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Effects of Turbidity on Growth and Feeding of Juvenile Crappies: The Difference is Black and White
 
by David Bogner, Sara Andree, and David H. Wahl, Kaskaskia Biological Station, Illinois Natural History Survey
 

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.
 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. 
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.

 Photo 3.  Various sizes of Daphnia used in a single prey study (top).  Dorsal and lateral view of Chaoborus used in a multiple prey study (middle) and an example of Chironomus used in a multiple prey study.
 Photo 3. Various sizes of Daphnia used in a single prey study (top). Dorsal and lateral view of Chaoborus used in a multiple prey study (middle) and an example of Chironomus used in a multiple prey study.
 

Contrary to the hypothesis, black crappie outgrew white crappie in the control, nutrient treatment, and nutrient/sediment treatment, and growth of black and white crappies was similar in the sediment treatment. The greater growth of black crappie compared with white crappie suggests competitive interactions between the two species with the black crappie having an advantage in most scenarios tested. Siltation-driven turbidity appears to negate any advantage that black crappie have over white crappie, allowing white crappie to dominate in these systems.

To further explore mechanisms that may affect growth, we completed a second feeding study in a small aquaria to measure total consumption and size selection of a single free-swimming zooplankton prey type, Daphnia, in low, moderate, and high levels of turbidity (Photo 2). In addition, we measured the selection among three prey types: Daphnia, free-swimming Chaoborus larvae, and benthic (bottom-dwelling) Chironomus larvae (Photo 3), as well as consumption and total diet energy values in low, moderate, and high levels of turbidity. Black crappie were expected to forage less effectively in turbid treatments, while white crappie would be less negatively affected by turbidity. However, we observed the opposite.

Black crappie consumed more prey than white crappie in all situations, and had an increased diet weight in turbidity when only Daphnia were offered. In low turbidity, black crappie were not at all size selective of the Daphnia they consumed, while white crappie were size selective. When multiple prey types were offered, both species universally preferred Chaoborus larvae and avoided Chironomus, indicating a tendency to forage in the water column rather than near bottom sediments regardless of environmental conditions. Neither species preferred Daphnia, although black crappie particularly avoided them.  

Study results highlight potentially significant differences between juvenile black and white crappies. Turbidity does influence the growth rate of crappies, but only siltation-driven turbidity apparently affects black crappie. Overall, black crappie continued to consume more prey than white crappie, even in high turbidity. However, due to decreased visibility, the crappies likely expended more energy locating prey than they might in clearer water. The shift from larger to smaller zooplankton in the sediment treatment of the tank experiment would have made this strategy ineffective, which may contribute to previous observations indicating reduced growth of black crappie in turbidity. These findings help to inform management of these important sportfish species by highlighting lakes where each of these species may be more successful.

 
 
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