Average length at age of largemouth bass from Sample Lake compared to Louisiana state-wide averages.  This graph indicates severe stunting of Sample Lake bass.

Cross-section of an otolith showing the nucleus (N) and annual growth rings (dots).  This bass

was four years old.

Knowing the growth rates of your fish is important, especially if large fish are desired.  An accurate idea of the growth rates of your fish can be very useful in assessing whether you have adequate forage and habitat available.  Most largemouth bass in the southern US live for eight years or less, so the faster they grow, the better chance they have of attaining trophy weights. 

Much like trees, annual rings on scales and other “hard parts” of fish are developed during alternating periods of fast (spring, summer, and fall) and slow (winter) growth.  Fish from the northern hemisphere are assumed to have a hatch date of January 1, so a fish is considered to be age 0 (young-of-the-year) from the time it is hatched until the following January 1.  The same fish is considered to be age 1 for an entire year beginning on January 1 of the next year, age 2 during the following calendar year, and so on.

Although ages derived from otoliths (ear bones) of bass are considered to be very accurate, the fish must be killed for this procedure.  Therefore, we often use scale samples, which don’t harm the fish.  Although the age obtained from an individual scale may not be as accurate as that from an otolith, we try to get as large a sample size as possible to obtain a useful average.

Fish are normally aged in autumn.  We make plastic impressions of several scales from each fish.  The impressions are viewed under a microscope so the annual rings can be counted.  However, if extremely accurate ages are desired, we can collect otolith samples.  The otoliths are broken though the center, sanded, imbedded in clay, and the cross section viewed under a microscope.