For more than 15 years, Luis Fernando Sánchez Caicedo had dedicated himself to human rights in Colombia, supporting young people and advocacting for Afro-descendant and campesino – small farmer – communities in the Pacific region. A prominent local leader and adviser to the area’s administration in Nariño, he was also a longtime collaborator with the Institute for Development and Peace Studies (Indepaz), working to promote dialogue in a country torn apart by decades of war.

That all ended in September when the boat carrying him and the mayor of Mosquera, Karen Lizeth Pineda, was fired on, reportedly by the Colombian navy. Sánchez was killed and the mayor’s bodyguard was seriously injured in the attack.

The incident, which is under investigation by the Colombian authorities, has added to the unease within the local community about a project to turn nearby Gorgona island into a military base.

Species such as the marbled poison frog make up the island’s richly biodiverse fauna. Photograph: Antonio Cascio/The Guardian

Gorgona’s marine protected area (MPA) is at a crossroads, with the installation of a coastguard station, promoted by the Colombian navy and supported by the US government, that critics say could jeopardise 40 years of hard-won environmental progress.

The £1.4m project, which is being financed by the United States, includes a pier, radar and facilities for navy personnel. It is expected to produce 587kg of waste during construction, a significant portion of which is considered hazardous.

The purpose of this area is protection – and having military personnel beside researchers and tourists contradicts this ideaProf Alan Giraldo

Campaigners say Gorgona has one of the most unspoilt MPAs in the region. It is a refuge for biodiversity, as its natural reserve comprises more than 60,000 hectares (150,000 acres), a third of the area of Greater London.

Before becoming a national park in 1984, it was an Alcatraz-like prison. Now, it is home to several marine species and ensures food security for coastal communities.

The high-security prison that was on Gorgona Island until 1984

Prof Alan Giraldo, a biologist at Valle University in Cali, who first visited the island in 1989, says: “The purpose of this area is protection – and having military personnel beside researchers and tourists contradicts this idea.”

Marine parks such as Gorgona contribute to the “30×30” global agreement to conserve 30% of land, water and ocean by 2030, as outlined in the Convention on Biological Diversity.

According to the Protected Planet database, joint project of the UN Environment Programme and the International Union for Conservation of Nature, Colombia is officially conserving 41% of its marine territory, achieving the 30% target years ahead of schedule.

However, according to the more detailed Marine Protection Atlas complied by the Marine Conservation Institute (MCI), only 6.7% of those waters are “fully” or “highly” protected.

Gorgona’s MPA falls into this much smaller category due to the ecological restoration it has undergone in recent years. Nature has reasserted itself: large trees tower over an unused prison compound, whales are regularly seen gliding along the coast, and the surrounding coral reef is the most extensive and biodiverse in the eastern tropical Pacific Ocean.

Humpback whales off Gorgona. They migrate from the Antarctic to the warm seas around the island to breed between June and October

Its success is not accidental. Strict conservation measures, such as a fishing ban, a ban on tourist boats chasing whales, and the complete restriction of terrestrial and marine areas to human activity, have led organisations such as the MCI to grant Gorgona a Blue Park award in June.

Local people say the decision to base a coastguard station on the island was made without consulting them, which has increased a sense of exclusion from the decision-making process and fuelled opposition.

Mercedes Colorado, government secretary of El Charco, a municipality on the mainland near Gorgona, says: “This project to set up a military base was not consulted with any of the municipal authorities, Indigenous territories or the [Afro-descendant] community councils.”

The radar mast, part of the coastguard project, visible at the top of Trinidad Hill on Gorgona. The Colombian army is also building a pier and barracks on the island

The Colombian government says the coastguard station and its radar are crucial for fighting drug trafficking and illegal fishing. However, scientists and activists fear the base could make Gorgona a military target in an area already known for armed conflict.

Attacks on Colombian military infrastructure have become more common in recent years. In August, 18 people were killed in two separate attacks, including a car bomb at a military aviation school in Cali and a drone attack on a police helicopter in Medellín.

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Trump’s policy involves deploying military forces to other countries to take control of territories that aren’t theirsMercedes Colorado

US naval forces have built up in the southern Caribbean Sea off Colombia and Venezuela, and President Trump has authorised the extrajudicial use of military force against drug cartels in the region. Since early September, US attacks on boats allegedly carrying drugs have killed more than 80 people, according to Pentagon figures.

Tensions between the two countries are growing, as the US has imposed sanctions on Colombia, accusing President Gustavo Petro of allowing drug cartels to flourish. Petro sees the US move as an attempt to influence Colombia’s forthcoming elections.

Back in El Charco, Colorado says: “We know that Trump’s policy involves deploying military forces to other countries to take control of territories that aren’t theirs.”

A memorial service held after the Cali attack, which killed six people and injured more than 60

Whether the US will eventually place military personnel on the island is unclear. Yet its involvement is clear as the Bureau of International Narcotics & Law Enforcement, a branch of the US state department, was in charge of the pier’s design and the vessels designated for the Colombian coastguard.

Experts fear the new construction could also affect the island’s recently restored ecosystem, as the pier could increase sedimentation and alter water flows, potentially damaging the coral reef and compromising the health of the seabed.

The Colombian navy’s environmental impact assessment has acknowledged possible effects, including the loss of soil structure, fertility and stability, as well as potential harm to fish, turtles and marine mammals. The project also includes a 20,000-litre fuel tank that must be transported by boat from the mainland and could spill fuel into the ocean.

For a biologist such as Giraldo, the project “puts at risk ecosystems that have developed over hundreds of years and could be lost in a second”.

Blue-and-gold snappers and a sergeant major in the reef off Gorgona. Thanks to conservation efforts in recent decades, the coral is the eastern tropical Pacific’s most extensive and biodiverse

Although the navy has insisted that the pier construction will respect the whale season to avoid disturbances, it is unclear how this will be implemented, as the environmental assessment indicates that the pier construction would take more than nine months. That leaves only 76 days of the year unaffected, while the whale mating and breeding season takes place between June and October, equivalent to more than 100 days.

Fishers, as well as scientists, also worry about the navy’s new restrictions on their movements and fear being mistaken for the military by armed groups when fishing at night. They fear that the navy will restrict when they can fish or even whether they can access the island at all.

A fishing boat from Bazán village. Some fear that as Gorgona becomes militarised, the waters around the island will be off-limits. ‘This base could mean the death of fishers,’ says one man

Although local communities are not allowed to fish within the MPA, they are allowed to use an authorised shelter to rest on the island periodically.

“That all creates a risk now,” says Espaciano Aguirre, a veteran fisherman. “This base could mean the death of fishers.”

Besides the security risk highlighted by the killing of Sánchez, the coastguard project could threaten the island’s Blue Park recognition. According to Dr Sarah Hameed, director of Blue Parks at the MCI, the organisation was completely unaware of the coastguard project.

“Any project that poses a threat to biodiversity conservation … can trigger the early re-examination of the five-year review,” Hameed says.

If the MCI were to decide that the coastguard station clashes with the conservation criteria of the Blue Parks initiative, this could mean that the Gorgona marine protected area loses its international recognition, undermining Colombia’s 30×30 ambitions.