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Freshwater fisheries biologists calculate condition to observe the overall health of fishes.

Freshwater fisheries biologists with the Fish and Wildlife Research Institute (FWRI) have many tools to assess sport fish populations. They can count the number of fish they collect in a sample to learn how many fish are in the lake, and they can count the rings of ear bones (called otoliths), much like the rings of a tree, to learn how old the fish are and how fast they grew. What do biologists use to determine a fish’s overall health? Often the first step in assessing health is calculating something biologists call condition. Condition is measured using the length and weight of the fish. Biologist can compare these values with other fish to determine if it has relatively good or poor condition. Basically, condition tells a biologist if a fish is fat, skinny or average for its length. 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 such as stocking bait fish species 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 averages from thousands of measurements of fish collected throughout their geographic range. They then divide the actual weight of a fish by the standard value 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. Values over 100 indicate the fish is healthier than a typical fish of that size. A fish’s condition can change throughout the year, so this value should only be used as a general benchmark.

Biologists have used standard weight for decades, but these equations were primarily developed for popular sport fishes such as largemouth bass or bluegill. More recently, biologists have started developing standard weight equations for other species, including rare and geographically limited species. For example, FWC researchers recently helped to develop a standard weight equation for the Suwannee Bass (Micropterus notius) which has one of the smallest distributions of black bass species, only occurring in a handful of rivers in Florida and Georgia. Calculating condition is quite simple and allows biologists and anglers alike to quickly assess the condition of fish in a lake.

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

Size (inches) Bluegill Redear Black Crappie Largemouth Bass Channel Catfish
6 0lbs 3oz - - - -
7 0lbs 4oz - - - -
8 0lbs 7oz 0lbs 6oz 0lbs 4oz - -
9 0lbs 10oz 0lbs 8oz 0lbs 7oz - -
10 0 lbs 14oz 0lbs 11oz 0lbs 9oz - -
11 1lbs 3oz 0lbs 15oz 0lbs 13oz - -
12 1lbs 10oz 1lbs 4oz 1lbs 1oz 0lbs 14oz -
13 1lbs 10oz 1lbs 7oz 1lbs 2oz -
14 1 lbs 13oz 1lbs 6oz -
15 2lbs 4oz 1lbs 13oz -
16 2lbs 4oz 1lbs 6oz
17 2lbs 12oz 1lbs 11oz
18 3lbs 5oz 2lbs 0oz
19 3lbs 15oz 2lbs 7oz
20 4lbs 11oz 2lbs 14oz
21 5lbs 8oz 3lbs 15oz
22 6lbs 7oz 4lbs 9oz
23 7lbs 7oz 5lbs 3oz
24 8lbs 8oz 5lbs 15oz
25 9lbs 12oz 6lbs 13oz
26 7lbs 11oz
27 8lbs 11oz
28 9lbs 12oz
29 10lbs 14oz
30 12lbs 2oz
31 13lbs 7oz
32 14lbs 14oz
33 16lbs 7oz
34 18lbs 1oz
35 19lbs 13oz

Which of these fish appears to be in better condition?

In the table above you can see that the standard weights are 8 lbs 8 oz (8.52 lbs) for Fish “A” and 7 lbs 7 oz (7.41 lbs) for Fish “B”. By dividing the actual weight by the standard weight, we can then determine relative condition of the fish.


Fish A: (8.27 / 8.52) x 100 = 97

Fish B: (9.33 / 7.41) x 100 = 126


Although both of these fish are in good condition, Fish “B” is very plump for her size and is well over the 100 mark. She is definitely eating well!