Trends and mechanisms behind the invasion of Coilia brachygnathus (Actinopterygii, Engraulidae) in one of the world’s largest reservoirs

Shortjaw tapertail anchovy, Coilia brachygnathus , has invaded and expanded in the Three Gorges Reservoir (TGR) over the last few years. The population size and biological characteristics related to this invasion are poorly understood. To investigate its invasion mechanisms, we studied spatial and temporal changes in C. brachygnathus biomass and abundance and its reproductive traits and diet. Evidence of progressive directional invasion suggested that C. brachygnathus invaded the TGR from downstream in 2016. The TGR provided an initial blockage and later facilitated C. brachygnathus dispersal. In 2020 and 2021, C. brachygnathus accounted for 11.66% and 16.06% of biomass and abundance, respectively, of the TGR fish community and both biomass and abundance decreased with distance to the dam. The fecundity in the TGR was higher than that in downstream native areas. Although their contributions to diet exhibited spatial differences, C. brachygnathus mainly consumed shrimps ( Exopalaemon modestus and Macrobrachium nipponensis ) and zooplankton in the TGR. Within a short period, C. brachygnathus has successfully colonized and expanded into the whole reservoir. Its fast maturity and high fecundity, coupled with the lentic habitats and abundant food availability, are probable key factors that explain its invasion. We propose that prevention measures to block the expansion of this species are urgently needed after the impoundment of cascaded reservoirs with navigation passages located upstream of the TGR.