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FISH FARMING TECHNOLOGY #2 Operating freshwater salmon RAS by Ben Green, MA (Cantab), CEO, The Supreme Salmon Co., UK

Operating a RAS is quite a daunting task; the salmon are like astronauts of life support. I haven’t had a disaster for over a decade thanks to robust design, back up systems and a good alarm, but I still can never be quite sure what I will find in the morning.

In this, the last of my three articles, I’ll describe how to run a RAS in the best way possible. A lot of effort goes into the design and building of the unit but that’s just the start, there’s the smooth running of the system with a proper maintenance schedule and crucially, the care of the stock. I can’t say I’ve read a huge amount about fish rearing but I can’t ever recall seeing an article devoted to the wellbeing and behaviour of the fish reared and how crucially important it is to for the smooth running of a RAS unit. Carp are very tolerant of bad environments and are easily domesticated but salmon are like racehorses and will take every opportunity to die that is offered.


I would imagine most people know about pH, ammonia, nitrites etc. levels so I won’t dwell on this for very long assuming the system is up to the job. I don’t monitor these anymore unless there might be a problem, my systems have a large cushioning water volume and I keep the bio-filter alive between crops. A bio-filter should be coping with ammonia after a day or two; the nitrites can take up to a month to stabilise and in three months the filter is fully mature. A RAS is home to two animals, the fish and the filter, if one dies then the other dies with it. For this reason it’s very dangerous to have anything around that might accidentally kill either, chemicals like Chloramine T destroy filters instantly and the fish will die soon after if there isn’t enough fresh water to call on. The filter can grow with the fish if a batch policy is operated, a continuously harvested system has a stable biomass but is probably always running near its maximum capacity like fully laden truck going top speed all the time, good for efficiency but more likely to crash. I use a batch process with discreet units and harvest over a period of time; this gives my static filters a rundown phase in which to clean themselves. Once the RAS is running it settles down to a life of its own, the operator will get used to the flow rates of the water and the sounds of the machinery. These things can be noticed from day to day. A single operator will notice any slight changes that are a precursor to an equipment failure such as a noisy bearing on a pump, which can then be replaced in good time. I look after my RAS 365 days a year (which is probably a bit sad!) but I have sole responsibility and no one else to blame if warning signs are not heeded, I can’t see how this can work so well with multiple personnel. A tip here, don’t change anything on the RAS then leave it! It’s after an equipment change that something can fail, for example a pipe might not have been properly secured after a pump change, and will come off a few minutes later. There will still be some natural variation of water parameters over time; temperature varies with the seasons and with it an evolution of the filter flora and fauna causing changes in water clarity or oxygen levels. This doesn’t seem to bother the fish but it can have implications for the smooth running of the system. For example if the water goes cloudy, the mortalities can’t be seen to be removed if they are normally visually speared or scoop netted, a rotting dead salmon in an RAS will cause chronic health problems in the rest of the stock. It’s not always the case that addition of fresh water will help the system, I find I have the urge to add more borehole water ‘because it must do some good’ but this isn’t necessarily the case. If the temperature is right and the fish are happy changing the water conditions will actually disturb them, spring or borehole water might look wonderfully clean to us but it’s often not a very good environment for fish in its raw form.

Husbandry and salmon behaviour

There are a lot of enthusiasts out there who make a very good job of running complex aquarium setups so it would be reasonable to assume that there’s no shortage of staff who could do a reasonable job of operating a commercial salmon RAS. I would say that that’s the easy bit; it can make huge difference to the harvest outcome if the salmon are ‘happy and contented’. This is especially important with RAS mainly because the subtle behaviour of the fish is the best indication of how well the system is running. I don’t think salmonids can be domesticated very easily especially as they aren’t bred for this trait; Atlantic Salmon are only a few generations from the wild anyway. I’ve grown a few commercial Rainbow Trout recently and was surprised that even they weren’t more comfortable with human presence. The more they are disturbed and scared the more nervous they will be. If you go out a night and shine a torch into a tank of salmon, the first time they will just calmly swim about, second or third time they will 42 | November 2017 - International Aquafeed

NOV 2017 - International Aquafeed magazine