Measuring Fish
Most folks know about the weight-estimation formula based on a fish's fork length and girth. This formula can vary from spot on to way off.
To get the proper measurements for this formula, get the girth in inches (measuring just behind the pectoral fin) and the straight-line fork length from the tip of the fish's lower jaw to the center point of the tail.
Inexperienced weighmasters also often include part of the dorsal fin when measuring fish — but this usually results in a big difference between the calculated weight and the fish's actual weight on the scale. Including the dorsal fin, or the flap of skin that hides a non-erect fin, bases the formula's calculations on muscle mass that does not exist.
Taking a girth measurement while the fish is hanging results in an erroneous, much larger girth due to internal organs slumping down inside the body cavity. Consequently, I always measure fish lying flat on their side, measuring one side for half the girth and then doubling the number. This results in a much more accurate estimate than trying to encircle the fish with the tape.
I've probably measured more than 300 marlin exceeding 1,000 pounds and find I get the most accurate estimates using measurements starting at the slit holding the pelvic fins and going only to the base of the fold that encompasses the dorsal fin. I double this half girth and then square it according to the formula.
To get the approximate weight, first square the girth (girth x girth). Multiply the result by the length. Divide by 800 to obtain the weight in pounds (girth x girth x length / 800 = weight). This is usually correct to within 10 percent on tuna or 15 percent on marlin.
While measuring many hundreds of marlin exceeding 1,000 pounds in Cairns (and even more topping 800 pounds), we learned to add another measurement. If a second girth measurement of a marlin, taken near the anus, is within a couple of inches of the girth at the pectoral fin, we add 10 percent to the formula's weight.
The reason this formula doesn't work on all species is that the volume of muscle, which correlates directly to weight, depends on the length and the cross-sectional area of a slice of the body, which differs hugely between fish with different body shapes — even if the length and girth are the same. (See the Circle/Oval illustration above that shows a circle and an oval with the same circumference but very different internal area.)
The "highly compressed" body shape of a tarpon, which is flattened and skinny like an oval, would make it weigh only a fraction of the weight of a "rotund" tuna, which is more round and circular, even if both had the same length and girth.
