BoatUS ANGLER: Fishing Basics
How To Measure A Fish
Image and information courtesy of the US Fish & Wildlife Service and Duane Ravers, Jr.
The above image depicts the most commonly used measurements for fish. The total length is the maximum length of the fish with the mouth closed and the tail fin pinched together. The best way to obtain this length is to push the fish's snout up against a vertical surface with the mouth closed and the fish laying along a tape measure, then pinch the tail fin closed and determine the total length, DO NOT pull a flexible tape measure along the curve of the fish.
Conversely, most marine (saltwater) regulations refer to the "fork length", and scientists often use "standard length" which is to the end of the fleshy part of the body. "Standard length" has the advantage of not being affected by minor damage to the tail fin, nor does it give too much credit to a fish for the relatively light weight tail when calculating a fish's condition.
"Girth" is best measured with a fabric ruler, such as tailors use. It can also be determined by drawing a string around the fish at its widest point marking where the string overlaps and then measuring the distance between the overlapping points on a conventional ruler. Knowing the girth is important when trying to certify a fish for a record, and provides useful information to biologists about the relative condition of a fish.
Using total length and girth you can get a rough estimate of a fish's weight using various formulas.
Length-Weight Formulas to Estimate Fish Weights:
Log (weight in grams)= -4.83 + 1.923 x Log (total length in millimeters) + 1.157 x Log (girth in millimeters).