Sneaking past locals lingering on a smoke break and into a gully alongside an aging general store is not how one expects to begin a quest for a rare fish.

“Most of the streams along the Hāmākua Coast that I have been to seem to support populations of ‘o’opu,” wrote Tim Grabowski, head of the Hawaii Cooperative Fishery Research Unit of the US Geological Survey, in an email about where to find the fish in question. The trouble is, despite dozens of streams along this 50-mile stretch of winding scenic seafront, very few – if any – are publicly accessible.

So, I snuck down the precariously steep and muddy path into a canopy of leafy green to one of the most discreet and least obstructed access points. Here, the rural Hawaiian countryside fades into wild nature; wind and the rumble of pick-up trucks crossing the bridge overhead are replaced with humidity, nagging mosquitoes and a burbling brook.

“‘O’opu are pretty easy to detect with a decent pair of polarised sunglasses,” Grabowski said. And, yet, upon reaching the stream and looking into the clear brown water, not a single finned creature moved.

‘O’opu – the Hawaiian word for fish in the goby family – most often refers to several species of unusual freshwater fish that navigate into inland waterways, some by scaling the islands’ waterfalls using their mouths and suction-cup shaped fused pelvic fins. Four species of goby and one species of sleeper goby (a family related to gobies but that lacks the fused pectoral fin common to all gobies) comprise the remote island chain’s only native true freshwater fish. Scientists say ‘o’opu are mostly endemic, understudied and threatened by development around their native stream habitats.

These fish are small, unassuming and usually brown and camouflaged, mottled or striped (though the male of one species, ‘o’opu ‘alamo’o, is a notable exception as it can be half black and half bright orange during spawning season). And each species appears to prefer a specific habitat along a stream’s length, with the most impressive waterfall climbers preferring the most remote and inland pools as adults.

With more than 200cm of annual rainfall, Hawaii Island’s verdant Hāmākua Coast drips with waterfalls. North of eastern-facing Hilo, it’s on the youngest of the main Hawaiian Islands; only around 500,000 years have passed since it emerged from the sea. This region is the steep backside of majestic Mauna Kea, at 10,211m, the tallest mountain in the world when measured from its underwater base. The volcano acts like a giant wall in the open ocean; the only surface against which clouds collide after more than 4,000km of nothing but Pacific heading west from Mexico. Here, streams are steep and short, and prone to flooding.

Only five native species of ‘o’opu have evolved to tackle Hawaii’s ragged jumble of freshwater streams and four of them have evolved the remarkable ability and drive to rock climb. 

Despite this, said Grabowski, “Hawaiian freshwater fishes are very understudied. There is not a lot out there in the literature about ‘o’opu and there are still a lot of aspects of their basic biology and ecology that are largely unknown.” What scientists do know, though, is fascinating.

They sort of leap out of the water and cling to the side of the falls

“They sort of leap out of the water and cling to the side of the falls,” said Richard MacKenzie, an aquatic research ecologist with the Pacific Southwest Research Station, part of the US Forest Service. “Then they sort of scoot up the waterfall in short bursts of energy, typically at the edge of the stream.” Like other fish in their goby family worldwide, the fishes’ pelvic fins are fused together to create a suction cup. The Hawaii fish use their suction and their rasping mouths, evolved to scrape algae from rocks, to haul themselves up the rocks in rapids, and further upstream, sheer vertical cliff faces.

Adaptation, separation

How far up a stream and inland each of the four species of climbing fish go seems to be related to their adult size. “The biggest ‘o’opu,” said Grabowski – referring to ‘o’opu nākea (Awaous guamensis), which can grow up to a foot in length – “may not get past the lower reaches of most streams.” As you head further inland and up the mountain, larger species peter out and the smaller fish dominate.

While juvenile ‘o’opu nopili (Sicyopterus stimpsoni) – which top out at around 18cm as adults – have been spotted scaling 41m drops, it’s o’opu ‘alamo’o (Lentipes concolor), the black and orange bicoloured goby, that’s the strongest climber. It’s been found in ponds atop some of the Hawaii’s highest falls, including the more than 300m-high plummet of Hi’ilawe Falls in northern Hāmākua’s Waipio Valley. That’s a long haul for a fish that grows no larger than 13cm. If a human were to accomplish the same feat, they’d have to scale a surface twice the height of Yosemite’s El Capitan, while being pummelled in the face by boulder-sized water globs after swimming upstream a full 4km longer than the length of a marathon.

Part of efficient climbing is finding the path of least resistance. For the ‘o’opu, that often means clinging at edges of waterfalls rather than in the middle. This poses its own challenges. “The real trouble for a fish being out of water isn’t breathing [as long as their gills and skin are wet],” explained Grabowski. “The real danger of being out of water is waste disposal.” Fish, it turns out, also use their gills to dispose of nitrogenous waste. Without the ability to “pee across their gills”, Grabowski said, a fish is in real danger of ammonia toxicity. Though scientists don’t know exactly how, the waterfall-climbing gobies seem to have adapted to overcome this problem, too.

So, the question remains: Why go through the trouble?

Again, scientists aren’t certain. But many who’ve studied Hawaii’s streams say it could be to carve out a niche for each species and avoid competition with one another. In other words: to be different and to survive. 

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Waterfall-climbing gobies are present in perennial streams on all of the six visitable Hawaiian islands but Bob Kinzie, an emeritus zoology professor at the University of Hawaii at Mānoa on Oahu said, “We have almost no idea of real population density or distributions on any of the islands.” This makes it challenging to determine how populations might be changing over time.

And yet, understanding how the population fluctuates may be critical to saving it. Hawaii’s native ecosystem has experienced breakneck change in the 200-plus years since Europeans stumbled upon its shores. Invasive plants have claimed 60% of the landscape; feral pigs, goats, sheep and cows trample the landscape eating native plants and causing erosion; and towns and cities now thrive in former valley forests. As the archipelago is one of the world’s most remote inhabited island chains, many creatures have evolved to thrive here and nowhere else, but those specific adaptations often prove detrimental when change comes fast.

Because of this, Hawaii has earned the unfortunate duelling nicknames of the “extinction capital of the world” and “endangered species capital of the world,’ with good reason: some 75% of all US plant and animal extinctions to date have taken place in the state and virtually all of its endemic wildlife is threatened in some way.

The waterfall-climbing gobies are particularly at risk because their unique stream habitat

The waterfall-climbing gobies are particularly at risk because their unique stream habitat – also home to two native shrimp and two native mussel species – runs through all of the islands’ ecosystems from the shoreline to the high mountains. Any radical change along its length could alter the stream enough to impact the fishes’ delicate ecosystem. For centuries, Native Hawaiians understood the fragility of the island environment and protected land in pie-shaped wedge communities called ahupua’a that extended from the mountain to the sea. Modern community development has not been as kind.

Though change and habitat loss continue to be a real and growing threat, that wasn’t the problem with the stream in the gully behind the general store, but I didn’t realise it at the time when I couldn’t spot any ‘o’opu.

The problem was the stream’s terminus – instead of mingling gently with the sea, it rained down a cliff face in yet another waterfall. The fishes’ unique life cycle begins as an egg flushed downstream into the sea by seasonal floods. Because juvenile ‘o’opu aren’t born climbers – they swim against the current into streams and spend several days growing, and in some instances, morphing physically before manifesting the ability to climb – this stream was incompatible with the fishes’ unique life cycle.

Further south down the coast, the paved ginger- and fern-lined paths of Akaka Falls State Park lead to its impressive 135m namesake cataract and known challenge to waterfall-climbing gobies. Its torrent is audible well before the waterfall creeps into view across a valley, but it’d take a telescope to spot a 13cm fish from here.

At the viewing platform, signage says “‘o’opu ‘alamo’o” – the regional name for the most skilful species of waterfall-climbing goby – and contains a nod to legendary Polynesian shapeshifting water dragons, or mo’o (hence the “mo’o” in “alamo’o”). It’s a sort of regional wish-granting Loch Ness monster: feared and admired, though rarely spotted. The two creatures share a head shape, the sign says, but I see another parallel: though the tiny ‘o’opu can’t bring floods or change the weather like the mo’o is said to do, it isan indicator of native stream health. So, like the mo’o, the climbing fish are denizens and rightful guardians of the islands’ precious freshwater resources.

Perhaps it’s best that these unique fish are left to climb away from human eyes – their mythical feats matching their mythical name. 

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Nature’s Curiosities is a BBC Travel series that offers a close-up look at the natural world, taking adventurous travellers on an unexpected journey of exploration.

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