Stanford, California - Conservation of large marine animals like bluefin tuna and great white sharks is challenging for a number of reasons. "They cross open ocean areas, so it's just difficult to track, observe and get information about the behavior, physiology and ecology of an animal," explained Stanford Professor Fiorenza Micheli, a marine biologist involved in the research.
And since many of these creatures - manta rays included - are ranked vulnerable or endangered, their rarity is also an issue. Even if one is caught and tagged, the GPS technology commonly used for tracking animals on land doesn't work as well because water blocks the signal.
"You only get information for where animals surface, but there's a lot you want to learn about what happens under the water," said Doug McCauley, a former PhD student at Stanford and now an assistant professor at the University of California, Santa Barbara.
Progress in paradise
To get beyond these issues, McCauley and Stanford student Paul DeSalles went to Palmyra, a marine protected area a thousand miles south of Hawaii, and used a novel combination of techniques to discover the daily life of manta rays in paradise.
An acoustic camera placed at the entrances of lagoons turned sonar signals into videos of manta ray traffic. The animals were tested for diet – turns out they enjoy lagoon plankton – and acoustic trackers allowed the researchers to get up close and personal, following the manta rays around.
"It hypnotically pulls you into the secret world of manta rays underwater," McCauley said. "It was both a spectacular and tedious experience." In rain or shine, they tracked the mantas for hours at a time – the record was 36 hours – and recorded their movements.
In the end, the team discovered that the manta rays were in and out of the lagoon constantly and posited a number of reasons: food, safety from predators or perhaps as a nursery for younger rays. "It's a habitat that they rely on fairly heavily, which raises then the question and issue of their vulnerability because this is where they're most likely to interact with people," said Micheli of Stanford's Hopkins Marine Station.
Hope for conservation in the big blue
"More and more there is evidence that even the most mobile organisms have some areas they return to or some preferential routes of migration," Micheli said. And this gives conservation policymakers a concrete goal for protecting the manta rays and other large marine animals.
"We can do something about the habitats that are relatively close to home and relatively easy to manage," McCauley said.
Large marine animals are often cogs in an intricate wheel and taking them out can cause whole ecosystems to collapse. Other, more invisible, cycles may break down like the one that McCauley and DeSalles (who has since graduated) observed on an earlier trip to Palmyra: an unexpected relationship among the manta rays, seabirds and plants on the island.
Stanford marine scientist Barbara Block, a professor at Hopkins Marine Station, is already taking the next step, moving offshore from Palmyra and seeing how manta rays use both lagoons and open ocean habitat.
"We don't know which multiple mechanisms draw them to the lagoon, and we don't know what the consequences are of removing these steps," Micheli said. "How are populations affected by the loss of these habitats?"