Patterns in abundance of fishes in main channels of the upper Mississippi River system

Abundance of fishes of the main channels of the upper Mississippi River sys tem and of other large North American rivers is largely unknown because his toric sampling methods have been inadequate. We used a bottom trawl to esti mate spatial and temporal patterns in abundance in the navigation channels of Pool 26 of the Mississippi River and the lower Illinois River. Total bio mass density averaged 21 and 29 kgha(1) in the navigation channels of Pool 26 and the lower Illinois River, respectively. We identified spatial and te mporal patterns in catches of key species using a generalized linear model based on the negative binomial distribution. Some species, including shovel nose sturgeon (Scaphirhynchus platorynchus), are persistent residents of th e main channel. Multiple-season residents, including freshwater drum (Aplod inotus grunniens), rely heavily on the main channel during most of the year but leave it briefly, for example to seek thermal refugia in backwaters du ring winters. We suggest revision of the prevailing notion that main channe ls of large temperate rivers serve mainly as corridors for movement among o ther habitat types.