The fish biomass correspond to the total weight of fish present in a fish tank but why do we calculate the fish biomass?

Knowing your Fish tank total biomass is important as this value will be necessary for the management of your Aquaponics setup. In this article we will see 2 points that require the fish biomass.

  1. Feeding following the feeding tables

Some species of fish raised in aquaculture have been raised for generations and lots of studies have been done on those fish. The fish supplier are able to supply “feeding tables” which are data giving you the quantity of fish food to give to your fish depending on the species, the water temperature and the size of the fish. The data unit give you a quantity of fish food to distribute to your fish and the unit is in “fish biomass percent”. For example a table giving you 1.2% will mean that you need to feed your fish 1.2% of the total fish biomass. It means that if you have a fish biomass of 10kg in your fish tank, the table recommends to feed 120g of fish food pellet to your fish. You see in this example that we needed to know the total fish biomass (here 10 kg) of our fish tank in order to calculate the quantity of food to distribute.

  1. Remaining under the maximum limits of the system

Every Aquaponics system has a maximum capacity to transform the fish poo into plant fertilizer. This maximum limit is determined by the quantity and quality of the bacteria present in the aquaponics setup. In a classic growbed aquaponics system this quantity of bacteria is present on the surface of the media into the growbed. If you build your aquaponics setup following the advices provided into our free 6 STEP Aquaponics guide you will put a flood and drain system into your growbed. This will ensure the environment is 100% aerobic which will allow the probiotic to grow and be efficient into the transformation of ammonia into nitrate. The total quantity of bacteria will therefore be dependant of the growbed volume. The larger the growbed and the more media volume you will have which mean more room for the bacteria to grow. At Melbourne Aquaponics we like to give a maximum biomass limit of 1kg of fish per 50 litres of growbed (this limit is only applicable if your media is highly efficient, see our video about media in Aquaponics for more information). Here we come back to the fish biomass… Let’s take the example of an aquaponics setup which is has a growbed of 250 Litres. If we apply the above rule we understand that the maximum quantity of fish we can stock into our fish tank will be 5kg (250L/50L = 5kg). Now the only way to know we are not too close to the limits of the system is to calculate the fish biomass of the system.

Now that we understand the utility to know the total fish biomass of an aquaponics system let’s see how to calculate it.

Fish Biomass calculation

So here is the simple calculation, the fish biomass is equal to the total weight of fish in the fish tank which is equal to the number of fish multiplied by the average weight of the fish (fish size). The question is “how do you get those 2 numbers which are the number of fish and the average weight…”

Number of fish: In aquaponics like in aquaculture we generally know how many fish we stock into the fish tank from the day we buy them. Indeed we generally buy fish fingerlings by 50 – 100 or much more so we know exactly how many fish we start with. During the raising process you will probably lose a few fish and I recommend to keep track of the number of dead fish in a little Aquaponics notebook. In this case you can withdraw the number of dead fish to the number of fish initially bought and obtain the current number of fish in your fish tank.

Average fish weight: The average fish weight is continually evolving and (generally and hopefully increasing excepted when you go on holiday and in winter if you live in a cold country). You should keep track of this value and especially when the fish get close to the plate size and the biomass increases. In order to do an average weight simply prepare a bucket with water and weight it thanks to a scale. Fish some fish from your fish tank (10 to 30) and put them into the water bucket. Now weight the fish bucket. Once you get this number withdraw the weight of the water bucket and you obtain the weight of your fish. Divide this number by the number of fish into your bucket and you obtain the average fish weight. Easy J.

Now you have your number of fish and your average weight so you multiply those 2 numbers and you obtain the fish biomass!