Black crappie (Pomoxis nigromaculatus) and white crappie (Pomoxis annularis) are two popular sportfish species distributed widely throughout much of the United States. Maintaining crappie populations is a concern because their numbers tend to fluctuate considerably in terms of recruitment (number of young fish surviving to adulthood), abundance, and growth. Such fluctuations often result from several environmental and biological variables working concurrently. However, one important variable affecting crappies is turbidity, a measure of water clarity. Turbidity alters the behavior of many aquatic species, including crappies, often resulting in reduced growth, changes in abundance, and shifts in feeding preferences.

	Photo 1. Tank array at the Kaskaskia Biological Station used to evaluate effects of sediment and nutrient turbidity on growth of black and white crappie. Closeup photo shows an example of a turbid and control treatment.

Humans often influence turbidity, and thus its effects on aquatic organisms are particularly worrisome. Practices such as boating and reservoir water-level changes can suspend bottom sediments and reduce visibility by increasing inorganic turbidity. Runoff from the surrounding land can add nutrients to nearby aquatic systems, increasing the production of phytoplankton and raising organic turbidity. High levels of both types of turbidity have been linked to changes in aquatic organism biology.

Responses to turbidity are not universal and may vary, even among closely related species, such as black and white crappies. In fact, although crappies were traditionally managed as one functional group, recent studies have indicated that life history differences between the two species may vary more than previously thought, potentially resulting in differences in response to environmental conditions such as turbidity.

Photo 2. Experimental aquaria showing low (left), moderate (middle), and high (right) turbidity treatments.

In general, white crappie are thought to be more tolerant of turbidity than black crappie, resulting in reduced growth of black crappie. Additionally, the two species tend to display differences in prey choice, with white crappie switching from zooplankton to fish prey early in life and black crappie preying largely on insects for an extended period. Researchers from the Illinois Natural History Survey’s  Kaskaskia Biological Station have conducted several studies to investigate the impact of turbidity on growth and foraging of juvenile crappies to identify potential mechanisms that drive the recruitment success of crappie populations.

The first study was conducted to determine if the type of turbidity (sediment versus nutrient) elicited different effects between the two species. Nutrients and sediment were added to tanks (Photo 1) to replicate Illinois reservoirs and create four different treatments: control, nutrient, sediment, and nutrient/sediment treatments. Because black crappie are associated with clear water, researchers hypothesized that black crappie would exhibit the largest growth in the control and nutrient treatments, and white crappie would outgrow black crappie in the sediment and nutrient/sediment treatments.