The most popular shallow reef sites can be quickly pulverized by trampling and aren't able to recover because of continued contact. The result is a clear pattern of decreasing coral cover and lower fish populations with increased visitor use.
However, researchers found that divers and snorkelers used less than 15 percent of the total reef habitat at each site, staying within relatively small, well-defined areas associated with access points.
Information from the study could be used to predict where people are most likely to go based on a site's topographical features. Once those areas are identified, designated access points and boat moorings could be used to focus activities away from the most sensitive habitats.
Reef damage from swimmers not widespread
Study finds that snorkelers, divers stay in limited areas
Christie Wilson, Honolulu Advertiser 26 Apr 09;
A new study based on secret observations of snorkelers and scuba divers at four protected marine sites in Hawai'i revealed that those activities have low impact on coral reef habitats and are contained to relatively small areas.
"Although Hawai'i marine protected areas were heavily used in comparison to those in other geographic locations, this did not translate into high recreation impact because most fragile corals were located below the maximum depth of impact of the dominant recreational activity (snorkeling)," according to the report by Carl Meyer and Kim Holland of the Hawai'i Institute for Marine Biology.
The research used handheld Global Positioning System units to map the movements of people in the water and identify "hot spots" where the most contacts with reefs and other substrate occurred at four marine life conservation districts: Honolua-Mokule'ia on Maui, Kealakekua Bay on the Big Island, Manele-Hulopo'e on Lana'i and Pupukea on O'ahu's North Shore.
"A lot of work on marine protected areas has focused on what the marine life is doing, not people," Meyer said.
Researchers found that divers and snorkelers used less than 15 percent of the total reef habitat at each site, staying within relatively small, well-defined areas associated with access points.
Meyer said information from the study could be used to predict where people are most likely to go while in the water based on a site's topographical features. Once those areas are identified, designated access points and boat moorings could be used to focus activities away from the most sensitive habitats.
"If you manage those access points somehow, you can determine where people go," Meyer said.
The study, funded by NOAA Fisheries and the Department of Land and Natural Resources' Division of Aquatic Resources, provides data on a subject for which little information exists. A shortage of scientific information on baseline conditions at Hawai'i's coral reefs, how many people visit snorkeling and diving spots, and impacts from recreational activities has made it difficult for marine resource managers to set policy aimed at protecting these precious habitats while supporting public access and the state's $800 million ocean recreation industry.
"There isn't a whole lot of good data that scientifically quantifies the impact from recreational activities. One of the things we find is that we tend to get a lot of push-back (when proposing management rules) just because we don't have a lot of science to back it up," said Emma Anders, who coordinates efforts to address recreational impacts on reefs for the Division of Aquatic Resources.
Although coral trampling by snorkelers and divers affects a tiny portion of Hawai'i's total reef resources, it most affects beaches and offshore sites of high recreational value and importance to the visitor industry, said Ku'ulei Rodgers, another scientist with the Hawai'i Institute for Marine Biology.
She said the most popular shallow reef sites can be quickly pulverized by trampling and aren't able to recover because of continued contact. The result is a clear pattern of decreasing coral cover and lower fish populations with increased visitor use.
"It can do really heavy damage to have people standing on the reef, but the good news is there are few places where there is a heavy impact from tourists. It's mostly concentrated in places like Waikiki and Hanauma," she said.
Of more concern is how trampling can weaken a reef's resistance to the more severe, looming threats of global warming and ocean acidification, Rodgers said.
The study by Meyer and Holland calculated that Pupukea is visited by 22,500 divers and 47,700 snorkelers annually; Kealakekua, 1,440 divers and 103,300 snorkelers; Manele-Hulopo'e, 1,750 divers and 28,200 snorkelers; and Honolua-Mokule'ia, 2,050 divers and 84,000 snorkelers.
By comparison, 780,000 people visited the lower beach at famed snorkeling site Hanauma Bay in the last fiscal year.
Scuba impact wider
Diver tracking revealed that scuba divers have a greater impact on coral per dive than snorkelers, largely because scuba dives last longer, cover larger areas and venture into deeper zones, the study said.
The research also determined that boat-based activities have less impact per dive than shore-based activities, primarily because boat-based snorkelers and divers don't access sites from land.
"There's no wading involved," Meyer said.
Divers on tour boats also receive pre-dive briefings about proper behavior in the water and are monitored by dive tour staff. "If people are doing things they are not supposed to be doing, I've seen them intervene, and that separates boat-based activities from shore-based," he said.
Although shore-based snorkelers and divers may get information from shops where they rent their gear, Meyer said there is less on-site management at dive spots accessed from shore.
Most of the substrate contacts that occurred were clustered at shoreline access points where people waded as they entered and exited the water, the study said. Even then, there were few impacts because the contacts that did occur involved sand or coral-free rock, although the study noted "it cannot rule out that (coral) colonization is being prevented by continued trampling."
Only 14 percent of contacts were with live substrates such as coral, coralline algae and invertebrates attached to rocks and coral. Fewer than 1 percent resulted in obvious damage such as broken branches and tissue abrasions, the study said. Most of the damage was done by shore-based snorkelers.
"One of things we noticed is that about half of the physical impacts that we observed in these areas resulted from only 16 percent of the people who are using it," Meyer said. "There is a subset of people who have a much higher impact. If you can reach that 16 percent, you could literally halve the existing impact."
That could be done by providing educational signs at popular dive sites or enlisting volunteers to stand by with information and advice.
"A lot of thought would have to go into how that would be done," Meyer said. "The tradeoff is trying to manage the impacts of the place without turning it into an urbanized situation where you have buoys.
"These are areas created with the overarching goal of maintaining an environment in pristine or near pristine condition. One of the ironies is that because it's such a nice area, a lot of people want to come and visit it, and that sets up the potential for the original goal of marine protected areas to be undermined by overuse."
Closure option
Managers of the 'Ahihi-Kina'u Natural Reserve Area in South Maui decided a tradeoff was unacceptable when they closed much of the reserve's 2,045 acres last October for a two-year period because people were straying onto unmarked coastal trails and trampling reefs at two popular snorkeling coves known as the Aquarium and Fish Bowl.
"It was a big step for us," said Bill Evanson, Maui District natural area manager for the DLNR.
Unlike beach parks, the 'Ahihi-Kina'u reserve is intended not for recreational use but for safeguarding of its resources, which include crystal-clear ocean waters that host some of Hawai'i's oldest reefs, rare anchialine ponds and numerous geological and archaeological features.
Before the closure, the reserve was getting 700 to 1,000 visitors a day. Evanson said that since the closure, there is visible evidence coral reef habitats are rebounding.
"We're seeing that many of the tidepools and coves where there used to be people are inhabited by fish in numbers and variety we haven't seen," he said. "Fish aren't being scared away by the presence of people. Now they are able to feed, hide from larger predatory fish and breed."
An advisory board is considering whether to allow future access by permit only or through guided hikes.
