Restructuring of littoral fish assemblages after drought differs in two lakes at the terminus of a heavily regulated river

Drought significantly shifts the mix of functional groups present in river ecosystems and abundances of many aquatic fauna decline. The consequences of drought for fish often include the decline of ecological specialists and the predominance of generalist species. However, factors influencing the restructuring of littoral fish assemblages following prolonged drought in heavily regulated rivers are poorly understood. We compared the restructuring of littoral fish assemblages in 2 large connected lakes, which differ in hydrology and habitat availability, after 15years of drought in their catchment. The once-abundant and diverse fish assemblages showed limited signs of postdrought recovery after 5years. There were differences between the 2 lakes related to the species present, their abundances, and the functional groups within fish assemblages. The functional groups present also shifted through time from alien-dominated to freshwater generalist-dominated. Only 2 of 5 native fish functional groups (freshwater generalist and diadromous) had increasing abundances (used as a proxy for recovery) following the drought, and these increases only occurred in the lake with the greater connectivity and diversity in habitat and hydrology. In contrast to patterns observed for natives, abundances of alien fishes in the littoral zone of the lakes declined after an initial spike in numbers associated with substantial river flows immediately following drought. This study shows that the recovery of native fish populations following prolonged drought in a heavily regulated semiarid river occurs over a time scale of years and recovery of some extant ecological specialists was incomplete after as many as 5years.