Image caption Lloyd Moskalik sells two tonnes of fish to wholesalers each week

Under eerie blue lights designed to simulate the ocean depths, hundreds of fish swim serenely through the bubbling waters of their circular tanks, 15 floors up in the sky.

There are 11 plastic tanks in total, holding a combined 80,000 litres of salt water.

They are full of grouper, a white-fleshed fish, which are all destined to end up on the plates of restaurant-goers across Hong Kong.

This is the scene at Oceanethix, one of the numerous so-called "vertical fish farms" in the special administrative region, which have become a key fixture of its supply chain.

If you like, this is rooftop farming on steroids Lloyd Moskalik, Oceanethix

For while most fish farms around the world are at sea, or at least, land level, in Hong Kong it is more often a necessity to put them many floors up in tall buildings.

This is because as one of the most densely populated places in the world, there is simply very little spare space. So fish farms have to fit in where they can.

For the small firms that dominate the industry, it is worth the effort, as Hong Kong has an insatiable appetite for fish and seafood. It consumes more than 70kg (11 stone) per capita every year, 10 times more than in the US.

"We're way above the hustle and bustle," jokes Lloyd Moskalik, managing director of Oceanethix, which is based in Hong Kong's New Territories. "If you like, this is rooftop farming on steroids."

Image copyright Thinkstock Image caption People in Hong Kong are big fish eaters

His business, which employs six people in Hong Kong, buys in the groupers as baby fish, or fingerlings. They then take between 10 and 13 months to get up to market weight.

If you come here on a Saturday it's an absolute theme park - there are people running everywhere Osbert Lam, Owner, City Farm

Oceanethix sells about two tonnes of groupers to fish wholesalers each week, and Mr Moskalik says he can get as much as 776 Hong Kong dollars ($100; £60) per kilogram.

As demand for farmed fish has soared in the region, wholesale prices have risen at a rate of between 10% and 15% per annum for the past five years.

Oceanethix also sells its water-recycling systems to other companies across Asia setting up similar fish farms in the sky.

"We've been selected by the Korean government as part of an ambitious plan to establish vertical farms in multi-story buildings... in Seoul," says Mr Moskalik.

The Singapore government has also bought a country licence for Oceanethix's water-recycling systems, and the company has its own sister facility in Shanghai.

Farm waiting lists

But it is not just fish farms that have been taking to the skies in Hong Kong, as a growing number of organic fruit and vegetable plots are being created on top of skyscrapers and other spare rooftop spaces.

Image caption There is a waiting list for space at Osbert Lam's rooftop farms

No doubt in part caused by a string of recent food safety scandals in mainland China, from where Hong Kong sources most of its food, a growing number of Hongkongers are wishing to grow their own produce as naturally as possible.

Helping to meet this demand is Osbert Lam, the owner of Hong Kong City Farms.

From just a hobby 10 years ago, he now runs three farms that convert thousands of square feet of rooftop space into organic plots he rents at about 190 Hong Kong dollars per month.

"We've got a list of about 30 people all waiting to get boxes," he says, from the top of a 14-storey industrial estate building in Quarry Bay, in the heart of one of Hong Kong's business districts.

Image copyright HK Honey Image caption Michael Leung's honey commands a premium price

"If you come here on a Saturday it's an absolute theme park - there are people running everywhere."

He says the urban farms reveal just how shallow Hong Kong's urban roots are.

"Many of the people that come here are not even two or three generations away from the land," says Mr Lam.

"In many cases, it's just one generation before they were from farming families. A lot of people come here with a lot of knowledge."

Expensive honey

The growth of rooftop gardens has also meant more business for Hong Kong's urban beekeepers.

Michael Leung, founder of HK Honey, is always on the look out for new places to put his hives, and to help him locate them, he looks up for papaya trees.

Image copyright AFP Image caption Green spaces are limited in central parts of Hong Kong

"The papaya tree grows very well in Hong Kong - most people, if they grow anything on the roof, it's a papaya," he says. "The height of the tree allows you from ground level to see that someone is using the rooftop.

"We're always looking for little trees that stick out. They're like a flag, a modern agricultural flag," he says. "Through that, we then try to approach the people growing on the roof."

Mr Leung then arranges to rent space for his hives.

He says that the honey his bees produce has a spicy tang, which reflects the biodiversity of Hong Kong's urban flora, and particularly the Chinese basil many people like to grow.

Such is the quality of Mr Leung's honey that he is able to sell it for a whopping 240 Hong Kong dollars per jar.

Pressure on land

For Hong Kong's larger commercial organic farming operations, which buy produce from Hong Kong's dwindling slivers of agricultural land near the border with China, the continuous pressure on agricultural land from developers could mean that rooftop farms will one day be all that's left to the special administration region, which even now produces 2% of the food it consumes.

Image copyright Thinkstock Image caption Hong Kong has a proud restaurant scene

Todd Darling, of Homegrown Foods, an organic grocery delivery business in Hong Kong, said permissive zoning regulations make it more cost effective for owners of agricultural land to store shipping containers on the space than to farm it.

In the meantime, however, the food scandals that creep across the border from mainland China have, perversely, been good for his organic business.

"I would never like to say that, but it does tend to encourage people to consider alternatives," he says.