A POPULATION-MODEL FOR THE ALKALI FLY AT MONO LAKE - DEPTH DISTRIBUTION AND CHANGING HABITAT AVAILABILITY

The densities of alkali fly larvae and pupae were measured in relation to depth and substrate type at six locations around Mono Lake. Sample s representing a mixture of different bottom features were taken to a depth of 10 m (33 ft) using SCUBA. This is at or near the depth limit of fly larvae and pupae. The biomass of larvae and pupae on hard subst rate were maximum and approximately equal at depths of 0.5 m and 1 m, substantially lower at intermediate depths of 3 m and 5 m, and over an order of magnitude further reduced at 10 m. Densities of flies on har d or rocky substrates (mainly calcareous tufa deposits), were signific antly greater than those found on soft substrates such as mud or sand, at all but the greatest depth surveyed. Bathymetric maps of the areas of hard and soft substrate occurring at different lake depths were us ed to estimate the fly population size over the whole lake, based on t he density distribution of larvae and pupae with depth on different su bstrates. The mapped areas of soft and hard substrates were also calcu lated for different lake levels, and applying the same procedure, a po pulation model comparing the abundance of flies at different lake leve ls was developed. This habitat-based population model predicts that th e abundance of the alkali fly is maximized at 6380 ft (1945 m) lake su rface elevation. Most of the tufa substrate submerged at this lake lev el will become exposed and unavailable as habitat as the lake declines to 6370 ft (1942 m). In late 1991, the lake level was just over 6374 ft (1943 + m).