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Fish consumption is rising. With the increase of the world population and the need for nutritious food, health-conscious consumers are looking for alternatives to “a nice slice of meat”. And they end up eating more fish or vegetarian food.

Specific species of wild fish are getting more and more scarce in open water due to the huge impact of industrialised fishing fleets and overfishing. The sea can not provide the increasing demand. Fortunately, in a trend towards sustainable food production, fish farming is gaining increasingly interest.

Fish farming

Fish farming is the aquatic peer of farming cows, sheep or chicken. For many, many years, we as humans have been farming our main food - have it grown in greenhouses, in stables, or in the fields. Whatever we need, we try to fulfil our demand - more and more in a sustainable way, with respect for natural resources. Fish farming is heading in the same trend.

When people hear about fish farms, they might think of an aquarium, a little pond or a floating net. But in Norway, a major player in fish farming, people think on a larger scale. A typical fish cage near the Norwegian coast has a diameter of tens of meters containing 200,000 to 300,000 salmon. In the near future, these designs will upscale to 1 or 2 million salmon. Only in Norway, at the beginning of 2018 more than 3500 cages for fish farming were floating in the sea. And ‘Norway’ is expanding their knowledge and technology across the world, where people are interested in large scale harvesting of fish in the sea - or maybe also on land.

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Salmon is a typical example of a fish that can be fish farmed. They need cold water - seven to nine degrees Celsius is what they like most - which is why this aquaculture is happening in the northern hemisphere, off-shore in the fjords. Moreover: salmon is a very popular fish, often found on the menu all around the world - so there is a high demand.

Aeration

In fish farming, aeration is literally of vital importance. In addition to food, the fish need oxygen that is supplied in the form of tiny air bubbles (‘aerated’) to the water. But aeration has more advantages.

Also in the early days, lice were a major disease that salmon suffered from. Since salmon lice had an impact on harvest, the fish farmers had to look for solutions. For some reason - maybe it was an experiment or it happened by accident - they started to purge air from the bottom of the cage. And they observed that the movement of the fish started to change. Instead of circling day in and day out - as salmon normally do - they started to move around the cage and became more agile. If the salmon are more agile, the muscles have to work more, and meat from moving animals has a better quality. At the same time, the fish farmers detected that aeration helped them to create a more thermal friendly water environment, with an advantageous temperature, conditions and amount of oxygen. With result that the occurrence of salmon lice reduced. So aeration had - and still has - two advantages: improving the salmon quality, and reducing the unwanted lice. By the way: the words purging and aeration have the same meaning. ‘Aeration’ has the word air inside.

Aeration of fish farms using mass flow controllers

The process of aeration is very simple - like in any aquarium you have at home - and yet can lead to very nice results as we saw above. The air bubbles can be generated by natural water currents (off-shore, down-hill), pumps, impellers, variable area flow meters or - as we do at Bronkhorst - by mass flow controllers and compressors. Here, a compressor generates compressed air from the surrounding atmosphere, and feeds this to the mass flow controller for controlled aeration of the water in the fish cages.

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To run fish farms remotely controlled and without much manpower, as much automation as possible is required. This also involves automated feeding. When the fish are fed, the air purging needs to be interrupted to give the fish the opportunity to hunt for the food before it floats out of the cage. In between the feeding periods, the aeration improves the condition of the water and the salmon. Here, it helps that mass flow controllers are remotely controlled from the control room at land. The aeration is stopped when the feeding starts, and when the feeding is over, the previous set point will automatically return and the water condition is as stable as it was before.

But there is more: mass flow controllers provide a potential for saving energy due to better conditions in the cage. The accuracy of the devices is important here. Every cubic meter of air you save by being more accurate - faster control or opening of valves - is of direct influence to your costs for running the compressor. Moreover, in stormy weather you can reduce the aeration, but during a long dry period without water movement, more air bubbles are needed. So essentially, this accuracy and flexibility leads to a better controlled environment.

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With MASS-STREAM mass flow controllers we have a robust instrument, which is performing well in the harsh northern surroundings. By Bronkhorst standards, this kind of aeration is a ‘high flow’. Typical air flows for a fish cage are in the range between 600 and 1400 liters per minute.

Mass flow controllers for other types of aeration

Mass flow controllers are suitable for other types of aeration - also again in aquaculture and agriculture. If you grow salmon, you need to breed the fish, which normally occurs on land. Fish eggs and young fish are even more vulnerable to changes, so the environment has to be more stable than for grown fish. Depending on the type of fish, the balance of oxygen in the water is delicate and has to be controlled accurately.

In algae farming, carbon dioxide gas is one of the food components for these species to grow, which needs to be supplied under defined conditions.

A very well-known application of aeration is in food & beverage industry. As you might know, every soda or carbonised drink is a liquid purged with carbon dioxide gas. Related to that: when packaging food, the packaging is purged with nitrogen to remove the oxygen before the food enters the packaging, as one of the steps to prolong the shelf life of the food.

“Fish farming with controlled aeration by mass flow controllers will support the focus on good fish quality, control of diseases and increase of the yield” according to Nicolaus Dirscherl, Managing Director of M+W Instruments GmbH.

For more information about the usage of mass flown controller in a fish farming application, please check our application story Aeration in Fish Farming.

Check out the products used in this application.