Growing fish in aquaponics is very enjoyable. What is even more enjoyable is to know that we are doing the right thing and the fish are grown in the best condition possible. Fish can suffer of a multiple of factors. I suggest to review the most common problems encountered in aquaponics affecting the fish and how to measure the fish condition.
Inappropriate feeding
If the fish are not fed properly because you don’t feed them with enough food, they will develop a long body with a very slim belly. It will limit the general growth of the fish but mainly the “height” of the fish shape.
Stress due to territorial fish
When a fish becomes territorial, he will chase the other fish in the pond and avoid them to access the fish food distributed. While the dominant fish will grow constantly larger, the stressed fish will get slimmer and slimmer.
Stress due to disease
Disease can have multiple symptoms on the fish. When sick the fish are generally eating and it is not good to feed them. During this period the fish will lose weight and become slimmer and slimmer.
Stress due to undersized tank.
If you keep large size fish into a limited size tank, their length growth may slow down while their body growth will continue. It generally result on a fish with very limited length but very large and height body. Those fish are misshaped and easily recognizable. It is not recommended to keep fish in a small tank and we should always make sure the fish have enough space in the tank to swim properly. If you keep your fish for a very long period of time, they may reach impressive size. It’s good practice to harvest them before any sign of misshape due to the tank size.
K-Factor
As explain, the fish condition impacts the general fish shape. Fish farmers studied this and Mr Fulton put in place a specific formula allowing to evaluate and compare the fish condition between fish of the same species. We call this specific factor the “K-Factor”. The K-Factor formula is as below:
K = (10^N * W) / (L^3)
where:
K is the K factor
N is a number adapted to the fish species in order to bring K close to 1. N must remain constant when comparing fish of the same species (we generally take N=5 for trouts).
W is the fish weight (g).
L is the fish length (mm).
Please note that in aquaculture the fish are generally put asleep before being handled and measured. To put the fish asleep we use a bath composed of water + eugenol (clove oil) and an air stone. As soon as the fish start swimming upside down they are considered asleep. It can be challenging to get accurate measurement on an awaked fish.

The K-factor interpretation
The result you will obtain need to be interpreted. If comparing wild caught fish, the taller the number, the better the fish conditions are. Here are few K results interpretation:
Fish grown with limited quantity of food will have a low K-factor.
Example: 0.9

Fish grown in good condition will present a high K-factor.
Example: 1.5

Fish grown in an undersized tank may present an unusually high K-factor that is not reflecting particularly healthy conditions but a problem in the fish growth.
Example: 1.9

The Eye size…
Fish stressed can be very limited in terms of growth and therefore you may wander how to recognize an old fish kept small because of stress factors and a young fish. Both of those are presenting the same size and possibly similar shapes but there is one organ that will be very different… The Eyes of the fish are the organs privileged by the organism. Even a fish in extreme conditions will continue to send enough energy to the eyes to allow them continue growing properly. The eye size will give you an indication on the age of the fish. When buying a new fish batch, always chose the ones with the smaller eyes for the same body size.
Conclusion:
The K-factor can be used if you want to compare the fish condition between different fish of the same species. It can help you to detect a problem in your production but you need reference points for the specific fish species that you are growing. You can keep an historic of the fish size and K factor evolution of your fish as they grow and it will give you some data that you compare year after year.

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