By Dr. Hal Schramm

Fish regulations are determined by length. For example, a “keeper” bass must be at least 14-inches long in some states or tournaments, but records, tournament standings and braggin’ rights are based on weight. Seems like a mismatch to me.

Length of fish can be measured several ways. Bass length for state laws and tournament rules is always measured as “total length”—the length from the tip of the snout to the tip of the tail fin. Squeeze the tail to make it touch that minimum acceptable length and I call it “maximum length.” Total length is easily, precisely and accurately measured. All it takes is an inexpensive measuring board or a ruler. Measuring boards can be easily compared for accuracy. Although small fish may measure a “skoch” longer on a belly board than a flat board, the positions of the inch marks are identical among all measuring boards. If three people measure the same fish, they will come up with the same total length, at least to within a tiny fraction of an inch.

Weight, on the other hand, requires more expensive scales, the scales have to calibrated, and even sensitive scales can give different weights when the wind is blowing, the boat is rocking or the fish is flopping. Because of this variation, record fish have to be weighed on certified scales by authorized personnel, and tournament weights are determined by weighing all of the fish on the same scale on the same platform. Exactly why records and winners are based on weight and not length is anybody’s guess, but it is well established that two largemouth bass of exactly the same length can differ in weight by a pound or more. Ditto for smallies. Which fish is bigger? I think all would agree that the heavier fish is bigger.
big show
If you really want to know what a bass weighs, buy a scale. Good ones cost $40 to $50, about one-tenth the cost of a good rod and reel. But for those who would rather spend the money on three or four new square bill crankbaits or a giant swimbait, here are some tables that can give you a good idea what a bass weighs. These tables can aid you if you’re considering conducting or participating in a “paper tournament” in which fish are measured, photographed and released.

Before using these tables, understand that there are several ways to estimate bass weight from just the length. Every bass angler knows bass come in a variety of body shapes from starving, hollow-belly bass, to torpedo-shaped river bass, to footballs in lakes with super-abundant, energy-rich forage. There are also regional differences independent of forage. Largemouth bass in Minnesota tend to have higher weight-to-length ratios (they are “chunkier”) than largemouth in the South.

Because body depth and thickness strongly affect weight, some anglers opt for formulas that consider girth—the longest distance around a bass. The simplest of these formulas is:

Weight in pounds = (Total length in inches) X (Girth in inches)2.

I have a couple problems with estimating bass weight from girth. First, you need a flexible tape measure that you are more likely to find at a sewing store than at a tackle shop. (If you borrow the one from your wife’s sewing basket, be sure to wipe the slime off before returning.)  Second, while the formula may be dead-on for some bass, I can’t evaluate the accuracy because there is no data base of fish lengths and girths with matching weights.

The tables that follow for weights of largemouth bass and smallmouth bass are based on standard weight equations used by fisheries biologists throughout North America. They are based on length and weight data for thousands of bass from dozens of lakes and rivers. But, the tables have limitations.

Although based on a huge data set, the weights will be less reliable for really large bass because biologists don’t sample many largemouth more than 8 pounds or smallmouth more than 6 pounds. Gargantuan bass can vary widely in weight-to-length. For example, Manbu Kurita’s recent world record largemouth was only 29 inches long; at 22.31 pounds this outsized fish was more than 7 pounds heavier than the biologists’ estimated weight for 28 inches. But, George Perry’s 22-pound -4-ounce largemouth was 32.5 inches long and would be only three-quarters of a pound heavier than the weight predicted by the table. On the other hand, the smallmouth bass standard weights almost perfectly predict David Hayes’ 27-inch, 11-pound-15-ounce world record smallmouth.

Refer to the graph to see the weight in pounds of largemouth bass from 12 to 33 inches total length in ¼-inch increments. To use the table, find the inch-group for the bass in the left column, then read across to find the length to the nearest quarter inch. For example, a 15-inch fish weighs 1.83 pounds; a 15 ¾-inch fish weighs 2.14 pounds. The Smallmouth Graph is read the in the same manner.
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What about the spawn? I hear it every spring:  “Man, if I’d caught her before she spawned, she would’ve weighed __ pounds (you fill in the blank -- 9 pounds, 10 pounds, or some weight a lot heavier than what the fish weighed on the scale at weigh in!).”  

No doubt about it, bass weigh more before they spawn, but not as much as some anglers think. To help minimize angler disappointment and to help replace fiction with biological estimates, I’ve provided a table that compares pre- and post-spawn weights of female largemouth bass. I only provide these pre-spawn weights for female largemouth bass because male largemouth bass testes weigh very little. Unfortunately, no smallmouth data like this exists.

Here’s how I estimated those prespawn weights. Biologists use an index of the weight of the gonads to the weight of the fish without the gonads to estimate the health of fish and determine when they spawn. This ratio is called the gonadosomatic index, or GSI for short.

Multiplying the bass weight by the GSI gives the weight of the gonads right before the bass spawns. Estimates for GSI vary among studies. I used maximum GSI values for female bass and included a function to allow GSI to increase with bass size. For example, the GSI for 3-pound female bass was 4.5 percent, but the GSI for an 8-pound female was 5.5 percent.  

The estimated prespawn weights are probably reasonably accurate for largemouth bass up to 24 or 25 inches (8 or 9 pounds), but I have little faith in the estimates for larger fish for two reasons. First, biologists handle very few large bass, and when they do they don’t whack them to weigh their ovaries. Thus, the accuracy of the GSIs for the large fish is not known. Second, at some point in a bass’ life, the production of eggs begins to dwindle. Unfortunately, data are lacking to estimate the age or length when GSI may start declining, so I have not included any decreases in GSI at old ages or large sizes.

As mentioned, if you really want to know what a bass weighs, invest in a good digital scale and keep it calibrated and handy. The table, however, has a couple of advantages. First, it is cheap. Second, if you think the table underestimates the weight of your fish, you can always argue that it was an extra heavy-bodied fish and bump the weight a little. That’s hard to do when your fishing partner is staring at the numbers on your super-accurate digital scale you have been bragging about!