Abstract:
In a study of cumulative effects of riparian disturbance by grazing on the trophic
structure of high desert trout streams, watersheds with greater riparian canopy had higher standing
crops of rainbow trout Oncorhynchus mykiss, lower daily maximum temperatures (range, 16-23°C
compared with 26-31°C), and perennial flow. Watershed aspect influenced the response of trophic
structure to grazing influences. Standing crops of rainbow trout were negatively correlated with
solar radiation and maximum temperature in watersheds flowing northward. A different relationship
was observed for a set of watersheds with a southern aspect, perhaps due to the presence of
spring seeps and stream desiccation in the heavily grazed stream. Trout biomass was negatively
correlated with solar radiation, whereas positive relationships were found for discharge and depth.
Algal biomass was positively correlated with solar insolation (r = 0.91), total invertebrate biomass
(r = 0.77), and herbivorous invertebrate biomass (r = 0.79) in all watersheds. Invertebrate biomass
was not significantly correlated with rainbow trout standing crop. High irradiance apparently
resulted in increased algal biomass and invertebrate abundance. However, temperature elevations
to levels close to lethal may impose high metabolic costs on rainbow trout, which may offset higher
food availability and affect the availability of prey.
