What makes this process difficult is the fact that a fish does not have an age dictated length. This is due to the fact that they grow based solely on what they eat. Therefore a 10 inch bass could technically be a few months or 10 years old.
Luckily there is a fairly strong correlation between the length and the weight of a bass. A 10 inch bass will not be 10 lbs instead a 10 inch bass will almost always be near .5 lbs. This means a bass individual weight can be predicted based on its length. A fish is considered underweight when it weighs less then expected for its length. Just as a fish is considered overweight when it weight more then expected for its length.
A 10 inch .5 lb bass would have a relative weight of 100% because it weighs what it should for its length. A 10 inch 1 lb bass would have a relative weight of 200% which means it is greatly overweight and will be increasing in length very quickly. A high relative weight like this example would be difficult to sustain unless forage was greatly abundant.
Conversely a 10 inch .25 lb bass would have a relative weight of 50%. This fish is underweight but most importantly this mean that fish have lost weight. The bass grew to a size of 10 inches and .5 lbs but has not eaten enough forage to sustain its weight and therefore has lost weight to become underweight. Just because this fish is underweight does not mean it cannot rebound. In fact depending on its age this fish can still have the potential to grow into a trophy fish one day.
What all this means is it is difficult to judge one fish and decide if it should be released or culled once caught. Instead you need data, and lots of it. The more fish you record the more trends you will find in the relative weights of the entire population. Eventually you will see groups of fish that are similar size. These fish may be underweight, stunted, or both.
Below you can find a standard weight chart to use to determine the relative weight of your fish.
Steven Bardin M.S.