Updated status of Bayou darter, a narrowly ranged endemic in a geomorphically active watershed

Abstract

Comparison of historical and current data are critical in establishing population trends for imperiled taxa. In this paper we revisit the status of the Bayou darter Nothonotus rubrum, an endemic fish restricted to the Bayou Pierre river system, Mississippi, USA. The Bayou Pierre has experienced substantial geomorphic change in the past century, leading to questions about persistence of this imperiled species. We employed historical field notes and museum records to identify collection localities, and we resampled 32 of these localities using methodologies comparable to the original samples. We further sampled an additional 10 sites with similar methodologies to fill in spatial data gaps. Rates of capture and numbers of individuals were similar between historical and contemporary samples; however, collections with multiple individuals in contemporary samples were largely restricted to the upstream periphery of their historical distribution. Qualitative comparisons of numbers of individuals caught over several decades using variable methodology suggest that declines in abundances have happened since the species was first described. Bayou darter occupancy was positively related to stream link magnitude (beta estimate = 3.07), and N-mixture modeling estimated contemporary abundance at 7.36 ± 3.83 individuals per site. Bayou darter counts were negatively related to variance in kilometer-scale stream elevation (beta estimate = -0.60). Our results suggest that this species remains imperiled and is experiencing a declining range, and that continued efforts to study and monitor this species, and to reduce geomorphic change in the system, are merited.