Many anglers are interested in how many fish of a certain species live in a body of water. Estimating population size is difficult and, even with extensive effort estimates, is imprecise. Growth rate is much more informative from a fishery management perspective and can be measured very precisely.
Growth informs managers about habitat quality and fish health. Growth rate is also essential to predicting the effect of harvest restrictions, like length limits and creel limits.
The simple rule of thumb: fast growth is good; slow growth suggests problems. Growth is fast when habitat and water quality are good and food is abundant. The benefits of fast growth are fish reach catchable size and sexual maturity quickly and often are more-productive spawners.
For young fish, fast growth yields higher survival. Piscivores — fish-eating predators — eat what they can fit in their mouths. Thus, larger fish have fewer predators than smaller fish. The faster a fish grows, the fewer the predators to terminate its life.
Growth rate is the change in size, usually length for freshwater fish, per period of time. Measuring the fish is easy. Determining time — how long the fish has been growing — can be difficult.
The simplest situation is shortly after fish are stocked into a new body of water, like a farm pond. You stock the fish as young-of-the-year fingerlings, like a catfish at 4 inches. Two years later, you catch a few fish, and they are 14 inches. These are known-age fish. They grew to 14 inches by age 2.
Rarely do fishery biologists have the opportunity to work with known-age fish. Even in the farm pond example, as soon as the fish reproduce, you have a mixture of ages. Biologists rely on distinct marks on hard structures that are present throughout the life of the fish. These marks, if they can be demonstrated to form once a year and at approximately the same time each year, are called annuli and can be counted to precisely determine a fish’s age.
Historically, scales were used to assign age to fish with scales, like minnows and fish in the sunfish family. Scales are easily obtained without injuring the fish, but they are hard to interpret, especially for older fish. Also, I learned many years ago that bass and crappie in the deep south don’t form annuli on their scales after their first year or two of life. One of my grad students working on crappie looked at the otoliths and could easily see distinct marks. We subsequently demonstrated that the marks were valid annuli.
Otoliths are bones in the inner ear of all fish. Some are quite large, like drum and fish in the sunfish family. Unfortunately, collecting the otoliths requires sacrificing the fish.
Many other bony structures have been used. Fin spines work well for catfish and sturgeon and can be obtained without sacrificing the fish. The spines are cut crosswise into thin sections and observed with a microscope. Annuli can often be seen on whole otoliths, but thick otoliths and those from older fish require sectioning.
Once age is known, measuring growth rate is simple arithmetic. Growth rate can be assessed by the change in length in years. For example, the average length of 4-year-old largemouth bass in a lake is 15 inches. The next year, 5-year-old bass averaged 161/2 inches. Four-year-old bass grew 11/2 inches per year in this lake.
Another way to express growth that is especially meaningful for comparing populations is length at age. In the previous example, 4-year-old bass were 15 inches.
Growth varies widely among populations, and that is why it is such a meaningful population metric. The table accompanying table average lengths in inches at age for several Mississippi sportfish. The lengths are averages for fish from many populations in the eastern United States and can be used to assess whether fish in a particular population have fast, slow or average growth.