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When the right formula and accurate measurements are used, a fish's weight can be estimated with little if any harm coming to the fish.
By Marty Garrell
Tags: inshore, freshwater, surf
Under the right conditions, fish can be measured without taking them from the water

A few years back, an article appeared in the January 10, 2013 issue of The Fisherman describing the catch of a 21.06-pound blackfish by Capt. Greg Nardiello. It contained lots of information about the catch and its circumstances, but most notable was the clear illustration of proper fish measurement. Apparently Nardiello would have released the fish if a customer hadn’t killed it, which brings to mind a simple thought: why not simply measure and photograph a catch of any size, a fish you intend to release, and calculate its weight later? If you know how to do this, you can get the weight pretty accurately.

Not weighing a fish has some real advantages. First and foremost it’s easy to injure a fish, especially a heavyweight, by hanging it vertically from a hook of any sort. First of all, breaking a spine or cutting a gill raker is a real possibility. Secondly you spare your catch precious seconds by simply slipping a tape along and around it, particularly in the water or even lying in a net or a cradle. This can be critical for the survival of a large fish that’s exhausted or pulled from warm water with reduced dissolved oxygen.

Many sources give a basic formula for weight, usually length times girth squared in inches, divided by 800. This is understandable since weight is the product of density and volume (a linear dimension cubed). But the upshot is interesting: girth is a more important factor than length, and the packing of a fish’s stomach is important.

If we take the Nardiello fish and use the basic formula, we get: 33 x (24)2 /800 = 23.76 pounds.

Apparently one should divide the predicted weight by another factor, here 1.13, to get the actual scale weight, 21.06 pounds. Looking at the photo of that giant tautog, this is hardly surprising. If the fish’s stomach had been packed with juicy crabs or small prey, it might have been a lot heavier.

The most important factor in calculating an accurate weight based off the IGFA formula is in acquiring true measurements of the fish.

If you weigh some of your catches in your logs, you can work out corrections from experience. Even here there are surprises. Fishing one night off Montauk’s Shagwong years ago with Al Ristori we helped him wrestle a pair of 50s aboard. Both fish weighed virtually the same, a few ounces either side of the magic number, but one was much, much shorter! We didn’t measure girths. If you calculate girths for a “fat” 50-inch fish weighing 50 pounds (without any correction factor---just use the original formula) you get 28.3 inches. A “skinny” 50-inch bass that weighed only 40 pounds would have a girth of 25.3 inches. Three inches doesn’t seem like much, but it sure makes a difference in weight!

You see this in certain species all the time. The pikes in freshwater are notorious binge feeders and their weight fluctuates a lot through the season. A couple of decades ago we had two wonderful autumn musky trips in a row with Montreal guide Mike Lazarus, and Jan caught giants on both trips. Her first fish taped out at 50.5 inches by 28.5 inches, the second at 54 inches by 27 inches (length x girth). Using the basic formula with a correction we use for pikes of 1.06 gave 48.37 pounds for the shorter fish, 46.42 for the longer fish. Both were, of course, duly photographed and released, as all large muskies are nowadays. Even more interesting was the fact that the stumpy fish spit out a half dozen gar, each weighing at least half a pound, when we took her from a cradle for measurement. Suppose we’d stuffed the bait back and had a scale aboard! We didn’t need to do so—the fish was big enough!

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