Growth is an important component of the ecology of fishes. It has been shown that larger fish are more capable of producing offspring, less susceptible to predation, and are often better competitors than smaller fish. Also, for most anglers, large fish are what fishing is all about.
Why do fish in one lake grow larger than those in another? Numerous studies that have attempted to answer this question indicate that growth can be affected by factors such as temperature, water clarity, food availability, and preferred habitat availability. However, most of this work has either focused on a single species or has been restricted to a solitary lake.
There is a need for the development of growth models comparing multiple lakes and multiple species. The purpose of our project is to develop general predictive growth models for three functional feeding groups of fishes: a piscivore (largemouth bass), an insectivore and planktivore (bluegill), and a benthic omnivore (channel catfish). Fisheries managers will be able to use readily available information from these models to make decisions about fishery operations.
Bluegill, the state fish of Illinois. (drawing by INHS artist Lydia Hart originally appeared in "The Fishes of Illinois" in 1908)
A number of lakes throughout Illinois were sampled monthly to assess several variables that might affect the growth of fishes, such as levels of food resources, limnological factors (physical and chemical characteristics), and fish abundances. These values were then examined for relationships with growth rates of the three fish species.
We examined several factors that appear to affect growth rates. Growth of small bluegill is relatively faster within lakes that have less littoral (shallow water habitat) zone. We believe this is due to predation pressures forcing small bluegill into littoral cover. Higher densities of bluegill in the littoral zone and a lower net energy diet than that in the open water results in slower growth. Lakes with less littoral zone have bluegill feeding on the higher net energy diet of open water zooplankton and higher rates of predation mortality due to a lack of cover. Lower numbers of bluegill and less competition result in faster growth. Also, results indicate populations of bluegill in southern lakes grow more in a year than those in northern lakes.
Largemouth bass growth, at least at smaller sizes, appears to be positively related to the density of forage fish in the lakes. Common littoral zone fishes, such as juvenile bluegill, and golden shiner and other minnows, are an important component in the diet of largemouth bass. Higher densities of these prey fish allow for faster growth of largemouth bass.
Analysis has also revealed that populations of relatively fast-growing channel catfish are found in lakes with larger littoral zones. We believe that with higher proportions of shallow-water habitats the catfish have a larger area in which to feed.
There is still much to learn about what drives growth rates in natural populations of fish. Our preliminary data indicate a number of important factors. Ongoing data collection and analysis will reveal if there are any additional environmental qualities that influence growth rates in fish. Identification of these factors will then enable us to develop growth models for use by fisheries managers.
Sean P. Callahan, David H. Wahl, and Clay L. Pierce, Center for Aquatic Ecology
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