Exploring the ecological differences between black crappie and white crappie

Black crappie (Pomoxis nigromaculatus) and white crappie (Pomoxis annularis) are two closely related species that support a vital catch-and-keep fishery in the central and southern United States. Management of these two species is often plagued by poor growth and unpredictable swings in recruitment, leading to undesirable size structures and population sizes. Although we have learned a great deal about these species over the last several decades, a universal solution to these problems has not been found. Oftentimes researchers and managers further complicate the issue by viewing both species as a single entity. Recent research has indicated that although these two species are closely related, they have many interspecific differences in their ecology. My thesis delves into the ecological differences between these two species with the goal of improving our ability to manage their populations. Through a multi-lake study, I analyzed how environmental variables affect recruitment and growth of black crappie and white crappie populations. I found that black crappie year-class strength was positively related to gizzard shad catch per unit effort (CPUE) and temperature, whereas white crappie year-class strength was negatively related to common carp CPUE and largemouth bass CPUE. First-year growth of both species was positively related to surface temperature; however, black crappie first-year growth was negatively related to zooplankton density whereas white crappie first-year growth was positively related to zooplankton density. A two-year telemetry study observing habitat use provided further evidence for interspecific differences in crappie ecology. My study found that black crappie were located more often in shallow, bathymetrically steep locations with sandy substrate and course woody debris, whereas white crappie were located in deeper, less steep locations. No significant difference was found between turbidity and temperature at the locations of the two species. I hypothesize that the interspecific differences I observed in both of these studies are a function of differences in ontogenetic diet shifts. Studies have shown that while white crappie switch from invertivory to piscivory, black crappie will switch at a later age, if they switch at all. I hypothesize that the interspecific differences in habitat use and factors related to growth and recruitment are all associated with differences in diet. Further research should investigate whether these differences are related to speciation, niche partitioning, or some other stimulus. Due to complications related to tag retention during the first year of my telemetry project, I conducted a laboratory test of three attachment methods for saddle style tags. I found that inserting the anchoring wire through the dorsal musculature led to severe dermal irritation and necrosis around the tagging location. Attachment of the tag to the dorsal spines did not lead to necrosis, and the attachment method was improved by the addition of a marine-grade epoxy. None of the methods I tested led to a scenario where healthy fish maintained the tag for more than an average of ten weeks, therefore further research is required before this style of radio tagging is used