The ecology of the net-spinning caddisfly Stenopsyche angustata (Stenopsychidae) was investigated in Tai Po Kau Forest Stream (TPKFS), New Territories, Hong Kong, over a two-year period (1977-79). Quantitative benthic samples were taken from different microhabitats in the stream on 45 occasions and, in combination with light-trap catches of adult caddisflies (taken during 1983-84 and 1985), were used as a basis for investigating the life-history pattern and estimating secondary production of this species. Annual production over the period 1977-79 was 940.4 mg AFDW 0.5m−2, and was slightly higher in 1977-78 (1004.1 mg AFDW 0.5 m−2) than in 1978-79 (926.1 mg AFDW 0.5 m−2). This variation reflected slight inter-year differences in mean population densities and biomass of S. angustata, which ranged from 41.1 individuals 0.5 m−2 and 113.3 mg AFDW 0.5 m−2 in 1977-78 to 38.6 individuals 0.5 m−2 and 105.9 mg AFDW 0.5 m−2 in 1978-79. Mean values of these parameters over the two-year study period were 39.8 individuals 0.5 m−2 and 109.4 mg AFDW 0.5 m−2. Stenopsyche angustata appeared to be bivoltine in TPKFS, but growth was rather asynchronous with some adult emergence and continuous recruitment throughout the year. Production: biomass ratios were almost constant (varying between 8.74 and 8.87) over the two years of the study.

Mean densities of Stenopsyche angustata varied significantly across TPKFS. Larvac were most abundant in midstream microsities, and least numerous close to the stream banks. Population densities in intermediate microsites were significantly less than in midstream, but significantly greater than at bankside microsites. Stepwise multiple regression analysis and multivariate statistics (principal components analysis followed by regression) were used to investigate the relationship between larval densities in each benthic sample and the sediment characteristics, detrital standing stocks, and algal biomass in the microenvironment where the sample was taken. Regression analysis revealed that S. angustata microdistribution was influenced by sediment grain-size characteristics (kurtosis of the grain-size distribution), and algal biomass (specifically, the amounts of chlorophyll b), and the relationship between algal biomass and larval densities was negative. Multivariate statistics followed by regression analysis confirmed the interactive effects of both biotic and abiotic influences upon S. angustata abundance. Approximately 20% of the variance in abundance in TPKFS was attributable to sediment characteristics and algae, but it seems plausible also that current speed had a direct effect on the microdistribution and abundance of S. angustata through an influence on the rate of supply of food to the filter-feeding larvae.