Salted mullet: Protracted occurrence of Mugil cephalus under extreme hypersaline conditions

Abstract The fish faunas of eight estuaries along 130 km of the south coast of Western Australia were sampled seasonally for one year, during which Beaufort Inlet became markedly hypersaline (salinities up to 122 and > 100 for six months). These conditions were caused by a combination of low amounts of saline river flow, the bar of this shallow estuary remaining closed for 27 months and high rates of evaporation. Fish faunas in the nearshore, shallow and offshore, deeper waters of Beaufort Inlet were depauperate compared to nearby estuaries. The number of fish species declined as salinity increased, with the highly euryhaline, estuarine-resident atherinid Atherinosoma elongata being the only species to survive throughout the study. The cosmopolitan flathead mullet Mugil cephalus survived for the next longest period, living in salinities in excess of 100 for at least four months and in a maximum salinity of 122. This is the highest known salinity in which a marine-spawning fish species has been found globally. This finding provides support for the cryptic species complex hypothesis pertaining to M. cephalus and that individuals in south-western Australia may be a seperate species. The survival of these species for a relatively protracted time reflects the fact that they are euryhaline and have a suite of adaptations that allow them to occupy environments from freshwater to extreme hypersaline conditions. The longer occurrence of M. cephalus in the extreme salinities than Acanthopagrus butcheri and Aldrichetta forsteri, is likely also related to their primary diet of detritus, particulate organic matter and diatoms, all of which are abundant in Beaufort Inlet. In contrast, piscivorous and zoobenthivorous fish were depauperate and in low abundances, which may reflect limited food resources during hypersaline conditions, and/or more reduced euryhalinity by these species. Clearly, hypersalinity has a marked effect on the faunas and thus ecosystem functioning of estuaries, and with declines in rainfall and hotter temperatures projected in future climate change scenarios, more estuaries are likely to become increasingly hypersaline.