Initiatives to protect marine ecosystems could do more harm to marine life than good, according to a recent PNAS paper. The work suggests that the announcement of one of the world’s largest marine protected areas sparked a sudden uptick in fishing, threatening the initiative’s goals before the protections were even in place.

In 2015, the government of the Republic of Kiribati, an archipelago situated in the heart of the Pacific Ocean, designated 408,459 km² of water as a marine reserve. The region, nearly the size of California, includes the waters around the remote Phoenix Islands. Fisherman became aware of the impending ban in late 2013, and upped their activities before fishing was outlawed, says coauthor Grant McDermott, an environmental and resources economist at Oregon State University. In fact, in the lead up to the ban, the researchers saw fishing activity more than double—the equivalent to an extra 1.5 years’ worth of fishing—before falling again to around zero after the ban was put in place.

McDermott says it’s not yet clear if the fishing surges will have long lasting impacts on marine ecosystems. If the only impact is a short-term fishing effort “then 1.5 years isn’t so bad in the scheme of things,” he says. But there could be hidden longer term impacts. “It could push fish populations past a critical threshold, or trigger irreversible habitat destruction,” McDermott warns.

The research is the first to examine how marine resources are affected by the announcement of new conservation initiatives. Previous work has found that land-based conservation initiatives and climate policies also tend to suffer from preemptive behavior that undermines the policy goal.

The PNAS study used open source satellite data on fishing vessels’ movements and positions from a variety of sources including their automatic identification system (AIS), a GPS-like device that large ships are required to use. McDermott and colleagues modeled total daily fishing hours per 1,000km² in the Phoenix Island protected area and in control regions around the Line and Gilbert islands, also part of the Kiribati archipelago.

The researchers were not expecting to find the surge in fishing because they assumed the activity was already at its maximum. “It’s a surprising result,” agrees Sarah Lester, who studies marine conservation and sustainable fisheries management at Florida State University. “Fishing was already unregulated, so you would have expected fishermen to take as much as they can already,” she says.

McDermott and his team suggest similar impacts could be affecting other marine protected areas. The International Union for Conservation of Nature (IUCN) is pushing to safeguard at least 30% of the ocean in marine reserves by 2030. If the IUCN goal is achieved, McDermott and his colleagues estimate that fishing bursts could lead to a 7% rise in the number of depleted fisheries, affecting nearly three quarters of all fishing areas.

Lester urges caution in broadly applying the results to marine protected areas. “The study is based on a single protected area so it’s hard to know how general the results are,” she says.

Marine reserves are still a useful conservation policy, she adds. “These results alone shouldn’t make us be reticent to establish marine reserves because they won’t be effective,” says Lester. “It might just take longer than expected to see the biological benefits.”