Recirculating Aquaculture System Fundamentals (Part 4 of 6)

Recirculating Aquaculture System Fundamentals (Part 4 of 6)

It is commonly advertised by system designers to prospective farmers that 20kg/kl is the norm.

Stocking density

It is commonly advertised by system designers to prospective farmers that 20kg/kl is the norm. The reality is that this is rarely achieved. I believe the reason 20kg/kl is advertised as the norm is that it makes a small amount of water a commercially viable option and we would all like to make money.

These financially impressive systems are not always able to deliver the stocking densities promised. Designing a RAS system should always start with the end goal in mind, tons to be produced per year. The design is either high tech with smaller bodies of water by adding oxygen and increasing filtration achieving high stocking densities up to 90kg/kl, or, low tech systems with lower stocking densities between 5kg and 20kg/kl with any volume of water. Both can achieve the same outcome of kg per year with an arguable/debatable cost for each.

It is important to remember stocking density is a relative term used for different applications.

A commercial farmer needs a crop to grow within a set period to be financially viable. If the system is designed to hold 10kg/kl with a water volume of 10 000l as well as fish that will triple in size in a month (from 10g to 30g), the farmer will stock the tank with 35kg of fry and then harvest the tank in a month with a yield of 100kg after a small mortality rate. When the farmer stocks this tank the filters have been seeded from the previous batch of fish and water conditions should be perfect to grow the batch.

Temperature, PH, NH₃, NO₂, NO₃, GH, KH and O2 are optimal for growth. During the month measurements are taken daily and as the fish grow the quality of water will deteriorate but still within the minimum parameters. The final test before harvesting will confirm the system capacity to hold 100kg of fish comfortably at optimal conditions. The farmer is confident in the system and the fish are healthy and have grown to the size required in the set constraints. They are now ready to grow and move to their next station 30g to 100g.

If the tests show that any one of the water quality parameters to be less than optimal the farmer can then reduce the stocking density to accommodate the next batch of fish or upgrade filtration or aeration accordingly. The farmer likewise can increase the stocking density if there is still room for more fish.

Unfortunately most farms are sold a system that is supposed to hold 20kg/kl and then stock 70kg of 10g fish into a 10 000l tank. After a month the harvest reveals only 1.5 times the growth. Potential reasons for the slow growth rate is low O2, high NO₂, NO₃ and a low PH. With resulting stressed fish that don’t eat, high parasite burdens, secondary disease and poor growth.

Stocking density will either make or break a farm. Know how much fish you are adding in kilogram not numbers alone and what you are harvesting in kilogram. Daily testing will show the regression of water quality and appropriate steps can be taken to address this regression. When a maximum stocking density is reached the farmer must be content with the system, or upgrade or expand.

Overstocking a system will yield less returns with bigger health issues. Remember the healthiest fry introduced into a suboptimal system will never reach full potential and your pocket will always be bare.


A fundamental milestone is reached when a body of water is harvested completely and easily. Every tank round or square, above or below water needs to be harvested at some time. Smaller bodies of water can be emptied quite easily and replaced quickly causing little disruption to filter media and fish. Anything larger than 5 000l becomes a laborious task.

Some systems allow for partial harvesting using screens sized for catching only certain sized fish, never needing to empty the system. Others will require the tank to be emptied to remove all fish as they need to grow beyond the constraints of the stocking density of the tank.

The system designer needs to have a system in place where the bio filters can stay active and well aerated while harvesting. Nothing could be more harmful to a commercial farm than the filter bacteria dying off every time the tank is harvested. Moving bed filters allow aeration to move the media while not having water flow through the filter, keeping it alive for a few hours while harvesting.

Pipes, drains, heating and filters must not be inside a tank when harvesting, so the design should cater for this. Flat even surfaces allow for easier harvesting in square and round tanks.

The harvest can be huge, up to 2 tons of fish per tank, per harvest. The labour needs to be able to remove this fish without causing damage to the tank, pipe, drains and filters.

In the next part of this series, I will address some challenges related to heating and fish stress.

Written by:  - 21 Oct, 2020  
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