Is there a crappie angler out there who doesn’t want to catch a lot of large crappie? Most crappie fisheries in Mississippi will, at some point in time, provide good numbers of large fish, but the quality fisheries don’t last.
Such is the nature of most crappie fisheries — they are highly variable, oscillating between fisheries with abundant but small fish and those with large but few fish.
The March Gettin’ Fresh column discussed how crappie in a high-density population competed for a limited food supply and had slow growth. As a result, the population produced few large fish.
Conversely, crappie have fast growth and quickly grow to large size when their density is low. The management challenge for sustaining crappie fisheries is stabilizing the abundance of crappie at a moderate level.
Abundance of crappie starts with the spawn. A good spawn and good survival of the young crappie produces a strong year class — the number of crappie produced in a particular year. Crappie that survive the first year in Mississippi waters are large enough that predation becomes a minor factor; and these fish will, in a year or two, recruit to the catchable population.
Therefore, stabilizing crappie abundance requires stabilizing recruitment. Developing management strategies to stabilize recruitment requires understanding the factors that cause the fluctuations.
The animal kingdom is replete with examples of population fluctuations. Some animals are, plain and simple, periodic, such as 17-year locusts. But many fluctuations are caused by predator-prey interactions — the prey population increases, the now well-fed predator population increases and suppresses the prey population. The abundant but now hungry predator population crashes. The prey population, lacking predation, increases, and the cycle repeats.
Crappie fit this classic model to a point. When crappie density is low and food is abundant, the crappie grow fast and survival is high. The abundant forage is conducive to successful crappie reproduction because the fish can channel energy to production of eggs and sperm. Successful reproduction leads to more crappie. The abundant crappie share a limited food supply, growth slows, mortality usually increases and reproduction may falter. This is called density-dependent population regulation, and was discussed in the March column.
But crappie reproduction is strongly affected by environmental conditions. Strong year-classes are often produced when spring temperatures are warm and stable, conditions expected to result in fast growth of the young crappie. Water level fluctuations also affect year-class strength. Weak year-classes often occur when water levels are low in the winter and spring, which possibly results in less spawning and rearing habitat. Weak year-classes also tend to occur in years with high reservoir flow-through in the spring and summer.
Although these relationships were developed from studies in reservoirs, I would expect them to apply to oxbow and abandoned-channel lakes connected to rivers like the Mississippi, Pearl and Yazoo.
Because environmental conditions suppress or benefit fish regardless of the population’s abundance or density, environmental conditions are considered density-independent factors.
The simple conclusion from more than a dozen studies is that both density-independent and density-dependent factors affect crappie populations, but no single factor consistently explains crappie cycles.
Mike Allen at the University of Florida and Steve Miranda at Mississippi State University used crappie data from throughout the country to model changes in crappie populations and explore the separate and interacting effects of density-dependent and density-independent factors on crappie recruitment.
Crappie recruitment fluctuated little when environmental factors were held constant but population size varied; the population model indicated density-dependent factors did not appear to trigger fluctuations in recruitment. On the other hand, crappie recruitment was highly variable when environmental factors fluctuated and density-dependent factors were held constant. The results suggested environmental factors, not crappie abundance, had the greatest effect on recruitment.
But, as is often the case in fishery systems, cause-effect relationships are rarely simple or straightforward.
Environmental factors strongly affected recruitment when crappie densities were low, but a different picture emerged at high crappie densities. High crappie densities suppressed high recruitment, a density-dependent effect, even when environmental conditions favored the production of a strong year-class.
Nice to know what over-the-top population models predict, but fishing doesn’t take place on the computer. Is there any validity to Allen’s and Miranda’s models? To test the model, they used actual environmental and crappie population data from Ross Barnett Reservoir. The recruitment predictions from the model closely followed the recruitment trends observed at Barnett. The model and the conclusions that environmental factors affect recruitment but the magnitude of recruitment is affected by crappie density appear valid.
Why high crappie density damped the environmentally driven recruitment fluctuations remains a question for further research, but the Allen and Miranda model provides some clues for where to start looking. The recruitment-damping effect of crappie density was greater when crappie had slow or moderate growth.
The slower growth is a consequence of greater competition for limited food. Age-0 (young-of-the-year) and age-1 (one-year-old) crappie are more likely to share food resources when crappie have slow growth and age-1 crappie are small. Competition for food may be one reason for high crappie density reducing the abundance of age-0 crappie and suppressing recruitment. A second possibility is cannibalism — age-1 and older crappie eating the very abundant age-0 crappie. A lot of hungry, slowly growing age-1 crappie can eat a lot of newly hatched crappie.
The more managers know about factors affecting fish populations, the better they can manage them. The third part of this mini-series will discuss what managers have done and are doing to sustain quality crappie fisheries.