BMI (Bass Mass Index) for Freshwater Fish

By: Justin Hill (FWC's Freshwater Fisheries Research Outreach Coordinator), with Kim Bonvechio

FWRI researchers are using these condition ratios along with TrophyCatch data and submission photos to find a way to stream line the submission process. Here a biologist uses just a ruler in an experimental attempt to verify if the fish is over 8 pounds.

Whether you want to pay attention to it or not, use of the body mass index (BMI) is well established. Though it may not be perfect, the concept is sound. It uses a weight to height ratio to let you know how you compare to an average healthy person of your size and age. When FWRI freshwater fisheries biologists are assessing the health of fish, they use a measurement they call “condition.”

Biologists compare the length to weight ratios with other fish of the same species to determine if they have relatively good or poor condition. Basically, condition tells a biologist if a fish is fat, skinny or average for its size like doctors use BMI on people. Differences in condition among sizes or ages of fish can provide important clues about what may be happening within the population. For example, if condition is good for smaller, younger fish, but goes down for larger, older fish, there may be a problem with the amount of larger prey items in the lake. Biologists can then direct their efforts to learning more about the food availability in the water and take actions to improve the fish population.

One of the most common factors for measuring condition is relative weight. To find a relative weight scientists first come up with a “standard” weight based on lengths and weights from thousands of fish collected throughout their geographic range. They then divide the actual

weight of a fish by the standard value for a fish that length found from the averages and multiply it by 100. Think of relative weight as a percent. A relative weight at or near 100 would be the normal weight for a fish of that length. Unlike humans, values over 100 typically indicate the fish is healthier than a typical fish of that size. A fish's condition can change throughout the year, for instance with gravid females during spawning season, so this value should only be used as a general benchmark.

The average weights at different lengths for several species are listed below. After checking those out remember to “like” us on Facebook at facebook.com/FWCresearch and visit us on the web at myfwc.com/research/freshwater to learn more about FWRI and freshwater fisheries research.

Standard weights for different sizes of common sport fishes caught in Florida

Bluegill

Length Weight

6 inches 0 lbs. 3oz.

7 inches 0 lbs. 4oz.

8 inches 0 lbs. 7oz.

9 inches 0 lbs. 10oz.

10 inches 0 lbs. 14oz.

11 inches 1 lbs. 3oz.

12 inches 1 lbs 10oz.

Redear (Shellcracker)

Length Weight

8 inches 0 lbs. 6oz.

9 inches 0 lbs. 8oz.

10 inches 0 lbs. 11oz.

11 inches 0 lbs. 15oz.

12 inches 1 lbs. 4oz.

13 inches 1 lbs. 10oz.

Black Crappie

Length Weight

8 inches 0 lbs. 4oz.

9 inches 0 lbs. 7oz.

10 inches 0 lbs. 9oz.

11 inches 0 lbs. 13oz.

12 inches 1 lbs. 1oz.

13 inches 1 lbs. 7oz.

14 inches 1 lbs. 13oz.

15 inches 2 lbs. 4oz.

Largemouth Bass

*A length-girth weight calculator is available on our website.

Length Weight

12 inches 0 lbs. 14oz.

13 inches 1 lbs. 2oz.

14 inches 1 lbs. 6oz.

15 inches 1 lbs. 13oz.

16 inches 2 lbs. 4oz.

17 inches 2 lbs. 12oz.

18 inches 3 lbs. 5oz.

19 inches 3 lbs. 15oz.

20 inches 4 lbs. 11oz.

21 inches 5 lbs. 8oz.

22 inches 6 lbs. 7oz.

23 inches 7 lbs. 7oz.

24 inches 8 lbs. 8oz.

25 inches 9 lbs. 12oz.

Channel Catfish

Length Weight

16 inches 1 lbs. 6oz.

17 inches 1 lbs. 11oz.

18 inches 2 lbs. 0oz.

19 inches 2 lbs. 7oz.

20 inches 2 lbs. 14oz.

21 inches 3 lbs. 6oz.

22 inches 3 lbs. 15oz.

23 inches 4 lbs. 9oz.

24 inches 5 lbs. 3oz.

25 inches 5 lbs. 15oz.

26 inches 6 lbs. 13oz.

27 inches 7 lbs. 11oz.

28 inches 8 lbs. 11oz

29 inches 9 lbs. 12oz

30 inches 10 lbs. 14oz.

31 inches 12 lbs. 2oz.

32 inches 13 lbs. 7oz.

33 inches 14 lbs. 14oz.

34 inches 16 lbs. 7oz.

35 inches 18 lbs. 1oz.

36 inches 19 lbs. 13oz.