Multispecies Occupancy Modeling as a Tool for Evaluating the Status and Distribution of Darters in the Elk River, Tennessee

Abstract

Sixteen darter species, including the federally endangered Boulder Darter Etheostoma wapiti, are known to occur in the Elk River, a large, flow-regulated tributary of the Tennessee River, Tennessee–Alabama. Since the construction of Tims Ford Dam (TFD) in 1970, habitat modification caused by cold, hypolimnetic water releases and peak-demand hydropower generation has contributed to population declines and range reductions for numerous aquatic species in the main-stem Elk River. We developed Bayesian hierarchical multispecies occupancy models to determine the influence of site- and species-level characteristics on darter occurrence by using presence–absence data for 15 species collected from 39 study sites. Modeling results indicated that large-river obligate species, such as the Boulder Darter, were 6.92 times more likely to occur for every 37-km increase in the distance downstream from TFD. In contrast, small-stream species were 2.35 times less likely and cosmopolitan species were 1.88 times less likely to occur for every 37-km increase in distance downstream from TFD. The probability of occurrence for darter species also had a strong negative relationship with the absence of cobble and boulder substrates and the presence of high silt levels, particularly for species that require boulder substrates during spawning. Although total darter species richness was similar across all 39 sample sites, the composition of darter assemblages varied substantially among locations, presumably due in part to species-specific habitat affinities and hydrothermal conditions. The use of multispecies occupancy models allowed us to account for the incomplete detection of species while estimating the influence of physical habitat characteristics and species traits on darter occurrences, including rarely observed species that would have been difficult to model individually.

Received August 23, 2015; accepted May 31, 2016 Published online August 12, 2016

Acknowledgments

Funding and support for this project were provided by the USFWS; TVA; U.S. Geological Survey; Tennessee Cooperative Fishery Research Unit; Tennessee Technological University; and the Center for the Management, Utilization, and Protection of Water Resources. This manuscript benefitted from the constructive comments offered by two anonymous reviewers. Many thanks are extended to Mary Jennings, Peggy Shute, and Todd Shaw (USFWS); Charlie Saylor (TVA); and Pat Rakes and J. R. Shute (Conservation Fisheries, Inc.) for sharing knowledge, ideas, and suggestions that greatly improved the quality of the research project and manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.