Overfished waters surrounding the main Hawaiian Islands are lacking the predator populations needed for a balanced ecosystem, according to a new report.

The study, titled "Predators as Prey: Why Healthy Oceans Need Sharks," called attention to the important role that sharks and other so-called apex predators play in the natural functioning of a healthy ecosystem.

However, commercial fishing, pollution and the practice of shark finning have left shark populations in a "grave state" across the state and worldwide, according to lead author Elizabeth Griffin.

While apex or top predators constitute more than 54 percent of total fish populations in the Northwest Hawaiian Islands, they make up only 3 percent of fish in the heavily populated main Hawaiian Islands.

Ulua jack fish are the most common apex predators found throughout the island chain.

KEY TO REEF HEALTH

Griffin, a marine wildlife scientist for the environmental advocacy group Oceana, said that declining populations of apex predators surrounding the main islands are "definitely a source of concern."

"The Northwest Hawaiian Islands are what it should look like in a healthy ecosystem. Not having those top apex predators does change things and disrupts the natural ecosystems," she said.

Alan Friedlander, a fisheries ecologist with the Oceanic Institute, describes the important role of apex predators in providing "top-down control."

Studies have shown that sharks and other predators regulate the distribution and diversity of their prey species by removing sick and weak individuals from prey populations.

Around the waters of Shark Bay, Australia, tiger sharks also help to control the distribution and movement of sea turtles, the Oceana study found.

Sharks likely play similar roles in regulating the movement of sea turtles around Hawai'i, Griffin said. Of 350 species of sharks, approximately 40 species inhabit waters surrounding Hawai'i.

"Large sharks in general can help to maintain the health of coral reefs," home to hundreds of endemic marine species in Hawai'i, she said.

The health of coral reefs depends on herbivorous fish such as parrotfish, which graze on algae that grows on the corals. Without thriving shark populations, the number of "groupers" — fish that eat herbivores — can increase. The loss of shark populations can thus trigger a chain reaction that results in the elimination of herbivorous fish, which can lead to algae overgrowth that permanently damages Hawai'i's reefs.

Shark populations have been depressed around the Hawaiian Islands for a long time, Friedlander said. Shark eradication efforts in the past have contributed to this decline.

"People fear sharks, and sometimes sharks and humans don't co-exist too well, but they're important to having a natural marine environment," he said. "It's a delicate line" between maintaining safe waters and conserving natural ecosystems, he said.

PROTECTIVE MEASURES

Earlier this month, the U.S. House of Representatives passed the Shark Conservation Act of 2008 to help protect shark populations. The legislation is intended to strengthen existing laws, including a federal ban on shark finning established by the Shark Finning Act of 2000.

Despite federal and state laws that prohibit shark finning, people have found ways to continue the practice, Griffin said.

"There have been enforcement problems with the law in Hawai'i," said Griffin, who cites a 2002 case in which 32 tons of shark fins were seized from a Honolulu-based vessel.

In March 2008, owners of the vessel won an appeal in the case, claiming that the ship was not a "fishing vessel" since it had bought the fins from other vessels on the open ocean.

Griffin hopes that new legislation will close the loophole and strengthen the enforcement of existing regulations intended to protect shark populations.