Aquatic Insect Abundance in a Regulated Stream under Fluctuating and Stable Diel Flow Patterns

Abstract

Aquatic insect abundance at water depths of 15-45 cm was examined in a fifth-order reach of the Skagit River, Washington from May to November in 1976 and 1977. The study site was subject to diel flow fluctuation in 1976 from hydroelectric power-peaking, and to a relatively stable flow pattern in 1977 while peaking was curtailed. Under fluctuating flow conditions, insect density increased from shallow to deep water and was negatively correlated (r = −0.76, P < 0.001) with hours of dewatering during the 2 weeks prior to sampling. Current fluctuation appeared to limit insect density at the 45-cm deep sampling locations that were not extensively dewatered during the daily peaking cycle. The highest density observed in 1976 was 1,788 insects/m 2 at 25 cm in July. Under stable flow conditions, the abundance of benthic insects was greatly enhanced and the densities at corresponding depths and months were 1.8-59 times higher in 1977 than in 1976. Insect densities increased steadily from May 1977 through September 1977 when they reached a maximum level of 16,763 insects/m 2 at 15 cm. Reduction in the amplitude and duration of power-peaking flow fluctuation can be a highly effective management strategy for enhancing aquatic insect standing crop, with a potential for increasing the survival and growth of fish dependent on insects for food.