For those anglers looking to set a lake, state, or even a world fishing record this year, there was a great question posed to the California Department of Fish and Game recently about the proper way to measure a fish to determine if it's legal.
Carrie Wilson, a 20-year veteran of the DFG, says:
The first rule when measuring fish is to lay the fish flat on its side and always pinch the mouth closed. All freshwater fish, including largemouth bass, are measured to total length. This is the longest straight-line distance from the tip of the head to the end of the longest lobe of the tail (CCR Section 1.62). The most accurate method is to place the fish's snout against a perpendicular surface and then measure along the intersecting horizontal surface to the end of the tail. Don't measure using a flexible "tape" over the fish itself or you will be given a longer false reading. All freshwater fish with a minimum size limit are measured this way.
On the ocean side, most saltwater fish with minimum size limits are measured to total length, but there are some that are measured to fork length instead (e.g. bonito, albacore, barracuda and yellowtail). Fork length is the straight-line distance from the tip of the head to the center of the tail fin (CCR Section 1.62). So again, lay the fish flat on its side, pinch the mouth closed and take your measurement from the tip of the head to the center of the fork of the tail. These are the only two measurements that you will need to know for the purposes of the regulations when measuring whole fish.
The rules and regulations for world record qualification with the International Game Fish Association, however, are a little more intense. According to the IGFA, world record submissions for the all-tackle length category (catch and release) must be measured using an official IGFA measuring device on a flat surface. The IGFA states that the "fish's snout must be touching the nose stop with the mouth closed and free of lures or lifting devices," and that "measurements will be taken from the most forward part of the fish's snout with the mouth closed to the rear center edge of the tail" in centimeters.
Visit the IGFA site for more information on measurement requirements for possible world record catches, and please practice catch and release when at all possible when hunting possible world record fish.