Changes in Chironomid Breeding Patterns in a Paved River Channel Following Adaptation of Cichlids of the Tilapia mossambica-hornorum Complex

Abstract

Densities of Chironomidae, principally Chironomus decorus Johannsen larvae, declined markedly in detritus habitats of the urban Coyote Creek drainage of the Los Angeles basin, with the establishment of Tilapia mossambica (Peters) and T. hornorum Trewazas fish populations. However, density changes in another group of non-annoying midges, Cricotopus and Tanypus species, were not pronounced over the 9-yr period of this study. The effective foraging on Chironomidae in certain substrates by very dense populations of the species of Tilapia influences the phenotypic characteristics of such substrates to produce chironomids. Typically the insect-produced fish biomass in autumn can exceed 4 × 105 kg over a distance of 18 km of paved river channel, a phenomenon apparently dependent indirectly on the availability of warm water effluent from a power generating plant. With the Tilapia species now ranging in the neritic zone along the southwestern California coast, their contribution to predatory marine fish biomass may be significant.