What does that bass weigh?

It’s all a matter of inches

Small bass are judged by length. Whether you are measuring a bass to comply with a length-limit regulation or to surpass some self-imposed threshold for a fish you would keep (even if you don’t keep the fish), it’s a game of inches.

But after a bass gets a little larger, size is all about weight. What self-respecting basser would brag that they caught a couple 16s, a 17- and a 21-incher? No way. That’s not how bass anglers talk — or think.

There are good reasons length limits are in inches. Bass are easily measured in inches with a cheap measuring board or an even cheaper yardstick. And length is accurate and repeatable. If I measure a bass to be 15 inches long, someone else would very likely measure it to be the same length, plus or minus a quarter inch or less.

Weight is more difficult to measure accurately. Scales, even good ones, often give slightly different weights. Live bass flop around, and weight goes up and down. And if the bass doesn’t weigh what the angler thinks it weighs, the scale is “off.” Have you ever heard anyone complain that the measuring board, yardstick, or tape measure was “off?”

Nevertheless, weight is what big bass are all about. If you really want to know what a bass weighs, buy a good digital scale and calibrate it by checking the scale reading against a range of known weights.

For those who don’t want to buy a scale and calibrate it, weight can be estimated from length. There are several formulas for estimating weight from length, and all claim to be the best.

Well, since they all can’t be the best, the attached table includes standard weights for largemouth bass that fisheries biologists use.

The table gives weights for largemouth that actually are a little heavier for their size than the average. It’s called the upper quartile weight.

Nevertheless, I’ll confess that the weights might tend to be a little low for bass of truly gargantuan proportions. 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. On the other hand, George Perry’s 22-pound, 4-ounce bass was 32.5 inches long and would be only 3/4 pound heavier than the weight predicted by the table.

Spawning and shrinking

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).

No doubt about it: Bass weigh more before they spawn, but not as much as some anglers think. These estimated prespawn weights are included in the length-weight table.

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 to 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 male and female bass, and included a function to allow GSI to increase with bass size. For example, the GSI for a 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 or testes. 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 and sperm 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 above, if you really want to know what a bass weighs, invest in a good digital scale and keep it calibrated. 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.