A deep reef that lies in the lee of French Frigate Shoals is a kind of singles bar for ulua — the trevally that are prized by Hawai'i shoreline anglers — and its discovery could be a key to the development of new marine reserves that are small but effective sanctuaries for the fish.

"Rapture Reef," as it's called, is a place where during the spawning season, ulua come from miles around to cruise in vast schools — and eventually to pair off and reproduce. Some swim as much as 20 miles to be part of the festivities on the full moons from April to September.

That information comes from a research program using ultrasonic transmitters that are surgically implanted into the fish, whose movements are then tracked on listening devices moored to the sea floor. It's a project of the Hawai'i Institute of Marine Biology, funded by the National Marine Sanctuary Program, run by biologists Carl Meyer and Kim Holland of the institute and Yannis Papastamatiou of the University of Hawai'i zoology department. Their findings were published in the journal Marine Ecology Progress Series.

Ulua, it turns out, are creatures of habit. They come in to shallow waters each day, and leave each night — the researchers don't yet know where they go at night, because they seem to swim somewhere beyond the range of the current locations of listening devices. They don't seem to swim from one island to another. The researchers tagged ulua at several of the islands in the Papahanaumokuakea National Marine Monument, but never had a fish from one island detected at another island.

But perhaps the most important piece of evidence from the research is the lunar migrations of the warmer months of the year. During the full moons of summer — the peak breeding season for the fish — hundreds of them show up. Divers have been in the water during these periods, and seen the amazing congregations. Great walls of silver fishes swirl. Males are often a darker shade — like chameleons, they have the ability to change their color from silver to almost black.

"What we've shown using these sophisticated technologies is that a central spawning area attracts ulua from all over the atoll. This (Rapture Reef) is the major if not the only spawning ground at French Frigate Shoals," Meyer said. No similar site has been found at other atolls and islands, but Meyer suspects that's only because his team was lucky enough to have placed a listening cylinder at Rapture Reef and still hasn't placed them in the right locations at other sites.

"I suspect each island has its own 'Rapture Reef,' " he said.

He also believes the fish may make similar mating migrations at each of the main Hawaiian Islands.

"We plan to make similar studies in the main Hawaiian Islands," he said. That work would be done in collaboration with the state Division of Aquatic Resources.

The aggregations are not unheard of. Fishing communities at some other Pacific islands have reported being aware of ulua gathering for spawning at the full moon. Anglers can use them as a way to increase their catch, but Meyer said the knowledge of these locations also may be a conservation tool.

"The wide-ranging behavior of giant trevally has clear management implications; either marine protected areas must be large enough to contain giant trevally movements or alternative management strategies must be used to prevent overexploitation of these coral reef top predators," the authors wrote in their paper.

In the case of the massive Papahanaumokuakea monument, the protection of more than 100,000 square miles is clearly sufficient to protect both their feeding grounds and their spawning sites. But few of the reserves in the main islands are big enough to contain the daily movements of the ulua, and even if they do, they would not likely protect them during their spawning migrations — if these massive group spawning gatherings occur in the main islands.

The authors argue that one alternative to massive preserves is to protect core activity zones with relatively small reserves, and to also identify and protect the fairly small spawning locations.

Meyer, Holland and Papastamatiou also suggest minimum size limits or slot limits to further protect spawning stocks. With slot limits, angers are allowed to catch middle-size fish, but must throw back the smallest ones to let them grow, and the largest ones, which are the most effective reproducers.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.