Connectivity, migration and recruitment in a catadromous fish

Understanding the influence of river hydrology and connectivity on the migration and recruitment of diadromous fishes is fundamental for species management and conservation. We investigated the downstream catadromous spawning migration of adult female congolli (Pseudaphritis urvillii) using acoustic telemetry, and subsequent juvenile recruitment, in the lower reaches of the River Murray, Australia, in 2009-2011. The years 2009 and 2010 were characterised by diminished freshwater flow, closure of tidal barrages, and disconnection of freshwater and estuarine habitats; however, a navigation lock was operated to facilitate downstream fish passage in 2010. In both years, >70% of individuals tagged upstream undertook downstream migrations, in association with day-of-the-year (June July) and moonphase (full), and accumulated upstream of the tidal barrages. In 2009, fish were unable to pass the barrages and remained upstream, but in 2010, an estimated >15 000 individuals passed through the navigation lock, including 40% of individuals tagged upstream. These transitioned rapidly (<24 h) through the estuary and into the ocean. In association, abundances of upstream migrant juveniles in spring summer 2010-2011 were up to 180 times greater than in spring summer 2009-2010. Our study illustrates the potential impact of tidal barriers on migrations and population dynamics of catadromous fish, and the importance of understanding species-specific migration ecology.