Modeling the Response of Salmonid Carrying Capacity to Changes in the Prey Base

We developed a framework for determining the co-occurrence of juvenile spring Chinook salmon with agricultural insecticides in the Willamette Basin, Oregon. As part of our assessment, we developed a prey base index (PBI) that incorporaties diet composition and prey availability to evaluate the indirect effects of the insecticide mixture on juvenile salmon production occurring as a result of a reduction in the prey base. Though related, diet composition and prey availability were considered independently. Diet composition represents the preference for prey taxa. It is expressed as the relative proportion of prey taxa ingested by juvenile Chinook salmon. Prey availability was allowed to vary in response to the concentrations of the insecticides. When considered in combination with diet composition, the PBI weights the importance of reductions in prey availability.  Our analysis focused on fish use of backwater and off-channel habitat units where they generally coincide with agricultural lands. About one-third of adult equivalent juveniles use backwater, off-channel habitat at some point for juvenile rearing.  Only 13% rear in backwater, off-channel habitat near agricultural land.  Using our framework, we estimated that percent reduction in adult equivalents as a consequence of reduced prey was about 5%.  Exposure to reduced levels of prey could result in several different responses accounting for reduce production, including: departure from the area by some juveniles to find better feeding opportunities, reduced growth of fish remaining in the area, and delayed smolting for some of the fish experiencing reduced growth. Overall, the PBI was a useful metric for quantifying the indirect effects of agricultural insecticides and has potential application for evaluating indirect effects of other stressors on juvenile salmonids.