Multiple studies find no effect
It’s that wonderful time on the bass-fishing calendar when bass move to predictable, shallow-water habitats to spawn. The egg-laden females are at their heaviest weight, and the nest-guarding males tend to be more aggressive.
Some anglers question the effects of bed fishing on bass populations. Studies in the 1990s demonstrated that bed fishing can adversely affect the production of fry for both largemouth and smallmouth bass. The longer a guarding male bass is kept away from a nest, the more predators like bream and shiners are likely to consume the eggs or fry. And the more eggs or fry removed by the nest-robbing predators, the more likely it is that the buck bass will abandon the nest completely, leaving the remaining eggs vulnerable to more predation, siltation, and fungus.
Fewer fry produced means fewer adult bass available in the future, right? Well, maybe yes, maybe not. Several studies indicate little effect of catching spawning fish on largemouth bass recruitment.
Pond fishing studies
Florida Fish and Wildlife Conservation Commission fisheries scientists evaluated the effect of controlled bed fishing on Florida bass in one-acre hatchery ponds. Ten male and 10 female bass were stocked into each of nine research ponds. Brush and cinder blocks were added for cover. The ponds were stocked with bluegill, Seminole killifish, and mosquitofish to provide forage for the adult bass and their offspring and to replicate the presence of potential nest predators.
Anglers attempted to catch the bedding bass in five ponds (fished ponds). Bass caught from the beds were held for one hour in a net enclosure and then released. Nests were monitored by snorkel divers and judged as successful if they produced swim up fry.
Across both years of the two-year study, bass were caught off 44 of the 91 nests fished in five ponds — not all spawning bass are catchable. Of the 44 nests from which bass were caught, 21 were successful (produced fry) and 23 were unsuccessful. Across both years, 41 percent of the nests produced bass fry in the fished ponds, and 54 percent of the nests produced fry in the unfished ponds.
The ponds were drained in the fall, and young bass counted. The difference in young bass per pond between fished and unfished ponds was not statistically significant.
University of 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.
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. Yet, 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 similar density of young bass at high and low numbers of spawning bass suggested density-dependent population regulation.
The average size of the age-0 bass and the number of small forage fish declined as age-0 bass density increased. At high densities, the age-0 bass appeared food limited and their growth slowed, making them more vulnerable to predation by the yearling bass. Further, because predators satisfy their energy requirements by weight — not numbers — consumed, the predators would eat more slow-growing age-0 bass than larger, faster-growing fish. A lot more small, slow-growing bass would be eaten in the high-density ponds, reducing their density to levels similar to the low-density ponds.
A water body will only produce a certain maximum number of young bass, and spawning bass in a healthy population can produce more offspring than needed to sustain the population.
Large-water modeling studies
Pond studies provide useful information, but do they apply to larger fisheries? University of Florida fishery scientists used a robust population model to identify conditions where spawning-season protection may benefit largemouth bass populations. Like most population models, their model used established values of bass growth, natural mortality, catch and harvest, and recruitment rates to predict changes in the bass population; but this model also incorporated new knowledge about the effects of fishing for spawning bass — namely, the loss of eggs and fry when females are harvested before they spawn or males are harvested while guarding nests — and functions for density-dependent mortality of young bass as summarized above.
For bass fisheries typical of those in most Mississippi reservoirs, catch-and-release-only fishing during the spawn increased the adult bass population eight percent compared to a year-round open fishery with a 14-inch minimum length limit. No fishing at all during the spawn increased the adult bass population 20 percent compared to the year-round open fishery. The greatest increase in adult bass abundance — 42 percent compared to the year-round open fishery — resulted from the year-round catch-and-release fishery. Importantly, spawning was sufficient to sustain bass populations under all regulation scenarios.
Despite documented adverse effects on nest success of individual bass, these three population-level analyses indicate no adverse effects of fishing during the bass spawn at present levels of fishing if fish are handled carefully. If anglers really want more bass to catch — a predicted 42 percent more — they need to consider year-round catch and release.