How to Measure Fish

Lee Schoech
How to Measure Fish

Throughout the summer, as an intern for Texas Parks and Wildlife Department (TPWD) Coastal Fisheries Division, I learned many biological as well as practical facts about fisheries management in the upper Laguna Madre. For instance, it is often perceived through the eyes of anglers that size and bag limits are meant to be restrictive attempts aimed at anglers. However, the factors used to determine size and bag limits are meant to help preserve and maintain current and future sport fish populations.

For example - the minimum size limit for spotted seatrout (trout) is 15 inches; the reasoning behind this size limit is to ensure that all members of the population spawn at least once before they become part of the harvestable portion of the population. Trout maturation is a size/age relationship so this limit is very important for the preservation of the fishery. If the minimum size limit was lower a portion of the trout population would not have a chance to reproduce and replace itself before being eligible to be harvested.

The "official" measurement of fish is very particular and takes the right gear and technique in order to ensure accuracy. Throughout the summer while conducting surveys for the TPWD Coastal Fisheries harvest-monitoring program I observed a number of discrepancies between measurements obtained by anglers and the TPWD staff. Most of these discrepancies could have been avoided if the proper equipment and measuring technique were used. Most anglers typically have some sort of measuring device, such as cooler lids or horizontal measuring tapes. These devices may be to the correct scale, but lack a ninety-degree backing making it hard to obtain precise measurements.

The measuring board used by TPWD is one that is constructed with an upturned end that is perpendicular to the measuring board. The end ensures consistent accurate measurement of fish. Local sporting goods stores carry similar boards made of aluminum that are lightweight and easy to mount on the deck of a boat. Measuring boards used by game wardens are similar to ones that are used by TPWD Coastal Fisheries employees while conducting a harvest monitoring survey. The pictures provided are examples of the TPWD board versus an igloo cooler lid. The official measurement of the fish in the photos is 414mm, or about 16.3 inches; however, when measured using the igloo lid the fish appears to be near 17 inches.

Having the right equipment does not ensure accurate measuring. The technique, though simple, is often conducted incorrectly which results in loss of accuracy. There are three main things to remember when measuring fish: closing the mouth, place the fish's head firmly against the upturned end of the measuring board, and pinching the tail. First it is important to ensure that the tip of the head or mouth, depending on the species, is flush with the upturned end of the board. While positioning the fish's head, ensure that the mouth of the fish in closed. Then pinch the tail and measure the longest part of the tail. By standardizing the equipment and technique there will be far fewer improper measurements.

A question I was often asked when measuring fish was, "how much do fish shrink when put on ice?" The short answer is not very much. According to a published study conducted by Coastal Fisheries staff in 1989 an average of three millimeters, which equates to about 3/25 inches, in length was lost due to shrinkage. Over half of the estimated shrinkage occurred in the first hour of the fish being on ice, and no fish experienced a decrease in size greater than 1/5 of an inch. Even if the worst-case scenario was met, the shrinkage should not create a potential law enforcement altercation.

By using a board with a ninety-degree backing, closing the mouth, and pinching the tail, recreational anglers can measure fish with the same amount of accuracy as TPWD game wardens and biologists. However, a number of dedicated anglers have advocated their own methodology to, without a doubt, alleviate most discrepancies in regard to fish length. By keeping trout that measure 15.5-16 inches they ensure, even with human/equipment error, their fish will be legal. This "half-inch extra" in minimum length can be applied to other fish species size limits. Not only is this a conservative action, it is also inadvertently a conservational action, which may allow more mature members of the fish population to spawn more than once before being harvested.