Tom Keelin, an engineering guru and fly fisherman, recently published a new, more accurate way to estimate the weight of fish. The article ran in the May 2008 issue of Fly Fisherman. The conventional formula: weight = length x girth (squared)/800 underestimates the true weight of steelhead, according to Keelin. The more accurate equation, based on collected data from 87 steelhead weighed on an IGFA certified scale, is: weight = lenthg x girth (squared)/690. Keelin says using a denominator of 690 correctly estimates a steelhead’s weight within one pound, 80% of the time.
Keelin’s Website, FlyFishingResearch.net, features an article explaining how to calculate fish weight, as well as an online fish weight calculator.
Keelin’s formula makes sense to me. On my return from a recent trip I estimated the weight of a 32 inch steelhead with 16 inch girth using the old formula and got an estimate of 10.6 pounds. I and my son had estimated the weight of this fish (based on previous experience) to be 12 to 13 pounds. Keelin’s formula gives a more believable (to us) weight of 11.9 pounds. A statistical comment: I note that the 20% confidence interval on the estimate is plus/minus 1 pound. Seems to me that one should be able to get a more accurate estimate based on a sample of 80 plus fish . What about length/girth (to some non-integer value)? The non-integer value could be estimated using non-linear methods.
Yes Roland, your observation is Very interesting. But remember, that some Steelhead ALMOST develop a body shape similar to Chinook!! And, even amongst Chinook Salmon, there are Significant Variations. For instance, in Garibaldi Bay, Oregon, the majority of the Chinook Salmon that many of my friends and I have caught are UNUSUALLY GIRTHY !!! But this was quite a few years ago!! My point is, that there is great Variation!! For one thing, as Anadromous fishes travel, they lose weight!! Because they don’t eat and use up their body fat!! Perhaps even Muscle tissue !! And, even considering fat loss, the other major factor stays the same (the Length).