Why industrial gases are important in fish farming

Fish is a nourishing, healthy food that is popular throughout the world. However, as the planet’s population grows, fish stocks in some oceans are dwindling. One way to address this shortage is fish farms. Popular in Europe, especially Nordic countries, aquaculture also occurs throughout Australia – from the tropical north to the more temperate climes of Tasmania.

Like any commercial venture, there are many facets to make it a successful enterprise. When it comes to fish farming, an essential ingredient are various industrial gases, which have many applications in aquaculture – from hatching the eggs through to when the final product is shipped for sale.

Air Liquide is a gas specialist that has a lot of information and experience when it comes to fish farming. Its Tasmanian sales representative, Grant Stingel, works closely with the industry, not only as a supplier of gases, but also giving advice on how much, what type and how often a certain gas needs to be applied to the various production processes.

The most prolific gas used in fish farming is oxygen. There are two main reasons it’s needed. The most obvious is to sustain the life of the fish as they hatch and are grown. The other is a little more interesting.

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“During the production of farmed fish, one of the high cost inputs is the food,” said Stingel. “It can cost up to $2,000 a tonne or more depending on the species and feed type.

Maintaining a stable level of oxygen in the tank increases the fishes’ metabolism, which in turn increases the conversion of food into fish mass. So the Feed Conversion Rate (FCR) reduces, meaning lower feed costs per kilogram of fish.

“And if you’re talking tonnes of fish, you’re talking tonnes of food per day. In the larger aquaculture systems, maintaining stable oxygen levels in the tanks will increase production. If you can increase the growth of the fish each day by adding oxygen, this reduces the time the fish are in the water, which in turn increases efficiencies within the whole production cycle.

“Typically, modern land-based aquaculture farms use what is called a Recirculating Aquaculture System (RAS). This is essentially a water treatment plant to circulate and reuse the water. This plant uses pumps to push water through a series of filters to help purify the water before going back into the fish tanks,” said Stingel. “Oxygen is also used in this process to produce ozone to sterilise the water.

“In the inlet water to each of the ponds or tanks, the oxygen level is elevated by injecting oxygen, typically using a pressurised oxygen dissolver, to 120 to 140 per cent of normal saturation, depending on the biomass. This ensures that the respiration demands from the fish are taken care of and a stable growing environment is achieved.”

There are other applications where oxygen is necessary. Just before the fish are harvested, whether in ponds or sea cages, higher doses of oxygen are needed due to the fish being crowded into a small amount of water within the harvest area. This ensures that the fish are not as stressed before processing, giving a better end product.

Also, in some farms, oxygen is used to supersaturate baths of water to treat the fish for pest and disease, such as sea lice.

With all the oxygen being used, what are the costs involved? Not as much as you would think, said Stingel.

“Oxygen is typically only about one or two per cent of the cost of your production but it’s very important,” he said. “It is an essential element to the fish farming process. In some cases, oxygen can be seen as just a commodity, but oxygen used efficiently can also add benefits to your production.

“Oxygen supply to fish farms is essential so we have engineering support available,” he said. “As far as technical support, we can calculate how much oxygen you will need for the quantity of fish in each system. Based on the calculated oxygen required, we also offer advice on the oxygen dissolving system best suited for the application. Measuring the efficiency of your existing oxygenation system is also something Air Liquide can offer.”

Other gases are also used once the fish have been processed. Oxygen goes from being the hero to the enemy once the fish are ready to be sent to Australian supermarkets or exported.

“After harvest, we use other industrial gases for packaging fish products,” said Stingel. “Some aquaculture companies use Modified Atmosphere Packaging (MAP). This is a mixture of nitrogen and carbon dioxide injected and sealed inside the trays often seen on the shelf at your local supermarket.”

The carbon dioxide inhibits bacterial growth, which will increase shelf life for the end product.

The nitrogen is to displace the oxygen and also maintain the package integrity so that it looks good on the supermarket shelf.

Another industrial gas used in the processing phase is liquid nitrogen, which is used to snap freeze the fish products by sending it through a freezing tunnel, which sprays the gas onto the product. This achieves a better quality product when thawed. This is because when a product is snap frozen, the cell structure of the food is maintained, meaning when thawed, the fish not only looks good, but tastes fresh.

“Even when it comes to the presentation of the food we can help. For example, dry ice produced from liquid carbon dioxide is used to add a bit of theatre at serving counters in restaurants or markets,” said Stingel. “As the dry ice thaws, vapour is formed, giving off a nice smoke effect. Dry ice is also good for keeping the product cold and fresh.”

In almost every stage in the production of fish in aquaculture systems there is potential to use an industrial gas of some type whether it is oxygen, nitrogen, argon or CO2. But the use of the various gases doesn’t stop there. Air Liquide can also provide gases for other, practical uses.

“The other application for industrial gases is for maintaining plant and equipment,” said Stingel. “With quite a lot of machinery involved in the process, you will also need oxygen and acetylene for heating and cutting, argon gas mixtures for welding, and LPG for heating and maybe also powering forklifts.”

Why mass flow meters are important in fish farming

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 scarce in open water due to the impact of industrialised fishing fleets and overfishing. In a trend towards sustainable food production, fish farming is gaining increasingly interest.

Fish farming is the aquatic version of farming cows, sheep or chicken. For many years, humans have been farming food by having it grown in greenhouses, stables, or fields. Fish farming is heading in the same direction.

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 metres containing 200,000 to 300,000 salmon. In the near future, these designs will upscale to one or two million salmon. In Norway, at the beginning of 2018, more than 3,500 cages for fish farming were floating in the sea.

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The country is expanding its knowledge and technology across the world, where people are interested in large-scale harvesting of fish in the sea – and maybe on land.

Salmon is a typical example of a fish that can be fish farmed. They need cold water – 7˚C-9˚C is what they like most, which is why this aquaculture is happening in the Northern Hemisphere, off-shore in the fjords. Salmon is a popular fish so there is a high demand.

Aeration
In fish farming, aeration is 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 other advantages, too.

In the early days, the salmon suffered from infestations of lice. 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 – the farmers 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, their muscles have to work more. This results in their meat being of 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, this resulted in a decrease in lice numbers. So aeration had two advantages: improving the salmon quality, and reducing the unwanted lice.

Aeration of fish farms using mass flow controllers
The process of aeration is simple. The air bubbles can be generated by natural water currents (off-shore, down-hill), pumps, impellers, variable area flow meters or by mass flow controllers and compressors.

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.

To run fish farms that are remotely controlled and without much manpower, automation is needed. This includes 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.

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.
Mass flow controllers provide a potential for saving energy due to better conditions in the cage. The accuracy of the devices is important. Every cubic metre of air saved by the device being accurate – faster control or opening of valves – is of direct influence to the costs for running a compressor. In stormy weather, fish farmers 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.

With Mass-Stream mass flow controllers, farmers have a robust instrument, which is performing well in the harsh surroundings. By the manufacturer’s Bronkhorst’s standards, this kind of aeration is high flow. Typical air flows for a fish cage are in the range between 600 and 1,400 litres per minute.

Mass flow controllers for other types of aeration
Mass flow controllers are suitable for other types of aeration in aquaculture and agriculture. If users farm salmon, they 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, CO2 gas is one of the food components for these species to grow, which needs to be supplied under defined conditions.

A well-known application of aeration is in food and beverage industry. 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.

Bronkhorst is represented in Australia by instrumentation specialist AMS.

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