Historical Changes in Large River Fish Assemblages of the Americas

Abstract

<em>Abstract.</em>—The Ohio River Valley Water Sanitation Commission (ORSANCO), along with cooperating state and federal agencies, sampled fish assemblages from the lockchambers of Ohio River navigational dams from 1957 to 2001. To date, 377 lockchamber rotenone events have been conducted, resulting in the collection of nearly three million fishes, representing 116 taxa, including 7 hybrids, in 19 families. We observed significant temporal trends in Ohio River fish riverwide at the assemblage, guild, and species levels. Modified index of well-being (MIWB) scores and changes in guild structure indicated significantly (<em>p </em>< 0.05) improving fish assemblages throughout the Ohio River. Quantile regression of the abundance of individual species by year revealed significant declines (<em>p </em>< 0.05) in populations of several pollution-tolerant species (e.g., <em>Ameiurus </em>spp., goldfish <em>Carassius auratus</em>) with time, while some intolerant species (e.g., smallmouth redhorse <em>Moxostoma breviceps, </em>smallmouth bass <em>Micropterus dolomieu, </em>and mooneye <em>Hiodon tergisus</em>) have increased in recent years. In all, 40 of the 116 taxa collected in the lockchamber surveys changed significantly over time. Sixteen species did not change. Sixty species could not be analyzed either because of incomplete data or insufficient abundance. Fish assemblage metrics that would be expected to decrease with improving conditions in the Ohio River (percent tolerant individuals, percent nonindigenous individuals, and percent detritivore individuals) also declined (<em>p </em>< 0.05). These changes coincide with marked improvement of the water quality in the Ohio River over the last 50 years, particularly in the aftermath of the Clean Water Act (1972). Some species and metric responses may also be due to the replacement of the 50 wicket dams by the construction of 18 high-lift dams.