Cold water fish species such as trout can be reared in aquaponics systems, but at much lower densities than tilapia in warm water aquaponics.
This is partly because these cold water fish species are less tolerant of the presence of ammonia and nitrates in the water than tilapia, but also because at lower temperatures nitrifying bacteria are less efficient at disposing of ammonia and nitrite.
Like all bacteria, they get their energy from their environment and with less heat in the water, the nitrifying bacteria can only dispose of the ammonia that is being produced by the fish in the aquaponics system at a slower rate than in a warm water aquaponics system.
You still need to protect the cold water aquaponics system from extreme temperatures, so it will still have to be in a greenhouse, since below 10 degrees centigrade the action of nitrifying bacteria is very much suppressed. Fish such as trout and perch also do not grow well below this temperature. So you are looking at a temperature range between 10 and twenty degrees centigrade.
This is usually easy to maintain in a normal greenhouse in temperate climates, but in Arctic conditions you will still need some way to heat the greenhouse. Your cold water fish species will not be comfortable if their water freezes solid!
This is where your electricity bill comes in, since you will need to heat the greenhouse so it stays constantly above 10 degrees centigrade, AND ALSO pay what it costs to run the pumps and regenerating air blowers 24/7 to keep your aquaponic fish and vegetables alive and growing at optimum rates.
In Milwaukee, both tilapia and coldwater great lakes yellow perch are raised at Growing Power and Sweetwater Organics aquaponic farms, by the thousands.
The market for sustainably farmed yellow perch, a favourite frying fish for the local population, has increased since pollution in the great lakes caused a massive die-off of the wild version of this species.
Which brings me to the problem of water filtration.
If you have cold water fish species such as trout and perch in your aquaponics system, you MUST make sure you have adequate filtration and solids removal, to keep dissolved ammonia levels to a minimum.
At the Herbs From Wales aquaponic and hydroponic farm in Anglesey, Wales, they have both aquaponic media beds filtering the water, and mechanical sponge filters that remove additional solids. These are regularly cleaned out to make sure that the water for the cold water fish species (in this case, brown trout) is not stacked full of ammonia and nitrite.
Stocking density (carrying capacity) there for their cold water fish species is much lower than for tilapia, at around 6 kilogrammes of fish to 100 litres of water to 200 litres of grow bed. This is basically the same as that advocated by Backyard Aquaponics in Australia. In Australia the ubiquitous tilapia fish is banned for fish farming, since it can be invasive if it escapes. So more nitrate sensitive kinds of fish including Australian native varieties of river perch are used, which are much more susceptible to disease if they are kept in dirty water, with medium to high ammonia levels. Using the above ratios and either growning media beds or deep water culture (DWC) raceways topped with floating raft growing systems, you can keep your ammonia levels down to the nearly nil levels required to raise these kinds of ammonia levels sensitive cold water fish species.
Make sure you have enough plants growing in the hydroponic part of the aquaponics system to soak up the ammonium nitrate that the nitrifying bacteria are digesting out of the fish waste water! Ammonia levels will soon rise dangerously if you don’t keep the fish and plant stocking levels of your aquaponics system in balance. This applies to cold and warm water aquaponics systems equally.
Making sure that the water is thoroughly aerated at all times both in the fish rearing tanks and the grow beds or DWC raceways is essential as well to provide a buffer against any nitrite or ammonia spikes that may accidentally occur. Excessive ammonia levels will certainly kill your fish. Having high levels of dissolved oxygen in the water at all times helps to prevent this from happening, since nitrifying bacteria need dissolved oxygen in plentiful amounts to digest ammonia efficiently. Also your fish and plants need to breathe oxygen too!
DON’T OVERFEED YOUR FISH. Uneaten rotting food in the system will cause major water quality problems and mean you may have to suddenly change out more than half the water in your aquaponics system to save your fish from dying rapidly of brown blood disease, or worse. This holds for any aquaponics system, cold or warm water. It’s best to feed your fish little and often, say, three times a day, and only as much as they will eat in half an hour, in a large tankful of fish, that is. For small hobby tabletop systems, make that as much as they will eat in two minutes.
Finally, remember fish have ears. They can hear. They do not like to be kept in noisy environments any more that you would!
If you like this article, I have edited the first six months of articles on this site as the ‘Aquaponics Global Anthology’ – available here: [paiddownloads id="1"]