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Thread: WEIGHT FORMULAS.... accurate?

  1. #1
    Member fishNphysician's Avatar
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    Jun 2006
    Aberdeen WA

    Default WEIGHT FORMULAS.... accurate?

    Are the typical formulas we use to estimate weight (based on simple length and girth measurements) really that accurate?

    As the author of one such formula, the answer is.... it depends. My formula is often criticized for overestimating the weight on large Kenai bucks. For every example someone shows me that the formula guessed too high, I can counter with a high profile example that was bang on or slightly less.

    Personally I think most of the error comes from poor measuring... or simply gross exaggeration of the actual measurements taken. Face it guys, 12 inches is a lot bigger than most of you think.... just read the frickin' tape at face value without highballin' it! No need to Dilly up!

    Another BIG mistake folks make with my formula is using TOTAL length. My formula uses FORK length.... snout to mid-fork. On a BIG Kenai king, that might mean a difference of as much as 2 inches or more. Those phantom inches are worthless. C'mon.... how much weight can a couple inches of fin rays add to the total? I refer to this as "empty length". Same goes for a buck with a really long snout. A couple inches of snout might add a few ounces tops, while a couple inches of mid-body steaks could add 3-4 pounds or more. Again the snout adds "empty length". Capiche?

    Then there' the issue of "empty girth". Think of a toothpaste tube which has consistent girth. When it's absolutely full, it will have a perfectly round cross section. If I squirt some toothpaste out by compressing it mid-tube, it will weigh less even though the girth has NOT changed. Bucks that have "slabbed up" (flattened from side to side with tall ridge backs) are just like that.... flattened mid-tube.

    Estimating the weight on hens tends to be MUCH less prone to these variations... tails tend to be less forked and heads are uniformly small. And hens don't really "slab up".... if anything they get a bit rounder as maturing eggs swell their bellies. The relative sameness of body proportions makes the formula much more consistent and precise for hens.

    Remember that these formulas are using only two variables to come up with the correct answer. While technically identical, the same two measurements may in fact be deceptively describing two totally different body shapes.

    Let's go back to that tube of toothpaste. A rotund tide-fresh powerhouse of a MEGA-buck caught at the Pasture is gonna be like a brand new tube out of the box. I catch him a week later at Harry Gaines and it's like squirting 3-4 inches of paste out of the tube by slightly flattening the middle. A fish I catch two weeks later in the middle river is gonna be more like a toothpaste tube where I squirt out 3-4 more inches of paste by flattening the tube. For even better effect, I unscrew the cap till it's just barely hanging on, and I squish the far corners of the tube closer together, adding empty length to the fish just like an elongated snout and forked tail would do.

    Get the picture?

    The first two examples would have identical L:G measurements yet yield very different weights. The third example would paradoxically give us even bigger measurements yet the absolute LEAST weight of the three examples.

    So folks have to be a little careful on using the formulas to get an accurate weight estimate. They're pretty good, but they ain't perfect. Just realize that for bucks, the longer that sucker's been in the river the greater the over-guess-timation factor is going to be.

    Here's a real live example of empty length and girth. One fish weighed 74.4# on a certified scale and measured total length 56 - 1/8 inches and girth 33.5 inches. The other fish with identical girth weighed 73.5# and measured total length 53 inches.

    That's over three inches difference in length yet less than a pound difference in weight!

    "Let every angler who loves to fish think what it would mean to him to find the fish were gone."
    The KeenEye MD

  2. #2
    Member willphish4food's Avatar
    Join Date
    Apr 2007
    Willow, AK


    Great explanation, doc! Thanks!


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