Growth to maximum size
Unlike birds and mammals that rather quickly, relative to their life span, grow to adult size, fish continue to grow throughout their lives. Every species of fish has a maximum size. For example, largemouth bass grow larger than smallmouth or spotted bass. Blue and flathead catfish grow larger than channel cats. Redear sunfish (shellcracker) usually grow larger than bluegill, but black and white crappie grow to about the same maximum size.
Although the maximum size differs among species, the pattern of growth to their maximum size is roughly the same for all fish. Initially, the fish grow quickly in length, but growth in length slows as they get larger. As the fish approach their maximum size, growth in length increases very slowly. For this reason, biologists call the maximum size that a fish attains asymptotic length - a length that an individual fish approaches but rarely achieves.
When a fish reaches the theoretical maximum length - the asymptotic length - increases in length are very small, but the fish often continue to gain weight.
While biologists recognize that every species has a maximum size, maximum size can vary within a species. These differences have a genetic basis and are heritable. For example, the Alabama strain of spotted bass has a greater maximum length than the Kentucky strain, and coppernose bluegill tend to grow larger than "plain old" bluegill.
Of course, attaining maximum size requires that the fish survive to older ages.
Growth rate is a measure of how fast a fish increases in size, and size is usually measured as length. Growth rate is measured various ways, but one of the more meaningful measures is called length at age. For example, largemouth bass in a fast-growing population may be 10 inches long at age 1, 14 inches at age 2, and 16 inches at age 3; largemouth in a slow-growing population may be 8 inches at age 1, 11 inches at age 2 and 12 inches at age 3. Fast-growing white crappie may reach 5 inches at age 1, 8 inches at age 2, and 10 inches at age 3, while the same species in a slow-growing population may only grow to 7 or 8 inches by age 3.
Many factors affect growth rate, but length of the growing season and food supply usually have the largest effect. The effect of food supply is obvious. Abundant food results in fast growth. If food is limited, the growth potential is not realized.
Abundant food, however, only benefits growth if the food is vulnerable to the predator - the forage must be of a size that the predator can consume and be located where the predator can catch it.
Fish in the southern states usually grow faster than the same species in northern states. The reason is that the growing season is longer. But just what is the "growing season?" The sport fish that we enjoy in Mississippi are classified as warmwater fish because they grow little, if at all, at temperatures below 60 degrees. A white crappie in Mississippi will grow faster than a white crappie in Minnesota because the number of days when the water temperature is above 60 degrees is greater in Mississippi.
The longer growing season results in faster growth rate only if vulnerable food is abundant.
Fish must survive to grow large. Natural mortality, principally predation, affects survival of small fish, but fishing mortality determines the survival of the larger fish.
Because each species of fish tends to grow to a maximum size, growth rate determines how long a fish lives. Indeed, fast-growing fish die young, slow-growing fish of the same species live longer. For example, largemouth bass in Mississippi may live to be 10 to 12 years old, while the same species lives 20 years in New York. Crappie live 5 or 6 years in Mississippi but 8 or 9 years in Wisconsin. The growing season is longer, the fish grow faster if food supply is abundant, and they reach maximum size sooner.
So why do fish tend to be larger in the South? In the case of largemouth bass, larger fish may be because the fish are the Florida subspecies of largemouth bass. The Florida subspecies has been proven to grow larger than the northern subspecies, and the 18-pound giant caught in Natchez State Park Lake was a Florida largemouth bass.
Florida largemouth bass aside, biologists don't have a tested answer to the question of why northern largemouth bass and other warmwater game species grow larger in the South. Possibly it is because they grow quickly to large size before mortality reduces their numbers. If that is the case, the amount of fishing effort and anglers' willingness to release fish have a lot to do with the average size of fish available to be caught.