|Let's compare the divisors for two steelhead of the same gender (male), the same river (Babine), and about the same weight (12 lbs). The first was caught and released on September 16, 2003: length 32 inches, girth 17 inches, weight 12 lbs. Divisor = 32 x 17 x 17 / 12 = 771. The second was caught and released October 11, 2004: length 31 inches, girth 15 1/8 inches, weight 11 3/4 pounds. Divisor = 31 x 15.125 x 15.125 / 11.75 = 604.
Our research establishes the simple fact that different individual steelhead (even of the same river, gender, and weight) may have very different divisors. The simple reason is that individual steelhead, just like individual humans, have different shapes.
The primary shape difference that leads to different divisors is what we call "girth taper." A fish that is particularly fat and whose girth tapers rapidly in both directions has a "large girth taper." A more tube-like fish that has a nearly constant girth over much of its length has a "small girth taper."
In the case of large girth taper, the girth measurement overstates the bulkiness of the fish (compared to more typical fish) and so a larger divisor like 771 is required to make the equation balance. In contrast, when guide Dave Holland and I measured and weighed the 604-divisor steelhead above, we remarked to each other that it had an unusually long section of nearly constant girth (i.e. it had a "small girth taper"). In the case of small girth taper, the girth measurement understates the bulkiness of the fish, and so smaller denominator like 604 is required.
The validity of these observations is confirmed by both our conical fish model and analysis of the data for fat vs thin steelhead.