When crowds left, reefs came alive at Hanauma Bay

Hanauma Bay Nature Preserve, a popular snorkeling destination that attracts nearly a million annual visitors, underwent a remarkable and rapid recovery when tourism ceased during the 2020 COVID-19 pandemic.

A study from the University of Hawaiʻi at Mānoa Hawaiʻi Institute of Marine Biology (HIMB), published in npj Ocean Sustainability, found that this period of human absence led to clearer water, increased sightings of endangered Hawaiian monk seals, and more active fish populations.

“We took advantage of a unique ‘natural experiment’ created by the COVID-19 pandemic,” said Aviv Suan, project manager in HIMB’s Elizabeth Madin Lab, which leverages cutting-edge technology to research human impacts on marine ecosystems.

“Hanauma Bay is normally a heavily visited site, but was completely closed to the public for seven months. We collected data on water quality, monk seal sightings, fish abundance, and fish behavior before, during, and after the closure. This allowed us to directly compare how the reef responded to different levels of human activity.”

By measuring the physical and biological health of the ecosystem, the research team found that when human visitors were absent, the reef in Hanauma Bay quickly returned to more natural levels. Fish densities changed, and vital herbivores such as parrotfish became more active in grazing algae.

“The ecosystem responded in remarkable ways,” said Elizabeth Madin, lead author of the study.

“These kinds of changes happened quickly, suggesting that everyday human presence can have a real and measurable impact on reef health. It’s a powerful reminder of just how sensitive and responsive coral reef ecosystems are to our activity.”

Resilience and recovery

This research serves as a valuable case study for marine managers around the world, and provides a science-based roadmap for sustainable tourism and effective conservation strategies that can benefit both the environment and the economy.

“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,” said 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 Hawaiʻi. 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.”

The team’s findings provide a clear and powerful message: coral reef ecosystems are incredibly resilient, and at least some key parts of the ecosystem can begin to recover quickly when human pressures are reduced.

“This study is an example of how research at the UH can directly serve the people of Hawaiʻi by helping to guide reef management, protect natural resources, and support a more sustainable future for both ecosystems and local communities,” said Suan.