A diver surveys the reef at Molokini, where reef fish increased amid a drop in visitors during the pandemic, according to a recent study. KEVIN WENG photos
The Maui News
A study that found that reef fish returned to Molokini as visitors disappeared in the pandemic suggests that the popular snorkeling and diving spot off Maui’s leeward coast is “being overused,” a state aquatic biologist said.
Researchers conducted the study during the COVID-19 lockdown that heavily limited travel to Hawaii. During August 2019, more than 40,000 tourists visited Molokini; in March 2020 the pandemic dropped that number to zero. With visitors gone, larger predatory fishes moved back in at Molokini before retreating again as tourism resumed, researchers noted.
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 Russell Sparks, an aquatic biologist on Maui with the state Department of Land and Natural Resources’ Division of Aquatic Resources and a co-author on the study.
The results of the study were published Wednesday in the peer-review journal “PLOS ONE,” and researchers hope it will help guide better management of Molokini and other threatened marine habitats, according to a University of Hawaii news release Thursday.
A school of fish congregate along the Molokini reef.
“Our research demonstrates that human presence alone can alter the community structure and possibly the functioning of an ecosystem,” said the study’s lead author, Kevin Weng of William & Mary’s Virginia Institute of Marine Science. “This means we can improve how tourism is configured in Hawai’i and around the world to reduce the impacts of human presence.”
Co-authors on the study include Alan Friedlander and Whitney Goodell of the National Geographic Society, who are both affiliated with UH-Manoa, and Laura Gajdzik and Sparks of DLNR.
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, according to the news release.
During the 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. Community structure refers to the type and numbers of species present in an ecosystem.
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 Hawaii Institute of Marine Biology.
The species that mainly drove the observed increase in 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.
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. For researchers, 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, the news release said. 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.
Molokini, which lies about 3 miles west of Makena, was designated as a “no-take” marine protected area in 1977 based on tour operators’ concerns regarding the impacts of fishing and other “consumptive” uses.
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 in 1987 and an anchoring ban in parallel with installation of permanent moorings in 1995 to protect corals.
“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,” Weng said. “Management of tourism should be guided by biological research, and include clear and well-enforced rules, adaptive management and broad stakeholder involvement.”
A diver surveys the reef at Molokini, where reef fish increased amid a drop in visitors during the pandemic, according to a recent study. KEVIN WENG photos 
A school of fish congregate along the Molokini reef.Today's breaking news and more in your inbox
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