Interactive Effects of Temperature and Predation on Juvenile Chinook Salmon

Predator-prey interactions are mediated by multiple abiotic factors such as habitat availability, water quality, refuge, and temperature; all of these factors vary both naturally and in response to ongoing changes in human land use and regional climate.  For freshwater fishes, the impacts of temperature are particularly vital to understand given projections of increased temperature and reduced stream flows for many regions due to climate change and water use.  Although many studies have been conducted on the separate effects of predation and temperature, the interaction of the two has seldom been investigated.  While the indirect effects of predation can be equally or more important than direct consumption in many systems, these are rarely tested experimentally. This represents a significant gap in our understanding of how predator-prey relationships will change with shifting temperature regimes.

Here, we examined the effects of increased temperature on the vulnerability of juvenile salmon to the direct (mortality) and indirect (behavior and reduced growth) effects of predation by smallmouth bass (Micropterus dolomieu).  Juvenile spring Chinook salmon (Oncorhynchus tshawytscha) were stocked into large, semi-natural stream channels with one of four treatments: control, predator present, increased temperature, and increased temperature + predator present.  Control temperatures were held at 15° C and increased temperature at 20° C.  Feeding, antipredator behavior, and activity levels of Chinook prey were observed over 2-day trials.  Predator activity and prey mortality were also monitored. 

Contrary to expectation, there was no significant difference in direct predation with warmer temperatures, but there were differences in prey activity and feeding patterns. At the conclusion of the experiment, surviving Chinook were sampled for blood glucose levels to indicate the effect of treatments on long-term stress and growth.  These integrated behavioral and physiological results will be discussed in the context of field studies which predict changes in temperature and predation pressure in salmonid rearing habitat over the next several decades.