Over 100 nautical miles off the coast of China in the Yellow Sea, a huge octagonal structure rests in the water. This is no new-look oil rig. It’s Deep Blue 1, China’s first offshore aquaculture base for farming salmon.
At its corners, the yellow octagon has steel columns extending 30 metres into the water. Enclosed by black mesh, they form a cage with a volume of 50,000 cubic metres, and room for 300,000 salmon. The yield is nearly 1,500 tonnes of fish per year.
Deep Blue 1 is merely a taster for future developments in Chinese offshore aquaculture. The Guoxin 1 aquaculture vessel is designed to produce 3,700 tonnes of fish annually. The ship cruises between the Yellow and South China seas, avoiding typhoons and “red tide” algal blooms, and keeping to waters in the 22C to 26C range suitable for the fish. This factory-type aquaculture vessel contains 15 tanks with a total volume of nearly 90,000 cubic metres. Stock density is four to six times that of traditional net pens.
Following successful trials of the initial project, upgraded versions of the vessel, in the form of Guoxin 2 and Guoxin 3 are due for delivery in March 2024.
China is developing various forms of offshore aquaculture. According to data released in June by the Ministry of Agriculture and Rural Affairs, coastal provinces have already brought into use more than 20,000 “gravity cages” – each formed of a net and a floating collar – 40 steel-framed sea cages, like Deep Blue 1, and four aquaculture vessels. China’s offshore aquaculture currently spans nearly 44 million cubic metres of water, yielding almost 400,000 tonnes of seafood – more than 20% of national mariculture, or marine farming, output.
The ministry says: “With another five years, we aim to increase the scale of offshore aquaculture nationally by 16 million cubic metres, to more than 60 million cubic metres, and achieve annual offshore aquaculture output of more than 600,000 tonnes – more than 25% of sea-farmed fish products.”
In the policy paper “The opinions on accelerating the development of offshore aquaculture”, jointly released in June by the Ministry of Agriculture and Rural Affairs and eight other Chinese government departments, offshore aquaculture is defined in a way that stresses the use of large-scale installations such as gravity cages, steel-framed cages, aquaculture platforms and aquaculture vessels, supported with mechanisation, automation and smart technology. They should all enable aquaculture to be undertaken at scale and efficiently in deep offshore waters, the paper states.
Records show that extreme waves along the coast of the South China Sea can be as high as 16 metres and exert 50,000 tonnes of force per 100 metres of wave front. The destructive power of the sea under such conditions is far worse than can be imagined. Take the example of HDPE (high-density polyethylene) deep-water sea cages, which are common in China’s offshore aquaculture industry. Analysis by Lin Ming of the Chinese Academy of Engineering indicates that cages of this kind would deform under catastrophic sea conditions, rendering their structure and anchoring system unsafe, and very likely resulting in the death or escape of farmed fish.
As a solution, Lin Ming’s team has proposed the construction of wave-dissipation installations close to aquaculture areas, cutting them off from the roughest of sea conditions behind gigantic “walls”. Earlier studies indicate that such installations can reduce a 16-metre wave to about 2 metres.
Constructing offshore defences of this kind may in future mitigate the risks to offshore aquaculture posed by climate change and its impacts.