Two fisheries scientists from the University of Miami Rosenstiel School have come up with a new and accurate method of estimating tarpon weight that doesn’t harm potential trophy- or world-record-sized fish.
In an article just published in the scientific journal Fisheries Research, authors Jerry Ault and Jiangang Luo lay out a mathematical model that uses a tarpon’s length and girth measurements to make a reliable calculation of weight without having to remove the fish from the water.
“A very cool formula,” Ault said in a telephone interview last week. “It’s a major conservation measure. Hanging [fish] up is a problem everywhere. The short story here is they don’t have to do it.”
Tarpon is a major game species in Florida, the Gulf of Mexico and Southeastern United States that helps support a $6 billion sport fishing industry. Although tarpon tournaments in the Keys have rules requiring minimal handling of fish, some contests in Mexico and elsewhere still allow anglers to hang dead fish up on scales. In southwest Florida’s Boca Grande Pass, the Professional Tarpon Tournament Series has come under fire for encouraging contestants to drag large, live fish by a lip gaff to be weighed in a sling-type scale before release.
The new Ault-Luo ellipsoid formula allows anglers to use a simple measuring tape to get the fish’s fork length and dorsal girth (an all-around measurement of the fish’s body starting just below the dorsal fin), then view a spread sheet or Tarpon Master version 1.0 app on an iPhone to instantly find its weight.
The mathematical calculation that resulted in the model is a bit too long and complicated for the layman. The non-profit Bonefish Tarpon Trust provides waterproof cards with the algorithm, similar to scuba diving tables, for those who don’t have the app.
Before the new formula, anglers who wanted to calculate a fish’s weight without killing it relied on a calculation developed by 19th-century fisheries scientists using tarpon from Boca Grande — girth squared times length divided by 800.
But Ault said that formula is inaccurate because it assumes the fish’s body structure is more angular than rounded.
“What I found was, ‘Oh, crap, it’s negatively biased,’ ” Ault said. “It underestimates weight by more than 15 percent.”
In the journal article, Ault and Luo describe the dismay of a fly angler in Belize who caught a really big tarpon, used the old method to estimate its weight, determined it wasn’t heavy enough for a tippet-class world record, and released it without documenting the catch. When the fisherman later plugged the measurements into the Ault-Luo formula, he found the fish would have been eligible for an IGFA world record.
Further assuring the method’s reliability, Ault said, is that it is based on a sample size of 1,400 tarpon from throughout their range — Florida, the Caribbean, the entire Gulf of Mexico and Africa.
Ault said he believes the formula can be used to estimate weights of other species, such as billfish, tunas and bonefish.