During my early years on the Great Barrier Reef, I helped Dick Fitzhardinge, the Cairns Game Fishing Club weighmaster, weigh and measure just about every fish that came in. When we first started using motherships on the reef, one of my anglers, Bill Chapman, bought a 2,000-pound scale and we weighed dozens of fish for ourselves and other game boats. My big one-ton scale now resides in Cooktown, but only rarely gets used by Jim and Anne Williams, the owners of the Cooktown Marlin Marina. I figure I've weighed and measured several hundred marlin over 1,000 pounds. 
 
Fitzhardinge got to where he could tell immediately if the marlin would make a grand when he put the tail chain on. If Fitzhardinge, a big man, could not get both hands around the tail with thumbs and middle fingers, he would proclaim, "This one will go!"
 
When I learned this secret, I started measuring tail girths. Since then I've seen very long fish make the magic weight with tails as small as 18 1/2 inches — but only one marlin I've weighed with a 20-inch tail didn't make 1,000 pounds. That short, very fat fish weighed 985.

The commonly used weight formula: Length x girth x girth divided by 800 (with length and girth measured in inches) is off quite a bit. A marlin with a true girth of 6 feet (measured just behind the pectoral fin) and a short-length of 11 feet (measured from bottom jaw to crotch of tail) should weigh 855 pounds by the formula. In real life, however, that fish will hit at least 900 pounds. And if it is very fat, the fish might even go over 1,000. 
 
The fallacy in this formula is that weight is a mathematical function of cross sectional area times length, and we have used circumference and length. A tuna with a length of 6 feet and a rotund (round) girth of 4 feet would weigh much more than a tarpon with the same measurements but whose girth is compressed (flattened). A foot-long chunk out of the middle of the tuna would weigh several times as much as a similar slice of tarpon. 
 
Marlin vary as much in shape as people do. In high school, at 5 feet, 11 inches with a 32-inch waist, I weighed 145 pounds. Jim Brown, the great NFL running back, shared the same height and waist measurements and weighed 235!
 
A marlin with a 6-foot, 2-inch girth, a 6-foot girth at the anus and 11-foot, 6-inch short length will weigh a grand, but the formula says 945. That's about what a fish with the same length and girth, but a 5-foot, 6-inch anal girth, would weigh. If a fish, like our first example, holds its girth well back, it weighs more. So we usually add 10 percent to the formula weight when we catch a fat one for a better approximation.