Among largemouth bass, wild and hatchery populations fight for genetic dominance

Many anglers headed to lakes, rivers, ponds, and streams have just one goal in mind: to catch a largemouth bass.

What are Largemouth Bass?

The largemouth bass, Micropterus salmoides, is practically synonymous with fishing in America, especially for anglers in bigger waters in the central, southern, and eastern parts of the country. Olive-green to greenish gray and marked by dark horizontal lines, these carnivorous freshwater fish can reach up to 2 feet or more in length and weigh more than 20 pounds. The fish put on a fight when hooked, often becoming dramatically airborne, and anglers can spend years perfecting their techniques.

The largemouth bass is so central to sportfishing, in fact, that it has spawned an industry centered around it, from bass boats that cost more than many luxury cars to bass-specific depth finders and targeted lures. There are tournaments, festivals, a chain of superstores and, perhaps unsurprisingly, a chart-topping country song (“I’m sure it’ll hit me/When I walk through that door tonight/Yeah I’m gonna miss her/Oh, lookie there, I’ve got a bite”).

But largemouth bass' popularity as sport fish cannot change one simple fact: stocked populations of largemouth bass are mixing with wild populations, and their genetics are not identical.

Species of Largemouth Bass

There are two recognized subspecies of largemouth bass: the northern (M. salmoides salmoides) and the Florida (M. salmoides floridanus). The two species diverged due to geologic changes. Both species have been stocked in waterways internationally, from Italy to Thailand, with record specimens being pulled from waters in Japan and South Africa.

The two largemouth bass species have blended genetically, reports the Texas A&M University AgriLife Extension, and crossbreeding, to create a species that is better adapted to cold waters while also being large and easy to catch, is popular. Crossbreeding results, the college reports, generally only last for a year, after which spawn backcross with the parent species and the new sets of genes dominate and one or more of the qualities introduced by hybridization are lost.

The widespread stocking of largemouth bass has influenced the species well beyond where they are introduced, according to a 2012 study in Transactions of the American Fisheries Society. In the study, researchers sampled the genetic structure of largemouth bass in nonstocked, wadeable streams in central Texas and found both largemouth and Florida bass mitochondrial haplotypes everywhere they looked.

“The presence of Florida bass haplotypes at the sampling locations indicates that the influence of stocking reaches far beyond managed reservoirs,” the authors wrote. “The admixture of nonnative genetic material can increase genetic diversity of native populations, but outbreeding depression, competition, and other negative impacts are of concern. Although the stocking of nonnative Florida bass in reservoirs may enhance fishing opportunities, it also has the ability to alter stream systems that are directly connected to stocked reservoirs.”

Species of Largemouth Bass and Genetic Diversity

Largemouth bass genetics is a matter of much debate for wildlife managers responsible for stocking, notes Dr. J. Wesley Neal from the Mississippi State University Extension, and especially so when it comes to F1 hybrids.

“In some cases, a phenomenon called outbreeding depression can occur, which causes slower growth, reduced survival, or low spawning success,” he wrote. “This can get worse each successive generation. As generations continue to interbreed, the lake ends up with a bunch of F_x generation bass (mixture of different generations), with genetics that are likely to be less well adapted compared to the first hybrid or either parent species.”

A moderate lack of genetic diversity may be invisible to the untrained eye, but if left unchecked, it can over time result in deformities, reproduction abnormalities, decreased growth rates, and decreased survival chances.

While such a lack of diversity may be slow to show in river systems, it may quickly be apparent in stocked ponds, Neal noted in a different article. For example, he said, a 20-acre pond may be stocked with several thousand bass which originated from a single hatchery, which in turn come from a small group of adult bass, which themselves came from a single source.

“Even if we assume good broodstock selection procedures and unrelated fish are spawned together, there is still significant risk of a genetic bottleneck,” he wrote. “For example, hatcheries usually put a few male and a few female bass together into each spawning pond to get fingerlings. Each female bass will produce at least 2,000 eggs per pound of body weight, so a 4-pound female may lay 8,000 eggs or more per spawn. Because small largemouth bass are highly cannibalistic, hatcheries like to maintain and sell fish that are very similar in size. This is easier to do when they all come from the same spawning event (i.e., the same parents).”

The end result: those thousands of fish dropped into the pond could have been bred from the same parents and suffer from a small founder population—the same problem confounding species today such as the bison and mountain lion. But in the case of largemouth bass and a small stocked pond, the solution may be as simple as throwing in a few new fish from a different supplier each year.

Genetic wrangling runs up against more problems when stocked fish are dumped into larger bodies of water, according to a 2012 article in Proceedings of the Southeastern Association of Fish and Wildlife Agencies. In that study, game managers stocked a reservoir in Arkansas with the Florida largemouth bass to boost sportfishing in favor of the native populations.

“Despite concerted and continuous stocking of Florida largemouth bass within tributary embayments of large reservoirs, incorporation of Florida largemouth bass and their alleles as demonstrated by q-values into the existing LMB population was consistently low in all samples,” the authors noted.

The idea that large hatchery stock sizes could boost genetic variation was also the finding from a 2019 study in Scientific Reports.

“Effective population size in hatcheries could promote high levels of genetic variation among individuals and minimize loss of genetic diversity,” the study found.

Despite problems with hatchery stock, there’s good news for those who like their largemouth populations wild.

“There is relatively high genetic diversity of wild largemouth bass populations in North America, and significant genetic differentiation among localities or populations in the largemouth bass, but obviously lower genetic diversity in hatchery stocks,” the study found.