Self-regulating bass?

Success of recruitment not related to the number of spawners.

Recruitment of young fish is essential to sustained fishing. Despite the amount and intensity of management directed at largemouth bass, America’s most-popular gamefish, the relationship between the abundance of adults and their offspring has not been clearly established. Pond studies from the University of Florida shed light on this critical question.

The Florida researchers stocked one- acre hatchery ponds with three to 20 male-female pairs of adult largemouth bass. Bluegill, redear sunfish, and golden shiners were stocked to provide forage for the adult bass and their offspring. Yearling largemouth bass were stocked as predators on the soon-to-be-produced bass fingerlings. Stocking rates for all fish were within the range of biomass of these species found in Florida lakes.

The bass nested in all ponds, and the numbers of successful nests — those that produced fry — was positively related to the number of adult bass. But when the ponds were drained in the fall, there was no relationship between the numbers of adult spawners and recovered bass fingerlings. The ponds stocked with three pairs of adults produced as many offspring as those stocked with 20 pairs.

Density dependent population regulation, which is commonly observed in many animal populations, occurs when some feedback mechanism — like disease, competition for food and starvation or predation — limits the number of individuals in the population to the number the environment can support. The density of young bass in the Florida ponds remained similar no matter how many nests were successful. Clearly, density dependent population regulation occurred, but how?

Relative density

The number of small forage fish declined as the density of Age-0 bass increased. Further, mean length of the Age-0 bass decreased as their density increased. At high densities, the Age-0 bass appeared food limited.

Food limitation does not directly affect survival of Age-0 bass, at least not through their first summer. When high densities of bass deplete the available food supply, the growth of the young bass slows. The longer they stay small, the more vulnerable they are to predators. In this experiment, yearling bass were the predator.

Predation on slow-growing, Age-0 bass is a double-edged sword. First, the smaller bass are vulnerable to a greater number of predators. Second, because predators satisfy their energy demand by weight consumed, not numbers, the yearling bass will eat more Age-0 bass in a high-density, slow-growing population with small Age-0 bass than in a less-dense, fast-growing population where the Age-0 bass are larger.

This simple and long-overdue study did not produce any revelations, but it provided solid scientific evidence to support the reasoning that good managers have applied for many years. There are two implications that should make anglers take notice. First, the fear factor about bed-fishing destroying bass fisheries promulgated by some scientists over the past two decades, and the need to protect spawning bass, are without substance in all lakes except those with so few adult bass that they probably aren’t worth fishing.

The second implication is that stocking bass into waters with existing bass populations is a waste. Indeed, excessive stocking could even lower the recruitment if the growth of Age-0 bass, both wild-spawned and stocked, is strongly suppressed.