Expanding marine protected areas by 5% could boost fish yields by 20%, but there’s a catch

Marine protected areas, or MPAs as they’re more commonly called, are very simple. Areas of the sea are set aside where certain activities—usually fishing—are banned or restricted. Ideally, these MPAs might be placed around particularly vibrant habitats that support lots of different species, like seagrass beds or coral reefs. By preventing fishing gear such as towed seabed trawls from sweeping through these environments, the hope is that marine life will be allowed to recover.

When used well, they can be very effective. MPAs have been shown to increase the diversity of species and habitats, and even produce bigger fish within their bounds. A new study argues that by expanding the world’s MPAs by just 5%, we could boost future fish catches by at least 20%. This could generate an extra nine to 12 million tons of seafood per year, worth between USD$15-19 billion. It would also significantly increase how much nutritious fish protein is available for a growing human population to eat.

So what’s the catch?

Spillover versus blowback

The scientific rationale is sound. We already know that MPAs can increase the numbers of fish living inside them, which grow to be bigger and lay more eggs. The larvae that hatch can help seed fish populations in the wider ocean as they drift outside the MPA, leading to bigger catches in the areas where fishing is still permitted. We know fish can swim large distances as adults too. While some find protection and breed inside MPAs, others will move into less crowded waters outside where they can then be caught. Together, these effects are known as the spillover benefits of MPAs.

The study is the first to predict, through mathematical modeling, that a modest increase in the size of the world’s MPAs could swell global seafood yields as a result of this spillover. But while the predictions sound good, we have to understand what pulling this off would entail.

The study maintains that the new MPAs would need to be carefully located to protect areas that are particularly productive. Locating MPAs in remote areas offshore, which are hard to access and typically unproductive, would have much smaller benefits for marine life than smaller, inshore MPAs that local fishing vessels can reach. Just 20 large sites in the remote open ocean account for the majority of the world’s MPAs. As the low hanging fruit of marine conservation, these MPAs are often placed where little fishing has occurred.

The MPAs themselves would also need to be highly protected, meaning no fishing. Only 2.4% of the world’s ocean area has this status. Increasing this by a further 5% would mean roughly trebling the coverage of highly protected MPAs, and that’s likely to provoke a great deal of resistance. Many fishers are skeptical that spillover can boost catches enough to compensate for losing the right to fish within MPAs and tend to oppose proposals to designate more of them.

People in the UK are often surprised to learn that fishing is allowed in most