Elise Chalcraft

7 kilos of Happiness

The ins and outs of weighing our fish

Every 6 months, we weigh our 128 African catfish. This is done to get a snapshot into the growth rate of the catfish and check on their general wellbeing. The Aquaponics Greenhouse currently has five tanks splitting the fish by size from smallest (approximately 550g) in Tank 1 to the largest in Tank 5.

Our biggest fish is similar size to a small dog, weighing just over 7 kilos! 


Two catfish being weighed - These two African catfish have been caught using the net and have just been weighed. They are being transferred into the weighing buckets until the rest of the tank has been weighed .  Elise Chalcraft

With: Abhay Villa

But how do you weigh a fish?

Firstly, the water from the tanks is pumped out into the large buckets that will hold the fish. Weighing a fish is done using a method of water displacement (explained below) where fish are placed in a see-through bucket with a set scale written on the side. After weighing, they are transferred into a larger bucket whilst we weigh the rest of the fish in that tank. A tally of how many fish enter each bucket is kept outside each bucket to ensure we don’t double count any fish. The weight of each fish is recorded along with any other anomalies we might spot.


Aquaponics fish weighing buckets - These are the buckets used to hold the fish after they have been weighed in the see-through square container. Each bucket has a tally sheet stuck onto its side to mark how many fish are in each bucket.  Elise Chalcraft

Bathtub mechanisms

There is a set amount of water in the bucket, and when you place a fish in the water, the water level rises due to the increase in biomass in the bucket. Think of when you sit in a bath how the water level rises when you sink in.

We measure how much the water has risen using the scale on the outside of the box and convert it to kilograms (see image below.

For example: if the water level is at 0 and we add a fish that raises the water level to 2, that fish weighs 2kg because 1L = 1kg.

Sometimes the difference is not high enough to get an accurate reading; therefore, we put multiple fish into the bucket and divide the water displacement by the number of fish.


Fish Weighing Scale - Scale on the outside of the bucket for weighing the fish This scale was made by adding water litre by litre and marking where the new water level rose to. This helps us have an accurate scale to measure and weigh the fish through the water displacement method.  Elise Chalcraft

Thick-skulled or smartest catfish on the block?

Pro Tip: Ensure that there is a lid for the bucket since African Catfish can jump out!

This is because in the wild, catfish are known to leave the water and move across land to the next water body when the previous one was too crowded or no longer contained the nutrients needed to sustain them. Therefore, they are not afraid of jumping out onto land! 

To prevent and injuries or unnecessary stress, we ensure that the greenhouse is catfish-proof by putting a tarpaulin along the tanks' edge to ensure that they don’t find themselves a new home under the tanks instead of in them...


Catfish-proof containers - Tarp covering any possible escape routes for cheeky catfish This tarp is very important to ensure that if any catfish escape during the weighing process , they do not get stuck underneath any of the tanks. The tarp also helps with protecting their skin if they jump out so that they don't injure themselves on the metal floor.  Elise Chalcraft

Pro Tip: Have a damp towel on hand to place over the catfish's eyes and protect their skin from our nails when we pick them up. Even if you do so carefully, it is better to use a towel to make sure you don’t drop them before reaching the bucket.

We also have to make sure to have weighted lids for the buckets since the catfish are thick-skulled fish, meaning they will try and jump out even if there is a lid! Sometimes they can knock the lid off a bit then jump out the gap, so it is important to have something weighing the lids down, especially when weighing the larger fish!

Relax, sit back, and chill fishy friends!

Weighing can be a stressful situation for fish as they are taken out of the water; therefore, we try and do this as quickly and as stress-free as possible to limit the amount of time in the nets.

We turn the water heaters off at least 24 hours before weighing the fish to ensure that the water temperature drops below 17°C. African catfish can adapt well to temperature changes, and by lowering the temperature, they go into a sort of hibernation to not waste energy on excess movements. They can now chill! 

Side note: it is important to not feed them the day before since with the temperature change, their metabolism slows down, and when being weighed, they may throw up their food and pollute the water, shocking the fish more than necessary.

Before the fish are put back into their respective tank, we make sure to divide any significantly larger/ smaller fish into separate buckets to be placed either in a higher or lower tank to equalize the sizes within each tank. This helps reduce bullying among the fish, where the larger ones eat more food and outgrow the other fish in the tank.