When size really does matter: Battle of the Japanese eelpouts (Ookazunagi)

Ten centimetres. That was the number on which we were hanging our hopes and dreams, as we prepared to film a sequence on Japanese eelpouts.

The eelpouts were small - really small. But at ten centimetres long and about one centimetre in width, they had been deemed (from the relative safety of the office in Bristol) - doable.

After all, we had a secret weapon in cinematographer Hugh Miller. A veteran of filming creatures that most people would struggle to see, let alone film.

So, ten centimetres? No problem. That was the theory anyway.

Shots of eelpouts had been taken by only a handful of photographers over the years, all at the same location, but the underwater habitat they depend on had changed and was now totally devoid of eelpouts.

These fish rely on wakame, a species of Japanese seaweed for cover during the breeding season. A combination of intensive harvesting at the site and warming seas had caused the wakame to completely disappear – and with it, the eelpouts.

We needed to find out whether there was anywhere left for them to live.

Eventually, a Japanese diver and photographer told us about a tiny patch of seabed, no more than five metres by five, scattered with the porous boulders the eelpouts need to lay their eggs.

Crucially, it was covered in wakame.

He had been diving here for the past ten years, watching the eelpouts return to the same small patch of habitat in breeding season. As far as we could tell, this unassuming corner of the seabed in a tiny Japanese village appeared to be the last stronghold of eelpouts anywhere in the world.

After many months of slow, translated conversations with our Japanese guide, a few flights and one long drive later, we were finally on the water.

We pulled on thick thermal undergarments and drysuits, before checking our underwater grip, comms equipment, and lights. Last came the rebreathers which would recycle the gas we breathed, adding oxygen as it was metabolised by the body, and scrubbing out the carbon dioxide we produced.

Unlike conventional SCUBA, they would allow us to remain underwater for many more hours. And with tiny subjects and low visibility, we were going to need all the time underwater we could get.

Cinematographer Hugh Miller, camera assistant Natalie Turner-Blackman and I descended onto the site. We could see only a few meters in front of our faces and as we settled onto the stony sea floor and instantly began to feel the all too familiar feeling of being pulled back and forth by strong swell. Four, then five hours passed. Not a single eelpout in sight.

Eventually, Hugh spotted what we had travelled so far to film. Two male eelpouts, their mouths wide open, but our excitement was soon replaced with more than just a little bit of dread.

They were even smaller than we had imagined they would be.

After the dive, although he didn't say it, I could see the concern on Hugh's face. This was going to be much harder than we had anticipated.

For the next few dives, we sat almost completely still, scanning the narrow pathways between the thick kelp, squinting, searching for the slightest movement, a wriggle of a tail, or the telltale flash of orange that might give away the location of an eelpout moving between the rocks.

After a few weeks, we began to get our eelpout eye in and became surprisingly good at spotting them. Hugh, meanwhile, had deployed a one-of-a-kind prism camera system that was starting to yield results.

It was hard not to feel for the eelpouts as we watched them day after day make their way to the nesting rocks. The journey was perilous and often left them exposed and more than once, we watched as an eelpout emerged from the shelter of the kelp, only to be intercepted by a passing rockfish that had been watching intently from a distance.

After hours underwater spent searching, a pot of steaming cheesy curry noodles once back on board was one of the few things that succeeded in bringing us back to life.

As the breeding season continued, the females finally began to battle it out, sizing each other up by comparing the size of their bright, oversized mouths. But just as the behaviour was becoming more dramatic, so too did the weather.

The visibility dropped to almost nothing. The mating rock became shrouded in a fine haze, appearing almost as though it had been dusted with snow. With a cyclone looming, we had no choice but to pack up our kit with the promise of returning the following year, determined to complete the story.

The following year, we arrived with some new tools, including a new probe lens and a series of long-deploy remote cameras. Returning to what had affectionately become known as 'Eelpout City' truly felt like returning to see some old friends, and every time I descended onto the site, it was like entering an extraordinary miniature forest, alive with a tiny cast of characters, each playing their part in a delicate underwater ecosystem.

I hope that by bringing the eelpouts' hidden world to the surface, we can give these charismatic little fish the attention they deserve and, most importantly, help to protect their fragile habitat and ensure that their pouty little faces remain here for years to come.