Illinois Natural History Survey - University of Illinois

Critical Factors in the Early Life History of Illinois Fishes

Fish communities in Illinois reservoirs can be influenced by changes in environmental conditions throughout the year. Within these fish communities, larval or young fishes are often the most susceptible to these fluctuations in abiotic and biotic conditions. During the critical early life stage of fish, slight changes in the environment can drastically affect the recruitment of young fish into the adult population. In fact, of the vast number of larvae that hatch, only a small percentage actually reaches the juvenile or adult stage. Even though biologists have recognized the importance of identifying the factors influencing recruitment, most studies have only dealt with a single lake or reservoir system. As a result, environmental variation is minimized due to restrictions of space and time. Our study was designed to examine several abiotic and biotic factors thought to determine larval fish growth rates across a series of Illinois reservoirs.

Locations of 21 Illinois reservoirs that were sampled to encompass range of
limnological and morphological characteristics.

We sampled 21 reservoirs (see map) every other week from May through July 1995. On each date, we collected larval fish and zooplankton samples and measured temperature, dissolved oxygen, and secchi disk depth. Lake morphometrics and water chemistry data were obtained from previous lake surveys. Principal Components Analysis (PCA) was used to reduce the large number of variables to a smaller set of factors. The PCA scores were then correlated to larval fish growth rates for each reservoir.

The most abundant larval fish species across reservoirs were gizzard shad (Dorosoma cepedianum), sunfish (Lepomis spp.), crappie (Pomoxisspp.), and brook silversides (Labidesthes sicculus). The peak densities of these young fish varied greatly across reservoirs and ranged from 1 to 250 fish per liter. Zooplankton levels (the major food source for young fish) showed similar high variation across reservoirs and through time.

The first principal component (PC 1) is composed of mean depth, shoreline length, and the surface area of the reservoir. These variables accounted for the most variation among the reservoirs but did not correlate with growth rates of any larval fish species. Lake size, then, did not influence larval fish growth rates in any way.

Otolith bones in larval fish used to determine age and growth.

The next component was described by zooplankton abundance and secchi disk depth (PCA 2). The correlation between the two variables supports the previously identified relationship between zooplankton dynamics and water clarity. The explanation is that an increase in light penetration results in an increase in algae, providing more food for more zooplankton. Since zoo-plankton are the primary food source during the early life of most fishes, we expected to find that this component (PC 2) would influence growth for all species; however, this was not the case. Gizzard shad growth rates were not correlated with zoo-plankton or turbidity (PC 2). In fact, the only environmental factors that slightly correlated with shad growth rates were temperature and latitude, the next component (PC 3). These variables also showed a similar weak correlation with growth rates for other species. Larval fishes in northern Illinois reservoirs grew slower than fishes in southern Illinois reservoirs likely due to temperature influences. Unlike shad, important factors in determining growth rates of other larval fishes were zooplankton, turbidity, and total larval fish densities.

Our study identified important factors influencing the growth rates of larval fishes and provides predictions regarding fish growth and success across lakes and reservoirs in Illinois. Gizzard shad was the only species not influenced by environmental factors, and may potentially explain why this species can pose overwhelming population problems in our lakes and reservoirs. In fact, larval gizzard shad are usually abundant and reach large sizes quickly, which is an argument for not using shad as a forage for native game fishes. In addition, the introduction of this fast-growing species may also have adverse effects on the growth of young native fish species. Future work will investigate the possibility that an early diet shift from zooplankton to detritus (organic matter) allows shad to grow independently of environmental influences.

Randall Claramunt and David H. Wahl, Center for Aquatic Ecology


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