Hawaiian monk seals are on the decline in the Papahānaumokuākea Marine National Monument protected area. Credit: James D. Watt/imagequest3d.com

Endangered seals in a marine protected area are heading towards local extinction, even while the same species thrives in an unprotected area nearby. Researchers report in Conservation Letters this week1 that the monk seal population in Hawaii's Papahānaumokuākea Marine National Monument is shrinking by about 4% a year, while a sub-population of monk seals in the main Hawaiian Islands next door — where fishing, development and boat activity are permitted — is increasing by 7% a year.

Encompassing all of the Northwestern Hawaiian Islands, Papahānaumokuākea covers an area of 362,000 square kilometres and is home to 85% of the 1,100 Hawaiian monk seals surviving today. The reserve is one of the largest in the world and has been heralded as a huge success, but this study, led by conservation biologist Leah Gerber of Arizona State University in Tempe, highlights one possible down-side to the reserve.

"It's impossible to know if the reserve is actually causing or even contributing to this effect, but it's deeply disturbing that it hasn't reversed it," says Les Kaufman, a conservation biologist at Boston University in Massachusetts who wasn't involved with the work.

Is it about saving endangered species, or preserving a functioning ecosystem? ,

The difference in the seals' survival rates could be a result of an increase in shark predation within the reserve. Locals in the French Frigate Shoals, a group of islands that are part of the protected area, have noticed a steep increase in shark predation, says Gerber. "They actually started a shark-culling programme" to control 'problem sharks', she says. "It's not something they like to broadcast," she adds, because of the conservation status of sharks.

Although the authors don't know for sure whether shark numbers have gone up, they speculate that the population could have been boosted by fishing discards before all fishing in the area was banned in 2006. A few of those sharks could now be wreaking havoc on seal pups. "All you need on each atoll is one really hungry rogue shark," says Kaufman.

Saving pups

Other possible explanations for the decline, which is being caused by a low survival rate of seal pups, include a change to the food supply owing to warming waters; competition for food from large fish called jacks; or the possibility that the reserve is simply too new for its ecosystem to have settled down. It can take 15 years of monitoring, says Kaufman, to properly understand the dynamics of an ecosystem.

Officials have been considering ways to help bolster the area's seal population. The Pacific Islands Fisheries Science Center in Honolulu is considering moving weaned monk seal pups from Papahānaumokuākea to areas with higher survival rates, then returning them to the reserve when they are older and less vulnerable. Gerber and her colleagues say that moving the pups is probably a good idea for now, but that the seals should not be moved back until something changes in the protected area.

What that might be is unclear. "Say you cull the sharks, and capture jacks, and transfer the pups — it's still probably not enough," says Gerber.

Researchers have noted in the past that conservation measures for certain species can have unintended effects on others. In the Pacific Northwest of North America, for example, reintroduced sea otters might be driving down threatened abalone numbers.

"It depends how you define success," says Gerber. "Is it about saving endangered species, or preserving a functioning ecosystem? It's worth taking a step back. Marine preservations areas may not be the solution in all situations."