Why 690 for Steelhead

Individual-Fish Divisors

Individual Variability

Fat vs Thin

Girth Taper

Improved Weight Estimator

Buck vs Hen

Variability By River

Conical Fish Model

Data Sources

Improved Weight Estimator

Our improved weight estimator identical to the our original 690 formula except that the 690 divisor is modified to the extent that a fish is fatter or thinner than typical.

               weight = length x girth2 / divisor

               divisor = 690 x (1 + 40% x (2 x girth / length -1))   or 45% ???

To interpret this formula, first consider a 40 inch steelhead with 20 inch girth. For this a "typical" steelhead (girth is half the length), the quantity in parentheses reduces to 1 and the divisor is 690.  Next, consider a 40 inch steelhead with a 24 inch girth.  For this "fat" steelhead, the quantity in parentheses is 108% and the divisor works out to 745.    For a thin 40 inch steelhead with 16 inch girth, the divisor is 635.

We determined the constant 40% from our analysis as the number that minimizes the sum of the absolute value of the differences between the actual weight of each steelhead and its predicted weight according to the above formula.  In other words, we found that 40% yields more accurate predictions than higher or lower numbers.  

Over 115 steelhead, the average error (absolute value of the difference between actual and predicted weight) with the formula above is 0.85 lbs. This number compares to 0.91 lbs for the 690 formula without this adjustment. So, while the improved formula above is more accurate, much variability still remains.

Click here for a table of weight predictions based on the improved-weight-estimator equations on this page.