Story by Dawn Southard
Photo by Elyse Butler
The pool beneath Maunawili Falls is a little otherworldly. It’s nestled deep in a keyhole-shaped niche overhung by banyan and strawberry guava trees, its steep, slippery walls covered in moss and ferns. You can scramble up the cliff and dive back into the swimming hole, but if you just cling patiently to the nearly vertical wall, you might see something even more otherworldly: small, oblong fish blundering into you as they scour the rocks for algae. These are ‘o‘opu, native freshwater gobies that are among the world’s most astonishing fish.
Hawai'i's Supergoby: 'O'opu are the Islands' only native freshwater fish. They live in saltwater as juveniles and move into streams as adults; they can even climb waterfalls to elevations over a thousand feet. This 'o'opu nakea at the Waikiki Aquarium is the largest of Hawai'i's five 'o'opu species.
There are five species in Hawai‘i’s streams; four of them are found nowhere else on Earth. All five are amphidromous— that is, they spend their adult lives in fresh water, where they spawn and lay eggs in the streambeds. When they hatch, the larvae are washed out to sea, where they drift for months. Only as fingerlings — and after a dramatic metamorphosis — do they begin their trip back upstream.
It’s this migration that makes ‘o‘opu so amazing. To reach Maunawili Falls, for example, ‘o‘opu have to climb almost four hundred feet, negotiating rapids and scaling waterfalls. But they’re built for the journey: ‘O‘opu nopili use their mouths to pull themselves along, and most species of freshwater ‘o‘opu have fused pectoral fins—they form a suction cup allowing the fish to climb vertical, even overhanging walls.
Each species occupies a different elevation depending on how well it climbs. ‘O‘opu nakea, a good climber, is found at mid-elevations. It’s also the largest ‘o‘opu (up to a foot long) and was a favorite food in old Hawai‘i. The champion climber, though, is the ‘o‘opu ‘alamo‘o. It’s been found in pools above towering, thousand-foot waterfalls. Some believe Hawai‘i’s ‘o‘opu are endangered, but it’s difficult to know for sure. It is clear, however, that there are fewer ‘o‘opu today than there were in the past, but where streams flow year-round you can still find these tiny, mighty fish clambering back to the pools of their birth.