Masters Thesis

Effects of turbidity on foraging efficiency and growth of salmonids in natural settings

Elevated turbidity and suspended sediment loads have been documented to negatively effect salmonids and their habitats. Laboratory studies have demonstrated the effect of elevated turbidity levels on the physiology and behavior of salmonids. However, the effects of turbidity on fish in a natural setting are largely uncertain. The goal of this study was to extend the understanding of turbidity effects to natural settings in coastal northern California where impacts to salmonids may occur both as a result of impaired visual capability and reduced prey availability. I compared foraging success and growth of rainbow trout/steelhead (Oncorhynchus mykiss) in reaches immediately above and below a sediment point source in each of two tributaries of the Mad River (Maple and Cañon creeks) during small storm events in 2004-2005 and 2005-2006. Movements of tagged individuals within and between reaches of Maple Creek were monitored using paired fixed antennas at the upper and lower ends of each reach. Increases in turbidity above background levels did not appear to have substantial negative effects on the feeding success or growth of salmonids. Fish actively fed during small storm events, as evidenced by stomach fullness values. Feeding was predominantly from the benthos, on oligochaetes and salmonid eggs, rather than from the drift. Growth of tagged individuals showed no apparent relationship with median reactive distance, and varied widely among sampling dates. Fish moved considerably during storm events, most often in an upstream direction. Retention of tagged individuals within a sampling reach was low.