Biologists recognize that hybridization between freshwater fish is more common than for any other type of vertebrate. Sunfish, including bluegills, redear, green sunfish, and pumpkinseed, readily hybridize, particularly in altered habitats, where spawning areas are limited, or when one species is introduced into waters where another species had solely existed. Some crosses have been recommended for stocking private waters.
Due to habitat alterations and widespread introduction of black bass species outside their native ranges, hybrid black bass are increasingly common. While anglers may find occasional catches of hybrids a curiosity, the loss of genetic adaptations honed over millions of years, which occurs with such genetic mixing, is not obvious; yet it represents a threat to the quality of bass fishing.
Meanmouth Bass: In the mid-1960s, Dr. William Childers and colleagues at the Illinois Natural History Survey began studies of centrarchid (sunfish family) hybrids. In the lab, they produced some oddballs—crosses of largemouth bass with warmouth, green sunfish, and bluegill. Crosses with crappie and rock bass failed.
The researchers noted that different black bass species didn’t hybridize when stocked in ponds with members of another species (i.e., all males of one species with all females of another). But fertilizing largemouth eggs with smallmouth sperm produced viable offspring that reproduced among themselves and with both parental species.
The term “meanmouth bass” was born when Childers observed a school of largemouth-smallmouths attacking a female swimmer. “The bass leaped from the water and struck her on the head and chest,” he wrote, “and drove her from the pond.” On another occasion, he watched meanmouths attack a dog that ventured into shallow water.
Though indications of hybrid vigor were evident in aggressiveness and fast growth, high mortality and low reproductive rates for the hybrids led to a halt of this investigation in the 1980s. Childers cautioned that backcrossing of hybrids with parental species would be harmful, since gene flow between the species would reduce the fitness of populations as maladaptive genes were introduced. Over 30 years ago, he urged caution in mixing bass subspecies and even geographically separated populations of fish of the same species.
In nearly all cases of hybridization outside the lab, smallmouth have been involved. Geneticist Dr. Dave Philipp, colleague of the late Dr. Childers, noted that fertilization of largemouth bass eggs with smallmouth sperm resulted in more successful crosses than the reciprocal cross (largemouth male and female smallie). The aggressive male smallmouth bass may be an instigator when introduced into waters outside its natural range where spawning sites are limited, or in altered habitats such as reservoirs.
When smallies were added to newly constructed Squaw Creek Reservoir in Texas, they soon hybridized and backcrossed with both northern and Florida subspecies of largemouths that were already in the impoundment. In 1993, Rich Fry caught an 8-pound 3-ounce bass from a Pennsylvania mine pit that was genetically identified as a first-generation hybrid of a largemouth and a smallmouth bass.
More Crosses: By the late 1960s, stocking of spotted bass in central Missouri had led to hybridization and genetic swamping of smallmouth populations. Today, backcrossed mixes of spots and smallies are increasingly common in central Missouri streams and in reservoirs such as Table Rock, where several state records have been set, up to 5 pounds 10 ounces. Due to their fighting power, they’re locally known as “meanmouth bass,” but this confuses the original meaning of the term. In 2006, an 8-pound 5.6-ounce spot-smallmouth hybrid was caught in Oklahoma’s Veteran’s Lake, a new state record and the largest black bass hybrid on record.
In north Georgia and Alabama, introductions of smallmouths into spotted bass water and of spots into smallie water led to hybridization and mixing of genotypes, compromising the adaptive characteristics of each species in these waters.
Beginning in 1974, smallmouths were stocked into central Texas streams where only native Guadalupe bass had existed. Within a decade, extensive hybridization and backcrossing occurred. To preserve the few remaining pure populations of Guadalupes from extermination by genetic swamping, smallmouth stocking has ceased here, sanctuaries have been established, and captive-bred Guadalupes planted in streams to buoy their numbers.
Hybridization of smallmouth and redeye bass also has occurred in the Upper Cumberland River watershed of Tennessee, where introduced redeyes hybridized with smallmouths, resulting in more than half the bass being crosses. A research group including Dr. Philipp has recently labeled the Florida bass a separate species—not a subspecies of largemouth as traditionally thought—based on analysis of mitochondrial DNA. Many studies of this cross have been done over the last 30 years.
While some short-term hybrid vigor may be noted, long-term loss of fitness is inevitable when black bass hybridize. Recent investigations show that “outbreeding depression,” a measureable descriptor of loss of fitness, occurs when populations are mixed.
While it may be hard today to find black bass populations unaffected by stock transfers, it’s imperative to keep them pure by restricting any transfers into those watersheds, and to limit further mixing of stocks of black bass.