A tiny coral reef lightens the azure waters off Papua New Guinea's Kimbe Island. For its diminutive size, the 0.3-square-kilometer (0.11-square-mile) reef supports abundant populations of orange clown fish and vagabond butterfly fish, two species separated by both appearance and reproductive strategies. Whereas the clown fish prefers to lay its eggs on a patch of bare rock that the parents then guard, the butterfly fish releases its eggs and sperm directly into the water, letting the current carry them where they may. The young of both, however, end up in the open sea, where they grow from larvae to juveniles. Where these juveniles come to live their adult lives has been something of a scientific mystery, but by tagging the larvae of each species researchers have discovered how they behave in this small corner of the world. Their work reveals that most of the fish come back to where they were spawned—spawning hopes that those that do not may help the marine reserve repopulate surrounding unprotected areas.

Marine biologist Glenn Almany of James Cook University in Queensland, Australia, and his colleagues captured adult females of both species while they were on the reef and then injected them with a rare isotope of the usually poisonous alkaline metal barium (137Ba) in December 2004. "The isotope is passed on from the female to her developing eggs and then gets incorporated into the bones of the larvae," Almany explains. "Because the isotope is so rare in nature, its presence in the larvae provides a very effective tag."

The larvae of each isotope-tagged fish developed normally. (Previous experiments had shown that the 137Ba was harmless to the fish as well as any humans who might catch and eat them.) The clown fish fry spent roughly 11 days in the open ocean while their butterfly fish counterparts spent over a month. Then in February 2005 the researchers returned to the reef and collected 15 young clown fish and 77 young butterfly fish. By examining the 137Ba in the fishes' ear bones, the researchers could determine whether or not their mothers had also been denizens of the Kimbe reef.

Nine of the clown fish and eight of the butterfly fish that were collected carried the barium isotope. In other words, 60 percent of clown fish and butterfly fish (after scaling to reflect their greater numbers) proved to be the children of mothers from the Kimbe reef, according to the paper in Science. "Because of the relatively small size of the reef we were working on, we were able to conduct extensive searches for any juveniles that had recently completed the larval phase," Almany says. "The number of juveniles we caught at the end of the trial were, we believe, sufficient to have a high degree of confidence in our estimate of the proportion returning home."

The finding reveals that the majority of fish do return home but a significant minority move on to other reefs. "The closest reef to the island we worked at is 10 kilometers [(6.2 miles)] away, which means that these juveniles traveled at least this far during their larval phase to reach the island," Almany notes. "Thus, while some larvae produced from the island's reef clearly came back to the same reef, others probably traveled much farther and enter different populations."

This idea of both self-seeding and connectivity is what makes the research interesting, says Eric Wolanski, a research scientist at the Australian Institute of Marine Science. It also provides hope that protected reefs may be able to rehabilitate their unprotected surrounding peers—good news for conservation and fishing. "Our study suggests that populations inside marine reserves can both sustain themselves through the return of some of the larvae they produce, and produce larvae that travel beyond reserve boundaries and enter other populations," Almany says. "[This] could help rebuild and sustain these non–marine reserve populations."