Calculations

The growth rate in fish can be calculated in the following two ways:

 

 

The growth of fish can be expressed as SGR (Specific Growth Rate) which is the daily weight gain in percent. This coefficient is calculated:

 
SGR = [ (ln W2 – ln W1) / (t2 – t1)]  100

where W1 and W2 are body weight at the start and end of the growth period, respectively, and t2 – t1 is the length of the period in days.

As the SGR does not depend on temperature profiles, it can only be used to evaluate the growth of a certain batch under ambient environmental conditions and extrapolation to other situations will be highly inaccurate.

 

 

TGC

 

Another and better way to express and predict the growth of fish is TGC (Thermal-unit Growth Coefficient) which is calculated:

TGC = 100  (Final body weight1/3 – Initial body weight1/3) / Sum(Days  Temp(C))

 

Calculation of growth rate is based on previous production records and water temperature profiles. Different strains of fish have different growth potentials. A great advantage of the TGC is that the growth of a specific or different strain of fish can be compared directly at different temperatures (locations) if other relevant environmental conditions are comparable.

 

Dependent on strain of fish, environmental conditions, feed and feeding strategy, TGC usually is in the range 0,13 – 0,20.

 

Example:
Start weight: 10 gram; end weight: 350 gram; average temperature: 11  C; length of period: 200 days.  TGC = 0,222

When the growth potential for a certain strain of fish is known, the calculated TGC can be used to predict growth rates (production times) if the environmental conditions are comparable.