomenon has been dubbed “fishing down the food web”, and it’s a problem all over the world: “You take the top predators out of any ecosystem and you disrupt the balance,” says University of Auckland marine scientist Andrew Jeffs.

An international group of scientists analysed 70 years of New Zealand fisheries data and found the pattern holds here, too. As bigger fish became harder to find close to shore, fishers started catching smaller fish, and moved offshore into deeper waters to target new species. Since 1950, larger inshore predators “were likely being overfished”, says the study’s lead author, Charles Patrick Lavin, from Nord University in Oslo.

The team also found New Zealanders have been catching more tropical species like skipjack tuna and blue mackerel in recent decades, suggesting climate change is already impacting fisheries.

For Jeffs, who was not involved in the study, the results underscore that New Zealand should be managing its fisheries more cautiously. “There are some major problems with how the quota management system is operating, and how it’s affecting the marine environment.”