Ecological response of an estuarine atherinid to secondary salinisation in south-western Australia

Secondary salinisation of rivers is a major threat to aquatic ecosystems, particularly in the southwest of Western Australia. Salinisation has changed the structure of freshwater fish assemblages in the south-west, with many estuarine species now being found well inland and outside of their historic range. The current study aimed to determine whether plasticity in life-history traits, in addition to physiological tolerance, may facilitate competitive advantage of estuarine invaders in secondarily salinised systems. Leptatherina wallacei is a teleost endemic to the region and was historically found in the upper reaches of estuaries and several coastal lakes. Several life-history, morphological and ecological traits of L. wallacei, in habitats of differing salinities in the Blackwood River were determined. Our study reveals that, complementing its broad physiological tolerance to salinity, the species has flexible life-history traits, such as a protracted spawning period, early maturation, fast growth and a broad diet. Its spawning period and growth rate also differed among populations in different sections of the river, with those in the more salinised upper catchment spawning at a similar time to estuarine fish, whereas in the less salinised lower catchment, spawning was delayed for a season, reproductive activity was reduced and growth was faster. The prevalence of macroparasites was significantly greater in fish populations in the lower catchment than in the upper catchment, and this may have contributed to reduced reproductive activity. We suggest that phenotypic plasticity in life-history traits, along with broad physiological tolerance to elevated salinity and an escape from parasitic infection, has facilitated a competitive advantage to the species that has enabled it to become a dominant fish in secondarily salinized habitats