The conservation word is spreading fast amongst tope and shark anglers, and rightly so. Even though the objective is to release the fish unharmed as quickly as possible, it's still nice to have an idea of roughly how big your catch was. Here's how to do it.
The following simple mathematical formula originated in Panama back in the mid 60's and was devised originally for billfish, at least that's the first reference to it that I've come across, but it's pretty accurate for most species. I've caught a large number of tope over the past 20-years, plus blue shark and porbeagles and found the formula to be accurate to within about 3% and that's on the high side, it's usually nearer than that.
I suggest using a modern fibreglass tape measure as it's more accurate and less prone to stretching than other types. If your maths are as bad as mine are, then a calculator makes life easier.
The girth is taken as the deepest part of the body, and the length is measuring from the nose to the fork of the tail for sharks and round fish. For billfish, you measure from the tip of the lower jaw to the fork of the tail.
1) Measure the fish's girth in inches, then square it.
2) Multiply the arrived at figure by the fish's length, again in inches.
3) Divide the final figure by 800 to achieve the approximate weight of the fish.
Confused? Here's an easy to follow example:
26 x 26=676
676 x 55=37180
37180 divided by 800=46.475lbs
It's worth writing it down on a piece of paper and keeping it in your tackle box or wallet.