Estimating tarpon weight means using a fish's fork length and girth in a proven equation, or a quick reference chart built from it, so you get an accurate number without ever pulling the fish from the water. In this How 2 Tuesday I brought on Dr. Mike Larkin, a fisheries scientist from the University of Miami, to walk through the improved formula for estimating a tarpon's weight. The girth in front of the dorsal fin matters as much as the length, because a one-dimensional measurement misses the full mass of a three-dimensional fish.
Listen now: press play in the player above and follow along.
You estimate it from two measurements taken at boatside: the fork length, measured from the tip of the snout to the fork of the tail, and the girth, measured right in front of the dorsal fin. Plug those into the improved equation Mike Larkin's team developed, or simply read them off the chart he built from that equation. You never have to lift the fish out of the water, which is both better for the tarpon and safer for you and your boat. That is the whole point of using a formula instead of a scale on a fish this large.
Because length alone is a one-dimensional measurement of a three-dimensional animal. A tarpon's girth captures the mass that fork length misses. As Mike Larkin put it, you can get away with judging a 16 or 17 inch trout by length because the girth barely changes, but a big tarpon's girth has a huge impact on its weight. Two tarpon of the same length can weigh very differently depending on how thick they are through the body, so the girth measurement in front of the dorsal fin is what makes the estimate accurate.
The Woods equation dates back to 1924 and is a clever, widely known way to estimate fish weight using length and girth. It works well for many species. Mike noted he would still use the Woods equation for tuna and football-shaped fish. For tarpon, though, his University of Miami team developed an improved equation because tarpon have a more compressed, ellipsoid body shape than a tuna or kingfish, so the general formula needed refining to be accurate for them specifically.
The improved formula is significantly more accurate than estimating by length alone or subtracting a flat percentage. Mike explained there is always some variability with a large database, anywhere from roughly five to fifteen percent depending on measurement error, but the equation gets you far closer to the true weight than guessing. The trade-off is that the full equation is complicated and not something you would do in your head on a boat, which is exactly why the chart exists.
No. The full equation is, in Mike's words, brutal, even though it is accurate. So his colleagues at the University of Miami built a simple reference chart that works like a slide rule. You find your fork length along the bottom axis, your dorsal girth along the side axis, and read the weight where they meet. You can print it, laminate it, and keep it in your tackle box or boat. I keep one on hand, and it means no math while a giant tarpon waits boatside.
Tarpon are a protected, regulated fish, and in many places it is illegal to remove them from the water. Beyond the rules, keeping a big tarpon in the water is simply better for the fish and for you. A giant tarpon flopping on the deck can break your console, hurt someone, and exhaust the fish. By taking the girth and fork length at boatside and using the chart, you get your weight estimate and release the fish quickly and healthy without ever lifting it out.
When I asked Mike how to size up a tarpon, the first thing he pointed out is that length is just one dimension of a three-dimensional fish. You can judge a trout by length because its girth hardly changes, but a tarpon is different. Its girth, measured right in front of the dorsal fin, carries a huge share of the weight. Two fish the same length can be very different animals. He explains exactly why that is, and how much girth changes the number, in the episode, so press play in the player above.
Mike walked through the Woods equation, a clever 1924 formula a lot of anglers have heard of, and why it still works great for football-shaped fish like tuna. The problem is that tarpon have a compressed, ellipsoid body, so the general equation needed refining. His University of Miami team built an improved version specifically for tarpon. It is more accurate, but as he admits, it is also brutal to calculate. He breaks down where it came from in the episode, so press play in the player above.
Here is the simple process Mike and I walk through for getting an accurate tarpon weight without taking the fish out of the water.
I unpack each of these with the details and stories in the episode. Press play in the player above.
The day after recording this, the part that stuck with me is how simple Mike made something that sounds intimidating. A complicated University of Miami equation becomes a laminated card in your tackle box, and suddenly you can tell a buddy exactly how big his fish was without ever lifting it from the water.
If you are lucky enough to catch a tarpon this season, take care of it. Get the fork length and the girth, read the chart, snap your photo, and let the fish swim off strong. That is a personal best you can be proud of and a fish that lives to be caught again.
How 2 Tuesday is my weekly series where I break down one fishing skill at a time, from knots and casting to gear, tactics, and the habits that make you a better angler. Watch and listen to every How 2 Tuesday episode from Tom Rowland.
Dr. Mike Larkin · University of Miami · tarpon · bonefish · kingfish · tuna · Woods equation · fork length · dorsal girth · Florida Keys · How 2 Tuesday · Saltwater Experience
I'm Tom Rowland, a professional fishing guide based in the Florida Keys, host of the Tom Rowland Podcast, and the longtime host of the Saltwater Experience television show. On the podcast's How 2 Tuesday series I break down one practical skill or lesson at a time, from fishing technique and gear to the habits that make you a better angler, in short, focused episodes you can put to use right away.
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