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scuba diver by reef
(Photo credit: Kevin Weng)

During August 2019, more than 40,000 tourists visited Hawaiʻi’s Molokini island—off of Maui’s southwestern coast—to snorkel or dive. In March 2020 the COVID-19 pandemic dropped that number to zero. The sudden and prolonged drop in visitors to one of the world’s most popular snorkeling spots provided scientists with a novel opportunity to study how underwater tourism impacts marine fishes. The results of the study, published in PLOS ONE, will help resource managers better care for Molokini and other threatened marine habitats.

The study’s lead author, Kevin Weng of William & Mary’s Virginia Institute of Marine Science, said “The COVID-related tourism freeze provided a unique natural experiment to measure the effects of decreased tourism on fish behavior in a high-use, no-take marine protected area.”


Co-authors on the study include Alan Friedlander and Whitney Goodell of the National Geographic Society, who are both affiliated with the University of Hawaiʻi at Mānoa, and Laura Gajdzik and Russell Sparks of Hawaiʻi’s State Department of Land and Natural Resources.

Overall, the team’s findings suggest that “Molokini is being overused, and that management is needed to improve not only ecosystem health but the visitor experience,” said Sparks.

Predatory species move back in

Community structure refers to the type and numbers of species present in an ecosystem. During Hawaiʻi’s COVID-19 lockdown—which began at full force in March 2020 and was then slowly lifted until visitation returned to pre-pandemic levels in May 2021—the researchers conducted SCUBA surveys on five separate occasions to record the species, abundance, size and location of predatory and herbivorous fishes within Molokini’s submerged crater. They also tracked the movement of the predatory species using electronic tags. Comparing these observations with data from similar surveys conducted in the years before and after the lockdown allowed them to detect differences in fish community structure caused by human presence. The researchers gathered data on human presence using log books kept by the 40 charter boat companies permitted to bring tourists into Molokini’s waters.

“When tourism shut down due to COVID, species that had been displaced from shallow habitats by high human presence moved back in on a timescale of months, increasing fish biomass as well as the proportion of larger predators,” said Friedlander, a researcher at the Hawaiʻi Institute of Marine Biology.

The species that mainly drove the observed increase in lockdown biomass were fast-swimming predatory fishes known as jacks, which learn to fear humans as they are often targeted by fishermen. When tourism resumed, “the predators moved to deeper waters, so fish biomass and habitat use dropped to pre-pandemic levels.” Biomass is a combined measure of fish abundance and size.

(Photo credit: Kevin Weng)

The observed changes in predator biomass were also reflected in their behavior. Because morning is the most popular time for tours to the island, jacks leave the inside of the crater during these hours. However, during the lockdown, they remained in the shallow sheltered interior. These predators were quickly displaced from this shallow-water habitat when tourism resumed. Their displacement is particularly concerning because their summertime spawning season overlaps with the annual peak in marine tourism.

The human-induced displacement of predatory fishes from Molokini’s crater likely sends ripples throughout the local food web. Previous studies have shown that a drop in the abundance of predatory fishes affects not only the herbivorous fishes they count as prey, but the algae and other primary producers eaten by the herbivores.

“Predators have diverse ecosystem roles, and their loss can reduce the resistance and resilience of ecosystems to other stressors,” said Friedlander.

“Our findings indicate that the business-as-usual conditions of high tourism alter community structure by displacing predatory fishes to deeper environments,” said Weng. Moreover, 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.

Marine protected area

Molokini, which lies about 3 miles west of Makena, Maui, was designated as a “no-take” marine protected area or MPA in 1977 based on tour operators’ concerns regarding the impacts of fishing and other “consumptive” uses.

“The tour operators have always been interested in the conservation of Molokini, and have worked with the State on several measures,” said Sparks.

As the volume of “non-consumptive” uses such as snorkeling and SCUBA diving increased, tour operators worked with the state to establish a limited-entry permit system for tour boats (1987) and an anchoring ban in parallel with installation of permanent moorings (1995) to protect corals.

scuba diver by reef
(Photo credit: Kevin Weng)

The study focused on the impacts of these non-consumptive uses. “Our research demonstrates that human presence alone can alter the community structure and possibly the functioning of an ecosystem,” said Weng. “This means we can improve how tourism is configured in Hawaiʻi and around the world to reduce the impacts of human presence.”

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

“Our study indicates that the intensity of non-consumptive uses, especially in heavily visited MPAs, should be considered for the long-term health and resilience of these ecosystems,” said Weng. “Management of tourism should be guided by biological research, and include clear and well-enforced rules, adaptive management and broad stakeholder involvement.”

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