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

A researcher measures fish

A biologist measures fish during aroutine sampling trip.

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 0 lbs 3oz        
7 0 lbs 4oz        
8 0 lbs 7oz 0 lbs 6oz 0 lbs 4oz    
9 0 lbs 10oz 0 lbs 8oz 0 lbs 7oz    
10 0 lbs 14oz 0 lbs 11oz 0 lbs 9oz    
11 1 lbs 3oz 0 lbs 15oz 0 lbs 13oz    
12 1 lbs 10oz 1 lbs 4oz 1 lbs 1oz 0 lbs 14oz  
13   1 lbs 10oz 1 lbs 7oz 1 lbs 2oz  
14     1 lbs 13oz 1 lbs 6oz  
15     2 lbs 4oz 1 lbs 13oz  
16       2 lbs 4oz 1 lbs 6oz
17       2 lbs 12oz 1 lbs 11oz
18       3 lbs 5oz 2 lbs 0oz
19       3 lbs 15oz 2 lbs 7oz
20       4 lbs 11oz 2 lbs 14oz
21       5 lbs 8oz 3 lbs 6oz
22       6 lbs 7oz 3 lbs 15oz
23       7 lbs 7oz 4 lbs 9oz
24       8 lbs 8oz 5 lbs 3oz
25       9 lbs 12oz 5 lbs 15oz
26         6 lbs 13oz
27         7 lbs 11oz
28         8 lbs 11oz
29         9 lbs 12oz
30         10 lbs 14oz
31         12 lbs 2oz
32         13 lbs 7oz
33         14 lbs 14oz
34         16 lbs 7oz
35         18 lbs 1oz
36         19 lbs 13oz

 

 

Which of these fish appears to be in better condition?

Standard bass
Fish "A"
Length: 24 in
Weight: 8.27 lbs (8 lbs 4 oz) 
 Large bass
 Fish “B”
 Length: 23 in
 Weight: 9.33 lbs (9 lbs 5 oz)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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!

 



FWC Facts:
Bass have been known to eat snook, and snook occasionally eat bass.

Learn More at AskFWC