Disturbance-driven changes in fish assemblages caused by a sudden increase in salinity in a perennial desert stream

Disturbance is a key factor in structuring fish assemblages in rivers; characterizing reactions to disturbance is especially important in systems containing endangered species due to their vulnerability to extinction. A natural disturbance event occurred in the Virgin River in 1985, when normal flow patterns changed due to the appearance of a sinkhole upstream of the hot saline Pah Tempe Springs. This led to abnormally elevated levels of temperature and salinity that propagated downstream. We examined changes in fish assemblages at five sites in a 21 km reach downstream from the hot springs in the mainstem Virgin River, which were monitored monthly (with the exception of 1 site monitored biannually) between 1984 and 1987. Specifically, we compared the responses of the different fish species in space and time. Of the six native species investigated, speckled dace, spinedace, and the endangered woundfin experienced declines in abundance at sites closest to the disturbance. In contrast, Virgin River chub, another endangered species, experienced increases in abundance in sites nearest the disturbance, and desert sucker and flannelmouth sucker showed no change. Redundancy analysis confirmed that changes in assemblage structure were significant. Differential responses to the disturbance appeared to correlate with differences in feeding ecology, with insectivores being the most affected group. In terms of recovery, species with higher abundances in neighboring sites experienced faster recoveries than those with lower abundances. This study characterizes disturbance reactions in the Virgin River, and emphasizes the susceptibility of rare species to natural disturbances and subsequent depression of recolonization rates.