A new study of the marine life at Molokini islet suggests the popular snorkeling destination off Maui is being overused and in need of additional management to improve not only ecosystem health but also the visitor experience.

Much of the research for the study, which was published in the latest issue of the PLOS One scientific journal, occurred during the pandemic lockdown in 2020, when snorkeling and SCUBA tours had pretty much dwindled to zero.

“COVID was terrible for a lot of things, but it did enable some interesting natural experiments,” said Kevin Weng of William &Mary’s Virginia Institute of Marine Science, the study’s lead author.

This is the second study to look at a popular Hawaii tourist snorkeling spot and compare its normal use to the lockdown period. A 2020 study found that Hanauma Bay on Oahu became healthier in both water quality and fish abundance with the absence of humans.

Weng, a former University of Hawaii oceanography researcher and part-time Oahu resident, said the state Division of Aquatic Resources had been looking for someone to study Molokini, and the pandemic presented a golden opportunity.

Raising the money to carry out the study was difficult on short notice. Such an endeavor might ordinarily cost as much as $100,000 a year, Weng said, but this one came together after DAR chipped in $7,000 and a GoFundMe page by the Maui Nui Resource Council raised $4,180. Others donated temporary lodging, the use of vehicles and other in-kind contributions.

A group of Hawaii-based scientists who also realized the value of taking a closer look at Molokini during the lockdown joined Weng in the effort.

In August 2019, more than 40,000 tourists visited Molokini island to snorkel or dive. Those numbers plunged to zero in March of 2020, when coronavirus restrictions were imposed in full force. Tourism slowly returned to pre-pandemic levels in May of 2021.

During the lockdown, the scientists conducted SCUBA surveys on five separate occasions to record the species, abundance, size, and location of fishes within Molokini’s submerged crater. They also tracked the movement of predatory species using electronic tags.

By comparing their observations with data from similar surveys in the years before and after the lockdown, the researchers were able to document the changes in fish community structure caused by human presence. To obtain data on human use, they examined the logbooks kept by the 40 charter boat companies permitted to bring tourists to Molokini.

Weng said that when tourism shut down, species that had been displaced from Molokini’s shallow habitats by snorkelers moved back within months, increasing the numbers and size of fish as well as the proportion of larger predators.

“Some species that moved inside were never observed before,” he said.

The species that primarily drove the increase in lockdown biomass were fast-swimming predatory fishes known as jacks, also known as bluefin trevally or omilu. Weng said under normal circumstances, the jacks would likely use the crater for shelter from larger predators and as a place to hunt.

When tourism resumed, the jacks moved to deeper waters, with fish biomass and habitat use in Molokini dipping to pre-pandemic levels, according to the study.

Scientist said the displacement of the jacks from Molokini’s crater is particularly concerning because their summertime spawning season overlaps with the annual peak in marine tourism.

The displacement of predatory fishes from Molokini likely sends ripples throughout the local food web, the study said. Previous studies have shown that a reduction in predatory fishes affects not only the plant- eating fishes they count on as prey, but the algae and other primary producers eaten by the herbivores.

Weng said the study does not suggest how the state should manage the pressures on Molokini’s ecosystem. But finding a way to limit the number of snorkelers using Molokini at the same time would help, he said.

“Management of tourism should be guided by biological research and include clear and well-enforced rules, adaptive management and broad stakeholder involvement,” he said.

A 2011 study found that more than two-thirds of visitors to Molokini felt crowded during their trip and supported actions that would reduce visitor numbers.

“As Hawaii formulates marine management plans and undertakes the Sustainable Hawaii Initiative, lessons from Molokini can help inform managers and help facilitate an effective response,” study co-author Laura Gajdzik said in a news release. “As part of this process, we need to think strategically about the scale and configuration of tourism in Hawaii to optimize earnings and employment without damaging the environment.”

Molokini, which lies about three miles west of Ma­kena, Maui, was designated as a “no-take” marine life conservation district in 1977.

Since that time, tour operators have worked with the state on several measures to limit pressures on the ecosystem, including a limited-entry permit system for tour boats in 1987 and an anchoring ban along with the installation of permanent moorings in 1995 to protect corals.

Study authors include Gajdzik and Russell Sparks of the state Department of Land and Natural Resources, and Alan Friedlander and Whitney Goodell of the National Geographic Society. Friedlander and Goodell are also affiliated with UH-Manoa.