Juvenile Salmonid Prey in the Lower Columbia River: Availability and Consumption of Invertebrate Prey Across Multiple Sites and Years

Macroinvertebrate samples were collected from multiple sites along the Lower Columbia River in 2008, 2009, and 2010, in an effort to characterize the diversity and abundance of prey available to juvenile salmon. We were especially interested in the spatial and temporal patterns of macroinvertebrates in off-channel and side-channel habitats that are critical for juvenile salmon rearing. We collected invertebrates in open water reaches and along river margins (where emergent vegetation was present) several times throughout the juvenile salmon growing season. Invertebrate sampling coincided with salmonid sampling, and when possible salmon diets were collected as well. Analyses of over 100 invertebrate samples and nearly 500 juvenile Chinook salmon diet samples indicate invertebrate diversity in both the habitats as well as the diets was highly variable in space and time. Juvenile Chinook salmon did appear to be selective, consuming primarily midge pupae (Chironomidae), even when other smaller bodied prey items were abundant (e.g. Cladocerans). Aquatic invertebrates were much more abundant in samples collected from margin habitats with extensive emergent vegetation (as compared to open water samples), indicating that the extent and condition of habitat margins may be important in understanding prey production and availability in these off-channel and side-channel habitats. Although riparian habitats were often intact and extensive at these sites, terrestrial insects derived from those areas generally made up less than 30% of the prey items. We anticipate that a better understanding of prey availability and consumption over space and time will help explain fish-use patterns and fish growth in the Lower Columbia River and Estuary.