Measuring Fish

Saltwater | Freshwater

MEASURING A FRESHWATER FISH

All freshwater Florida Fish and Wildlife Conservation Commission regulations depend on “total length.”

 The image below depicts the most commonly used measurements for fish. For freshwater fish, the measurements that you need to use are total length and girth.

Total length measurement

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 lying 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. These photos show a bass on a measuring board with the mouth held shut. 

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 lightweight tail when calculating a fish’s condition.

Girth measurement

Girth is best measured with a fabric ruler, like 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. The measurement should be taken perpendicular to the length of the fish. This measurement is analogous to measuring the circumference of someone's waist. 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.

How to estimate a bass’ weight

Although it cannot be used to certify an official weight, use of the length and girth can give you a good estimate of a bass’ weight. Scientists use a rather complex formula to attain the greatest accuracy. The equation is: Log (weight in grams) = -4.83 + 1.923 x Log (total length in millimeters) + 1.157 x Log (girth in millimeters). A 22-inch-long bass with a girth of 15 inches weighs about 6.0 pounds using this formula.

Fortunately, there are several other easy formulas that you can use, although they are not as accurate, they will give you a rough estimate. A quick, though very rough, estimate of torpedo-shaped fish such as young bass can be obtained by using total length (in inches) squared, times girth (in inches) divided by 1,200. A 22-inch-long bass with a girth of 15 inches weighs about 6.1 pounds using this formula.

Another common option used for estimating bass weights is girth (in inches) squared, times length (in inches) divided by 800. A 22-inch-long bass with a girth of 15 inches weighs about 6.2 pounds using this formula.

 (Note: Marine fish are sometimes measured using other techniques that are also used by freshwater fisheries scientists.)

MEASURING A SALTWATER FISH

Most finfish size limit regulations of the Florida Fish and Wildlife Conservation Commission (FWC) use either “fork length” or “total length.”   

These measurement methods provide a consistent, well-defined measurement technique. These methods encourage angler compliance with fishery management regulations.

Total length measurement

Total Length is now measured from the most forward point of the head, with the mouth closed, to the furthest tip of the tail with the tail compressed or squeezed, while the fish is lying on its side.

Fork length measurement

Fish regulated by fork length are measured from the tip of the jaw or tip of the snout with closed mouth to the center of the fork in the tail.

Lower jaw fork length measurement (Billfishes)

Fish regulated by lower jaw fork length are measured in a straight line from the anterior most part of the lower jaw (tip of the lower jaw) to the fork in the tail.

Measuring Stone Crab and Spiny Lobster

Stone Crab measurement

Stone Crab claws must measure at least 2¾ inches in length measured by a straight line from the elbow to the tip of the lower immovable finger. The forearm (propodus) shall be deemed to be the largest section of the claw assembly that has both a moveable and immovable finger and is located farthest from the body of the crab.

Spiny Lobster measurement

Spiny Lobster must have a minimum carapace length of greater than 3 inches and the measurement must take place in the water. The carapace is measured beginning at the forward edge between the rostral horns, excluding any soft tissue, and proceeding along the middle to the rear edge of the carapace.   

For more information please visit the Fish Length Measurement FAQs.