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

Fat vs Thin

We observed in our data that the average steelhead girth divided by length is 50.4%, and that fatter steelhead tend to have larger divisors than thinner ones.  To illustrate this effect, we use the following definitions:
  • "fat" steelhead:                          girth greater than half its length
  • "thin" steelhead:                        girth less than half its length
  • "typical" steelhead:                    girth equal to half its length

Of 115 steelhead, 13 were typical, 49 were fat, and 53 were thin. As the chart below illustrates (click here for larger printable version), fat steelhead have larger divisors than thin ones.  This observation is true not only for the average divisor (704 for fat, 675 for thin), but for every point along their respective probability distributions.  That is, the entire probability distribution of divisors for fat fish is shifted significantly to the right compared to the probability distribution for thin fish.

This effect is not just a random anomaly of the data, but rather a fact of nature.  The theory of girth taper explains it, our conical fish model shows how it works mathematically, and our improved weight estimator incorporates an appropriate adjustment to account for it.

Fat Steelhead Tend to Have Larger Divisors Than Thin Ones