Calculate Your Tarpon’s Weight

Figure out your tarpon’s weight without ever having to remove it from the water. Above, the specific fork length and dorsal girth measurements used to determine a tarpon’s weight.

Oh come on! No one believes your tarpon tango fish story when you tell of a 200-pounder caught off Key West, Boca Grande or Homosassa. But that’s all there is when it comes to tarpon fishing, right? Best guesses and estimates? The burden of proof is no longer backed by a scale, as the FWC has outlawed the possession of a tarpon without a $50 tarpon tag affixed to the fish.

Most all recreational anglers have no intention of killing a tarpon, but love to catch them. Fish on lip gaffs, fish pulled into the boat, and fish held vertically for weight measurement are no longer options. So what’s left are fish tales, and your buddy’s record-breaking catch that only took a half-hour to land.

Good news. Tarpon researchers have created a handy chart that accurately tells you your tarpon weight based off two key measurements. Jerald S. Ault, a Professor of Marine Biology and Fisheries at the University of Miami, has been at the forefront of tarpon research for some time now. His studies helped develop a chart that allows anglers to predict the weight of their tarpon based on two measurements: fork length and dorsal girth length. See the chart below:

Using input data from Florida and Mexico catches, along with IGFA records, Ault formulated the tarpon weight chart with help from an impressive algorithm. The key component is that anglers must accurately measure the fish’s dorsal girth and fork length in inches. The dorsal girth is the circumference around the fish, just in front of the dorsal fin. The fork length is from the tip of the snout to the fork in the tail.

With those two numbers remembered or recorded, locate them on the chart and find out where they intersect. A tarpon with a dorsal girth of 42 inches and fork length of 83 inches would weigh 210 pounds. The chart can be off, plus or minus three percent, but that’s a couple pounds at most.

My guess is that plenty of 120-pound tarpon suddenly turn into 60- or 80-pounders … still a good catch any day of the year!