Trout Condition Factor

By Dick Marquand

Trout anglers and those involved with the management of our trout and salmon fisheries are often heard to use the term “trout condition factor. ” What does the term actually mean?

Condition factor is a numerical value given to a trout or salmon that reflects its condition. This value is arrived at by using a mathematical formula that takes into account both the weight and length of the fish. A well-conditioned fish has a high condition factor, while one in poor condition has a low factor.

The condition factor is arrived at by dividing its weight (in grams) by its cubed length (in centimetres
from the tip of its nose to the fork of its tail) and multiplying the result by 3612.8. This may sound
complicated, but in this day and age of pocket calculators, it is very simple and takes little time.

For example, let’s take the trout in photo A, a typical good condition Lake Dunstan brownie with a length of 55.5 cm and a weight of 2700 grams.

For those of you who still think of your trout and salmon in pounds and inches, the formula is even
simpler.
For example, the poor conditioned brown trout in photo B is 23 inches long and weighs 1 lb 12 oz. (One ounce is equal to 0.0625 of one pound, so 1 lb 12 oz is 1.75 lb.)

The condition factor of a trout or salmon is important to fisheries managers. The reason for a low condition factor could be that the fish is stressed, diseased or starved because of overpopulation, or perhaps its the environment is unsuitable because of water temperatures, pollution or some other factor that affects either the trout or its food source. Likewise, a high condition factor indicates a healthy fish and healthy habitat with a low population and/or a plentiful food supply.

Pre-spawning trout will be found to be in better condition than those that have experienced the rigours of spawning.

I enjoy grading my catch using a value other than its weight and length. The condition factor is to me a very important value. I’d far rather catch a 1 kg brownie in good condition than a 3 kg slab that I could almost shave with – wouldn’t you?

The growth of rainbow trout (Oncorhynchus mykiss) in warm-temperate lakes from The University of Waikato.