The Marine Reserve Goldilocks Problem

To protect coral reefs, governments and conservationists are looking to establish networks of marine reserves, where fishing is prohibited. But for the reserves to work, they need to be the right size and distance apart from one another.

If a reserve is too small, it can’t accommodate enough newly hatched fish larvae to sustain populations. If it’s too big, larvae will simply stay within the reserve and not disperse to surrounding areas, including other reserves, and replenish fish stocks there, said Simon Thorrold, a biologist at Woods Hole Oceanographic Institution.

A key unknown is how far newly spawned fish travel away from their parents. To find out, Thorrold and colleagues from Australia, France, and Saudi Arabia undertook the most comprehensive study ever conducted of larval dispersal on coral reefs. In 2009 and 2011, they collected DNA samples from thousands of adult and juvenile clownfish and butterflyfish throughout Kimbe Bay, Papua New Guinea, across 3,000 square miles of the ocean—an area the size of Yellowstone National Park. The 30-person science team spent thousands of man-hours scuba diving over many weeks each year.

Their analyses matched up the DNA fingerprints of juveniles and their parents, Thorrold said. “Since we knew the respective locations of the adults and babies, we were able to come up with the exact linear distances that the larvae had dispersed.”

They determined, for example, that most juvenile clownfish journeyed a mean distance of 6 to 9 miles from their natal reefs before settling into their new habitats. Butterflyfish dispersed further, averaging distances of 27 to 40 miles.

Knowing how far fish disperse could allow fisheries managers to optimize the size and spacing of marine reserves and monitor their effectiveness, Thorrold said. In addition, “if you can trace larvae from one reserve to a place that’s fished, you can directly measure how many fish the reserve is contributing to exploited populations beyond the reserve.”

The multi-institutional study was published in May 2017 in the journal Nature Ecology & Evolution. The research was supported by the Australian Research Council, the King Abdullah University of Science and Technology, and the National Science Foundation.