Historical Changes in Large River Fish Assemblages of the Americas

<em>Abstract.</em>—The Saint John River is located on the mainland of eastern North America, forming in northern Maine–southeastern Quebec, and flows east and south through New Brunswick. Fish collections were conducted at sites located from 135 to 625 km above the mouth in 2000 and 2001. Methods development trials demonstrated that the highest success was achieved with a standardized netting protocol consisting of a combination of dusk seining, nighttime electrofishing, and short-term gill net sets. A total of 36 species of fish were collected, with the greatest diversity occurring at the farthest downstream site. Upstream migration of anadromous species is restricted by the absence or poor performance of fish passage facilities at the five hydroelectric dams along the river system. The downstream migration of introduced muskellunge <em>Esox masquinongy </em>and upstream range expansions of introduced smallmouth bass <em>Micropterus dolomieu </em>and rainbow trout <em>Oncorhynchus mykiss </em>were observed. Fish species and abundances varied along the river, but the cumulative effects of human activities were not easily identified within the fish assemblage. There may have been a critical threshold within the fish assemblage defined by an accumulation of 20 anthropogenic developments. Cyprinid species declined in abundance and yellow perch <em>Perca flavescens </em>and brown bullhead <em>Ameiurus nebulosus </em>increased in abundance downstream of this apparent threshold.