Big bass, big spawn?

The bragging-sized female bass that anglers love to catch are also the best spawners, producing more and healthier eggs and fry. (Photo by Roger Stegall)

Big, healthy fish are the most productive when it comes to repopulating a lake.

The odyssey to a trophy bass begins with a fertilized egg, but recent research suggests that factors that occur before that egg is released by the female may affect reproduction and recruitment.

Bass management usually isn’t complicated, at least biologically. A few principles keep bass populations producing the quality fishing that anglers enjoy: maintain good habitat for spawning and all life stages, keep growth rates high and keep harvest low enough that some bass survive and grow to acceptable sizes and a few to grow to memorable sizes.

The prevalence of catch-and-release makes harvest a non-issue in most waters. If good habitat is available, the second principle — keeping growth rate high — becomes the primary challenge. High growth rate among bass requires abundant, vulnerable-size forage, but the key to fast growth is abundant forage for each bass. Therefore, fast growth can be achieved by increasing the amount of forage or by reducing the number of bass. Research by Ohio Department of Natural Resource biologists indicates the same conditions needed for fast growth also benefit successful bass spawning.

Fueled by a diet of abundant shad, this well-fed bass can be expected to produce a lot of high-quality eggs.

The study

Sexually mature largemouth bass from 9 to 22 inches and 0.3 to 6.7 pounds were collected immediately before or at the beginning of the spawn in 19 reservoirs ranging in size from 37 to 1,280 acres.

The fish were sacrificed and ovaries and otoliths (ear bones) were removed. The ovaries were processed to determine the number of eggs that would be spawned in the current spawning season and the energy density: the amount of energy per unit weight of the ovary. Higher ovary energy is presumed to indicate better egg quality, as this energy is needed for proper development of the embryo and early survival after hatching. The otoliths were used to age the fish and allow calculation of growth rate.

Standardized electrofishing surveys were used to estimate the catch per hour of largemouth bass. Electrofishing catch rate is a common and effective way to index the density of largemouth bass in a population: more bass per hour electrofishing signals a higher density of bass.

The results

To better help understand the results of this study, the desirable conditions are high fecundity (more mature eggs) and high ovary energy density.

The estimated fecundity of female largemouth bass ranged from 687 to 176,224 mature eggs per female and increased linearly with bass weight. The ovary energy density increased with bass weight.

The relationships of fecundity with bass weight and ovary energy density with bass weight were always positive but differed between reservoirs. The fecundity-bass weight relations were greater for populations that had greater ovary energy density, indicating populations with greater fecundity also had greater energy density.

The relationship between fecundity and bass weight were less positive for slower-growing bass populations and in smaller reservoirs. Similarly, the relations between ovary energy density and bass weight were less positive for slower-growing bass populations and in smaller reservoirs.

Largemouth bass growth, fecundity and ovary energy density decreased with increasing bass density.

Finally, smaller reservoirs and those with lower productivity had more dense, slower-growing bass populations with lower fecundity and lower ovary energy density.

Summing this up, larger and faster-growing female largemouth bass have greater fecundity and higher ovary energy. These females are more likely to be found in larger reservoirs with bass populations that are lower in density.

Implications

Studies on other sportfish have also found positive relationships between female size, fecundity and ovary energy density. These studies have also found that the progeny of larger females also have greater chances of survival. A simple conclusion is this: more large spawners — individuals that are relatively rare in a population — better ensures successful spawning.

Similarly, fast-growing bass are better spawners, and their progeny have a greater chance of survival. These fast-growing bass are most likely to be found in lower-density bass populations in large, productive reservoirs.

This study offers an additional important piece of information for fishery managers: not all bass in a population have equal reproductive output, and the size structure and growth rate of bass should be considered in populations where reproduction may be limited.

A key question awaits an answer: are the offspring from the larger, faster-growing and better egg-producing females more likely to survive their first year of life — which includes their first winter — than those produced by small, slowly growing females from high-density populations? Considering the many environmental and biological factors affecting young bass from hatching and through their first winter, the question will not be easily answered.

The hard work of the Ohio biologists demonstrates that the same conditions that lead to excellent bass fishing — fast growth rate and appropriate harvest to produce a population with large size structure — are the same conditions conducive to successful reproduction.