Catfish feeding ecology and bioenergetics in a large river-floodplain ecosystem

A comprehensive bioenergetic assessment was conducted for blue catfish Ictalurus furcatus and flathead catfish Pylodictis olivaris in the lower Mississippi River (LMR). Objectives were to compare rates of food consumption and energy intake and examine energy partitioning patterns by catfishes in different LMR habitats through time. Catfishes were collected during 1997–1998 from three major riverine habitats—the main river channel, secondary (side) river channels, and floodplain lakes. Diet composition of both species varied among habitats and differed in the energy levels of the foods being consumed. Caloric (energy) densities of foods consumed by catfishes were generally greatest in floodplain lakes, least in the main river channel, and intermediate in secondary river channels. This energetic gradient among habitats was strong for blue catfish and weaker, though similar in direction, for flathead catfish. In spite of the greater energetic benefit of some riverine habitats (e.g., floodplain lakes), estimated caloric intake rates of both species did not vary statistically among habitats each year, which indicated that catfishes did not appear to differentially exploit off-channel habitats in terms of energy intake. Between-year variation in caloric intake was more significant for both species. Mean annual caloric intake rates of blue catfish increased by 31% in 1998. Between-year increases differed among habitats, being greatest in floodplain lakes (53%), least in the main river channel (15%), and intermediate in secondary channels (26%). In particular, blue catfish consumed 151% more calories in floodplain lakes between April and June 1998 compared to the same period in 1997. For flathead catfish, between-year increases in caloric intake averaged 37% between habitats in 1998 and were 56% between April and June. Increases in caloric intake observed for both species in 1998 were attributed to warmer thermal conditions that year, especially during the annual flood pulse between April and June. Despite the energy intake and thermal differences between years, however, partitioning of consumed energy by both species, including that allocated to somatic growth, did not vary greatly among habitats or between years. Results suggested that although different LMR habitats afforded catfishes different energetic returns in terms of foods consumed, realization of these benefits may be influenced by year-specific physical characteristics such as water temperatures (e.g., in terms of caloric intake). Furthermore, energetic benefits may not be accrued by catfishes every year (e.g., in terms of caloric growth). Although probable that different habitats and flooding afford fishes other benefits besides food and energy, energetics of both catfishes appeared only weakly affected by habitat or the annual flood pulse in the LMR.