Mitigating the impact of HABs on aquaculture - Part 2
March 2022
In our last blog post we discussed the impact that Harmful Algae Blooms (HABs) have on aquaculture. In this blog post we conclude this topic by discussing how compressed air is playing a pivotal role in the mitigation systems being used to prevent losses in fish due to HABs.
Mitigation Systems - using aeration and oxygenation
As we have seen then, HABs thrive in warm and nutrient rich water. Aeration and oxygenation therefore become part of the solution in mitigating the impact of the HAB on the fish. Here compressed air plays a pivotal role.
The Upwelling system
An upwelling system works on the principle that microalgae are buoyant and therefore only present in the top layer of the ocean. Therefore this system creates an artificial upwelling of cold, oxygen rich water that doesn't contain phytoplankton.
This is created by an air compressor that pumps air through either a single - or multiple - diffuser(s) into the sea pens. The air is often injected at depths of 15 to 30 metres. Through the injection of small air bubbles, the density of the water in the depths is lowered, which makes the water rise. When the water with the lower density hits the surface, it spreads out to the sides. By spreading out to the sides, it pushes the surface water outwards and with it the harmful algae. If controlled well, the fish will never be in contact with the algae bloom, as the air is constantly pumped into the sea pens.
Aside from mitigating stock losses due to HABs, aeration has a number of further beneficial effects for the aquaculturist:
- By bringing up water from the depths, warm water temperatures (which are more and more becoming a problem for aquaculturists - especially Salmon farmers) are cooled down. And generally speaking, cold water contains more oxygen than warm water. By increasing the oxygen content of the water, the Feed Conversion Ratio can be lowered which means less feed for 1 kg of growth therefore generating cost savings. And, as feed is the largest cost factor for an aquaculturist, that is a big win!
- Sea lice are a big problem for example in Salmon farming. Here, many farmers are using ‘lice skirts’ to prevent the lice from entering the sea pens. As these skirts block the waterflow, they can lead to low oxygen conditions in an area of the sea pen that needs to be frequented by the salmon in order to feed. Aeration can also solve the issue of having low oxygen conditions if lice skirts are used, as the upwelling system brings oxygen rich water from the depth inside the skirted zone.
- In addition, an upwelling system creates a water current which makes the fish more active (e.g. they swim more). This exercise makes the fish leaner (e.g. they carry less fat) and therefore more favorable for sale.
- Finally, the upwelling system pushes out other marine animals that are planktonically floating in the top layer of the water. This includes jellyfish, which are known to cause mass mortalities as well as the copepodite stages of sea lice (lepeophtheirus salmonis). The larval stage that would infest salmon for example, tends to float in the top layer of the water.
Bubble curtain
Combining an upwelling system with a bubble curtain presents another mitigation system option
Combining an upwelling system with a bubble curtain presents another mitigation system option. An upwelling system can only stop algae on the top layer of the pens due to the water movement. This means that only algae in the photic zone will be pushed out. To protect the entire water column a bubble curtain would be added. Here, a compressor pumps air into a diffuser hose which surrounds the entire farm at its lowest point. The bubble curtain this creates all around the farm acts as an underwater wall and stops particles drifting along with the current from entering the farm.
The suitability of these mitigation system solutions will depend on a farm's specific conditions and should be assessed case by case. For example, in some areas deep water can potentially have lower oxygen levels in which case an air driven upwelling system would not be the right choice. Here, increasing the oxygen content would be done by injecting pure oxygen. Another example is sites with strong currents which have the potential to eradicate the effectiveness of these mitigation systems.
Conclusion
AI - the holy grail of mitigation systems
In order to keep fish stock as healthy as possible and for these mitigation systems to be effective, measures need to be taken before a HAB hits the farm. Thanks to AI (artificial intelligence), it is now possible to predict algae blooms. By implementing AI based monitoring systems that measure water temperature at different levels, that monitor water quality - and by regularly analysing water samples under the microscope - aquaculturists are able to determine types and density of algae present. And, such a large amount of data makes it possible to then learn and predict blooms in the future.
When these systems are set up to connect with the control systems on the compressors, it’s also possible to start-up the mitigation systems early enough to prevent stress and losses in the stock. Such a combination makes it possible to save large amounts of money thanks to the accurate use of the system, controlled by sensors and a multitude of water parameters.
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Specialist aquaculture solutions
Upwelling systems and bubble curtains demand a reliable supply of compressed air. All Kaeser blowers and compressors are designed and built to provide maximum durability and reliability. Moreover, their impressive energy efficiency helps to keep the largest part of total system costs to a minimum, whilst the low maintenance requirement reduces costs even further. Kaeser provides special products for the aquaculture industry that are tailored for the rough conditions on fish farms.
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