Trout Condition Factor

By Dick Marquand

This brown trout was caught in the Pukaki Canal, near Twizel, by Paul Spicer. It is an incredibly fat fish that has “possibly” been feeding on pellets that have dropped through the bottom of the Mt Cook Salmon fish farm holding pens. Though relatively short in length it weighed an amazing 30lbs. We don’t know the exact length of the fish to calculate its condition factor but it is certainly going to be very high. Photo courtesy of Paul Spicer.

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.)

Photo A (above): Brown trout, length 55.5cm, weight 2700 grams.

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.

Photo B: Brown trout, length 23 inches, weight 1lb 12oz.

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.

Paul Spicer with another short fat brownie, this time from the Ohau A Canal. This fish weighed 18lb. Photograph by Allan Burgess.

This is a fibreglass replica of the winning fish from the 2000 Waitaki River Salmon Fishing Competition. This fish was very thin and its condition factor very low. It is widely believed that feeding conditions at sea were very poor that year. Fish numbers were well done in the 1999-2000 season and many of the fish that did return were under-weight like this one.