Improved formula for estimating steelhead weight

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.

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One Response to Improved formula for estimating steelhead weight

  1. Roland says:

    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.

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