That smoked salmon you bought for the New Year's festivities has a story to tell.

The salmon may have been raised in Scotland -- but it probably began life as roe in Norway.

Harvested at a coastal farm, the fish may have been sent to Poland to be smoked.

It may even have travelled halfway around the world to China to be sliced.

It eventually arrived, wrapped in that tempting package, in your supermarket.

Globalisation has changed the world in many ways, but fish farming is one of the starkest examples of its benefits and hidden costs.

The nexus of the world fish-farming trade is China -- the biggest exporter of fish products, the biggest producer of farmed fish and a major importer as well.

With battalions of lost-cost workers, linked to markets by a network of ocean-going refrigerated ships, China is the go-to place for labour-intensive fish processing.

In just a few clicks on Alibaba, the Chinese online trading hub, you can buy three tonnes of Norwegian filleted mackerel shipped from the port city of Qingdao for delivery within 45 days.

"There is a significant amount of bulk frozen fish sent to China just for filleting," said a source from an association of importers in an EU country.

"The temperature of the fish is brought up to enable the filleting but the fish are not completely defrosted."

The practice has helped transform the Chinese coastal provinces of Liaoning and Shandong into global centres for fish processing.

But globalised fish farming leaves a mighty carbon footprint and has other impacts, many of which are unseen for the consumer.

Don Staniford, an activist and director of the Global Alliance Against Industrial Aquaculture, called the fish industry's production and transportation chain "madness".

- Environmental cost -

"The iconic image of Scottish salmon -- a wild salmon leaping out of the river -- has gone. The Scottish salmon farming industry is dominated, 60-70 percent, by Norwegian companies," he said.

The biggest such company, Marine Harvest, is the world's largest producer of Atlantic salmon, some 420,000 tonnes in 2015.

Scottish salmon farms import eggs from Norway, the fish food from Chile and then send the fish to Poland -- "because it's cheaper" -- for smoking, said Staniford.

"Consumers don't realise that cheap supermarket salmon comes with a huge social and environment cost," he added.

One such problem is that integrated markets, with the free flow of fish and fish products across borders, may spread disease and new bugs.

Antibiotic-resistant diseases or parasites such as sea lice require bulk slaughter on giant fish farms, pushing up prices.

Chile, the world's second-largest producer behind Norway, suffered the ravages of an algae bloom in early 2016, resulting in high mortality, reducing its expected production by 30 percent.

Despite such setbacks, the economic potential remains enormous.

According to Allied Market Research, the global aquaculture market will be worth $242-billion (228 billion euros) in 2022, compared to $169-billion in 2015.

The Organisation for Economic Cooperation and Development (OECD) and World Bank go further and say that by 2030 two-thirds of the seafood on people's plates will come from aquaculture farms.

- Employment mirrors changes -

The tipping point may have come in 2014.

Only 81.5 million tonnes of fish were netted at sea -- down from a historic peak of 86.4 million tonnes, according to the UN's Food and Agriculture Organization (FAO). For the first time, more farmed fish were consumed than wild fish.

On a broader employment level, fishing and aquaculture account for an estimated 56.6 million jobs across the globe.

But a breakdown of the numbers again underlines change.

Those involved in catching fish has fallen from 83 percent in 1990 to 67 percent in 2014 -- with the corresponding numbers in aquaculture almost doubling.

The evolution of the industry has also been driven by overfishing, with fewer wild fish now being caught, leading to a concentration of large multinational fisheries.

"We are seeing a new phenomenon of deals that have consolidated the sector in Europe and the world," said Francois Chartier, an economic specialist at Greenpeace.