When the Covid-19 pandemic reached Hawai‘i’s Hanauma Bay in 2020, the popular snorkelling site – which attracts nearly one million visitors every year – was closed almost overnight. It remained shut for seven months.
The sudden absence of tourists created what scientists call a ‘natural experiment’ – it was a rare opportunity to study how quickly damaged coral reef ecosystems can bounce back when human pressures ease.
Monitoring changes to the reef’s water quality and biodiversity, the researchers discovered that even a short break from tourism allowed this famous ecosystem to recover remarkably quickly. The study, led by researchers at the Hawai‘i Institute of Marine Biology (HIMB), was published in the journal npj Ocean Sustainability.
A rare natural experiment
“Hanauma Bay is normally a heavily visited site, but was completely closed to the public for seven months,” says Aviv Suan, project manager in HIMB’s Elizabeth Madin Lab.
“We collected data on water quality, monk seal sightings, fish abundance, and fish behaviour before, during, and after the closure,” adds Suan, who explains that this allowed the team to directly compare how the reef reacted to different levels of human activity.
“The ecosystem responded in remarkable ways,” says lead author of the study Dr Elizabeth Madin. “The water became noticeably clearer, endangered Hawaiian monk seals started showing up more often, and fish numbers increased.”
According to the study, the percentage of days when Hawaiian monk seals were observed at the bay more than doubled during the Covid closure period, with the researchers suggesting human presence may disturb animal haul-out behaviour. Hawaiian monk seals are one of the most endangered seal species in the world and are endemic to the Hawaiian archipelago, making the increase in sightings particularly exciting.
The team also noticed that fish behaviour shifted noticeably when humans weren’t around, with vital herbivores such as parrotfish grazing more actively, a behaviour that helps keep potentially harmful algae in check.
“These kinds of changes happened quickly, suggesting that everyday human presence can have a real and measurable impact on reef health,” says Madin. “It’s a powerful reminder of just how sensitive and responsive coral reef ecosystems are to our activity.”
For conservationists, the results of the study reinforce how resilient coral reefs can be when given a reprieve from human pressures.
“Hanauma Bay is one of the most iconic marine ecosystems in the state,” says Suan. “By observing what happened when people were absent, we saw nearly immediate benefits for key parts of the ecosystem.”
According to the researchers, the study shows how limiting visitor numbers and promoting sustainable tourism, not just in Hawai‘i’ but all around the world, could help preserve reef health without damaging local economies.
“Putting caps on the number of visitors to reefs – especially those that are currently unregulated – could help restore lost ecological function and ease human pressures while still maintaining access,” says Madin.
“Protecting these ecosystems doesn’t have to come at the cost of the economy. Yes, reef tourism brings in billions of dollars each year, both globally and here in Hawai‘i. In fact, research shows that many visitors are willing to pay more to experience healthier, more vibrant reefs. That means we can potentially reduce crowding and still support the tourism industry and everyone who depends on it – if we manage it wisely.”
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