New Recruits

What limits survival of the great largemouth bass? Dr. Hal Schramm talks about young-of-the-year recruitment and habitat.

For a lake to produce great bass fishing over the long haul, you need a steady supply of young largemouths reaching adulthood to replace older fish dying from angling-related mortality and natural causes.

Biologists don’t fully understand the factors limiting bass recruitment (the term for small bass coming of age), but University of Florida research1 sheds light on one of the most important considerations, the relationship between the abundance of adult bass and the success of their off spring.

Density Dependence
In every water body, there’s an upper limit to the total weight of fish it can support per acre. It’s known as maximum density or maximum biomass. The poundage barrier varies from one lake to another and is set by some limiting factor, usually food. Other common culprits include critical habitat and the number of adult fish that spawn.

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There are two types of limiting factors: densitydependent and density-independent. As might be imagined, the independent flavor reduces bass survival regardless of how many pounds of fish per acre. Harsh environmental conditions are one example. Lacking such limits, density-dependent compensation kicks in to keep fish populations in check. For biologists, determining if this is the case and at what life stage density is regulated, is essential to effective management.

A Closer Look
Florida researchers stocked one-acre hatchery ponds with different densities of adult largemouths. Bluegills, sunfish and shiners were stocked to provide forage for the adult bass and their off spring. Yearling largemouth bass were stocked as predators on the bass fingerlings. Stocking rates were within the range of biomass of these species found in Florida lakes. Bass nested in all ponds. The number of successful nests—those producing fry—was positively related to the number of adult bass. Yet, when the ponds were drained in the fall, there was no relationship between the numbers of adult spawners and surviving bass fingerlings. Ponds stocked with three pairs of adults produced as many off spring as those stocked with 20 pairs. Some form of density-dependent population regulation affected the young bass, but how?

The number of small forage fish declined as the density of bass less than a year old (age-0) increased. Further, the average length of the age-0 bass decreased as their density increased. At high densities, the age-0 bass appeared to be food-limited. Food limitation does not directly affect survival of age-0 bass, at least not through their first summer. But when high densities of bass deplete the available food supply, the growth of the young bass slows. And the longer they stay small, the more vulnerable they are to predators.

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, they will eat more age-0 bass in a high-density, slow-growing population than in a less-dense, fast-growing population. Anglers should note two implications. First, fears of bed-fishing destroying bass fisheries—promulgated by some scientists over the last two decades—are without substance in all lakes except those with so few adult bass, they aren’t worth fishing anyway. Second, stocking bass into waters with existing bass populations is a waste. In fact, excessive stocking could even hurt the population.

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