Koh Ach Seh - Khmer កោះអាចម៍សេះ, generally shortened by everyone to Koh Seh, Horse Island - sits in the Gulf of Thailand off Cambodia's southern coast, part of the Kep Archipelago and administered by Kep province. Its recent history is written into the vegetation. During the Cambodian-Vietnamese War the Khmer Rouge held the island as an outpost, and the military bunkers they dug are still there, along with the fruit trees they planted to feed the garrison. Someone stood on this rock four decades ago working out where an orchard should go and how long a small garrison could feed itself off it. Those calculations belonged to a regime that had emptied Cambodia's cities and killed something close to two million of its people, and the fishing families of this coast were among those who paid. The trees kept growing anyway. They are about the most benign thing the Khmer Rouge left behind anywhere in Cambodia, and they still fruit.

Since 2013 the island has been home to Marine Conservation Cambodia, the country's first permanent marine conservation organisation, founded in 2008 by the British conservationist Paul Ferber and originally based far to the west on Koh Rong Samloem. The Kep government lent MCC most of Koh Ach Seh, keeping only the southern tip, where a small marine police station and a cluster of fishermen's shelters share the shoreline. It is an odd, productive arrangement - researchers at one end of a former military island, the state's own patrol boats at the other, both facing the same problem out in the same water.

The work that put the island on the scientific map began with an animal almost nobody had bothered to count. MCC identified four seahorse species living in Cambodian waters and became the first organisation in the country to study them seriously, working with Project Seahorse and contributing to the case that saw the species written into CITES protection. Seahorses are a good indicator and a poor survivor: they hold station in seagrass, they do not flee well, and a trawl net takes everything. Protecting them means protecting the meadows they live in, which happen to be the nursery grounds for most of what Kep's fishermen actually catch. The argument for conservation here has never needed to be sentimental. It is arithmetic.

In April 2018 Cambodia approved the Kep Archipelago Marine Fisheries Management Area - 11,354 hectares of protected water spanning nine islands, with two no-take zones inside it. MCC helped build the case for it and now manages it. Two months later the concrete anti-trawling blocks won the organisation one of the first three Marine Protection Prizes ever awarded by the National Geographic Society. In April 2019 the archipelago was declared a Hope Spot by Mission Blue, Sylvia Earle's global network of marine sites judged critical to the ocean's recovery. For a country whose coastline spent decades as a war zone and then as an open-access free-for-all, that is a remarkable turn in the space of six years, driven from a single island most charts do not name.

One caution for anyone navigating by name: there is another Koh Seh, sitting inside Ream National Park off Sihanoukville province, and the two are confused constantly. The Kep one is the island with the bunkers, the blocks and the seahorse counts. Look for it south of the Kep peninsula, low and green, sitting in shallow water not far from the Vietnamese maritime boundary - close enough that fishing disputes in this archipelago are never only about fish.