Growth, production and mortality of two species of Agapetus (Trichoptera: Glossosomatidae) in the Acheron River, south-east Australia

Abstract

1. Quantitative samples of Agapetus pontona and Agapetus monticolus larvae were taken at two sites on each of three rivers in the catchment of the Acheron River (i.e. Little River, Steavenson River and Acheron River). Both species were univoltine with A. pontona having a 5-6-month life cycle (spring to late summer) and A. monticolus a 10-month life cycle (autumn to early summer). 2. Population densities, biomass (B), growth rates and mortality patterns derived from these field data were used to calculate secondary production (P) and turnover (P/B). At each site, these features were measured for the whole of the A. pontona life cycle, but only for the last 3 months of the A. monticolus life cycle. 3. Growth rates were highest at the sites on the Little River during summer for both species: 1.8-1.9% dry weight day-1 for A. pontona and 2.0-2.2% dry weight day-1 for A. monticolus. Turnover ratios (P/B) were also highest at the Little River sites: 3.2-6.3 for A. pontona and 1.6-1.9 for A. monticolus. Production was variable and was not significantly different among rivers for A. pontona (28.4-222.1 mg m-2 per 6 months) but was for A. monticolus (70.5-123.8 mg m-2 per 3 months for the Little River compared with 14.823.3 mg m-2 at the other sites). 4. Two of the rivers were subject to higher levels of rock movement during summer than the third (Little River). It was suggested that the higher growth rates (and turnover ratios) in the Little River were caused by the lower levels of rock movement causing less disruption to the feeding of the larvae. 5. Little or no larval mortality of A. pontona was observed at any site. However, mortality occurred between instar 5 and the pupal stage. This varied in a density dependent-fashion, suggesting population regulation occurred: the higher the larval density the greater the mortality suffered by the pupae. No such density dependent pattern occurred for the mortality between instar 5 and the pupal stage of A. monticolus. 6. The population of A pontona was not food limited and larval densities were low. Competition appeared to occur for pupation sites. Low and relatively constant discharges during the late summer when A. pontona pupated appeared to provide more predictable conditions than those experienced by A. monticolus in the spring when discharge was very variable resulting in the stranding (and thus death) of pupae above the water line. Such unpredictable conditions would not foster density dependent population regulation via pupal mortality.