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When California implemented a string of marine protected areas (MPAs) about a decade ago, one of its goals was to bolster the number of fish available to recreational and commercial fishers. The state created a network of protected reserves, occupying about 18 percent of the state’s waters, that are close enough together that fish born in one no-fishing area can readily relocate to another reserve or adjacent waters where fishing remains legal.

Known as the spillover effect, this long-assumed consequence of marine protected areas has proven incredibly difficult to verify.

Traditionally, scientists looking to track the movements of wild animals have to catch them, attach a tracking tag, and, with luck, catch them again. But John Carlos Garza, a geneticist with the US National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center, knew that catching a fish as a juvenile and then again years later would be nearly impossible on the California coast.

“Because population sizes are so large and the ocean is so large, the sampling efforts would just be overwhelming,” Garza says. The fact that most baby fish do not reach adulthood further reduces the odds of recapture.

Garza and his colleagues, however, have found a way around this substantial logistical barrier. About 15 years ago, Garza and a research partner, geneticist Eric Anderson, developed a genetic sampling method to track salmon migrations. The approach, called intergenerational genetic tagging, allows scientists to identify the relatives of an individual fish by comparing its DNA to the rest of the population, looking for shared genetic markers.

For the new study, Garza and his team collected and analyzed DNA samples from about 6,000 kelp rockfish living in the water in and around MPAs off the California coast between 2013 and 2016. Using their genetic analysis technique, the scientists identified parent-offspring pairs and recorded the distances between the related fish.

The scientists chose to focus their study on kelp rockfish because they tend to remain in the same location for their entire adult lives. Because of that, Garza and his colleagues could safely assume that where the parent rockfish was caught was also the site where its offspring was born. If the juvenile fish was then caught elsewhere, it meant it had migrated there.

The research shows that juvenile kelp rockfish will move out of marine reserves and relocate as far as about 20 kilometers away—a finding that supports the spillover effect, Garza says. “This demonstrates that one of the primary proposed benefits of MPAs does actually occur,” he says.

Of the eight parent-offspring matches identified by the scientists, four offspring had relocated from one MPA to another, two had moved from an MPA into adjacent fishable water, and one had gone in reverse—born in an area where fishing is allowed, the juvenile fish had moved into an MPA.

California’s new MPAs—at least in the area Garza and his colleagues studied—seem to be adequately spaced, allowing the protected areas to function both individually and as a network between which individual fish can move.

Yet the published results have not necessarily turned MPA opponents into believers. Dan Wolford is a member of the Coastside Fishing Club, a sportfishing advocacy group that aggressively fought the implementation of California’s MPA system. Wolford thinks Garza’s paper “implies that the dispersal [of kelp rockfish] is the result of the MPAs, when in fact it is the result of the presence of suitable habitat and currents.” He says he is concerned that the paper “falsely attributes fishery benefits to the MPAs.”

Garza insists the protected areas are providing fishery benefits. “These MPAs are producing fish,” he says.

Marine biologist Crow White, who works at the California Polytechnic State University and was not involved in the new research, says the study is exciting and shows that fish populations are connected between MPAs, and between MPAs and fished areas.

But White notes that the MPAs in Garza’s study happen to be relatively close together—roughly 15 kilometers apart. He wonders if the results might have been different if Garza and his colleagues had sampled fish populations in areas where the state’s marine reserves are much farther apart.

Garza wonders the same thing. In fact, he’s already plotting ways to sample fish farther up and down the coast to hopefully learn just how far coastal rockfish can disperse. Much of the baseline fieldwork for this expanded study is already completed, Garza says, because many of the fish sampled in the current project will be alive for decades, generating uncountable offspring with recognizable DNA.

“Because each female can have a million offspring in her lifetime, we’ve now got a billion genetic tags out there,” Garza explains. “We could potentially find parent-offspring matches for many years to come.”