Juvenile crappie growth and bioenergetics: implications for management using blacknose crappie

Abstract

Collectively, black crappie Pomoxis nigromaculatus and white crappie P. annularis make up important harvest-oriented sport fisheries across North America, especially in the Midwest. Over the past several decades, the major management issues facing crappie populations have been identified as poor size structure and highly variable recruitment. Although stocking programs have not been historically common for crappie, some recent examples have been successful in supplementing year classes in systems with poor recruitment. In order to differentiate stocked from naturally occurring individuals for evaluations of stocking programs, several management agencies have cultured and stocked blacknose crappie, a phenotypic variant of black crappie. Continued use of blacknose crappie by managers has also stemmed from perceived differences in growth and survival relative to black crappie in rearing environments. My thesis evaluates growth-related differences among juvenile black, white, and blacknose crappies in both experimental ponds and the laboratory. In a common garden pond experiment, I found no differences in survival among strains or ponds. Blacknose crappie outgrew black and white crappies in both length and weight and black crappie outgrew white crappie in just weight. Growth was pond-dependent for all strains, with growth rates being highest in ponds with high zooplankton density, low macroinvertebrate density, and low vegetation density. Turbidity also may have had indirect effects on the growth of black and blacknose crappies, but not white crappie. Across a range of temperatures in the laboratory, I found no differences in metabolic rate among the strains. I also observed superior growth characteristics of blacknose crappie in the laboratory, as they exhibited higher relative growth rates and food conversion efficiencies than black and white crappies. Black and blacknose crappies also had higher maximum food consumption rates than white crappie. No strain-temperature interactions were detected for the suite of parameters measured, suggesting a lack of differential thermal adaptations among black, white, and blacknose crappies. Differences in growth and food conversion efficiency between blacknose and black crappies may be attributable to selective pressures associated with the multi-decadal culture of the strain, especially in a species where survival and handling mortality are highly size-dependent. My laboratory results indicated ontogenetic shifts in thermal optima for juvenile crappies when compared to previous studies with adult white crappie. Results of my thesis can improve future juvenile crappie bioenergetics models and advise management decisions using blacknose crappie. Blacknose crappie may provide a better alternative to other crappies for stocking programs, but potential negative impacts on resident crappie populations need to be investigated in the future due to the origin of the strain coming from a single source population. Blacknose crappie exhibit superior growth characteristics over black and white crappies in rearing environments and these relationships should be evaluated further at larger spatial scales and across varying environmental conditions