Index of Predation on Juvenile Salmonids by Northern Squawfish in the Lower and Middle Columbia River and in the Lower Snake River

Abstract

We examined relationships between lower Mississippi River physicochemistry and growth of young (^age 2) blue catfish Ictalurus furcatus, channel catfish /. punctatus, freshwater drum Aplodinotus grunniens, and gizzard shad Dorosoma cepedianum collected from dike fields (river kilometers 669.0-883.2 upstream from the mouth) in September and October 1987-1990. Growth increments of the four fishes were determined from cross sections of otoliths (freshwater drum and gizzard shad) or spines (blue and channel catfishes), and we used regression analysis to assess the effects of annual variability in 15 physicochemical variables on growth. We hypoth- esized that growth of these fishes would be positively related to the extent and duration of the annual flood pulse, as it is in tropical floodplain systems. However, growth increments of age-0 blue catfish, channel catfish, and gizzard shad were related only to length of the growing season. Further, positive relationships between growth and total organic carbon levels (age-1 blue catfish), and negative relationships between growth and total hectares flooded (age-1 gizzard shad) and discharge (age-0 and age-1 freshwater drum) indicated that these fishes exhibited highest growth during years of limited floodplain inundation. These results suggest that for some fishes in large rivers with reduced floodplains, mainstem primary and secondary production may influence growth rates more than allochthonous inputs from seasonal flooding.