Perils of a Drifter: Larval Drift and Entrainment of Freshwater Catfish (Tandanus tandanus) in the Murray–Darling Basin, Australia

Abstract

The freshwater catfish Tandanus tandanus, a species with riverine populations listed as endangered in the Murray–Darling Basin, faces significant conservation challenges. Knowledge about early life stage dispersal remains limited, yet recent larval netting over two spawning seasons in the Macquarie River and adjoining irrigation channels fed by river pumps has revealed important insights. Freshwater catfish exhibited actipassive larval drift dispersal during a short spawning and nesting period in late austral spring to early summer (October–December), coinciding when water temperatures exceeded 21°C. Larvae (9.3–23.9 mm in standard length) were sampled in drifting ichthyoplankton nets set in the main flow of the river, as well as in adjacent irrigation channels. Sporadic low catch rates during the second season, likely due to flooding events, suggest potential disruption of nesting and spawning. Most freshwater catfish larvae were collected in overnight drift samples, indicating that drifting may be a nocturnal dispersal method. No larvae were captured in the still water areas using overnight light traps, further underscoring the potential importance of drift as a dispersal mechanism. The detection of frequent loss of larval individuals at unscreened pumps highlights a previously undocumented threat to the species during the larval life stage. Downstream larval drift contributes to the dispersal of early life stages of this species, but also increases susceptibility to entrainment at pump intakes and water diversions, which often operate at peak capacity during the larval drift period. Threats may be particularly pronounced in lowland rivers, where long reaches and higher levels of water extraction are likely to increase the risk of larval entrainment. This reinforces the need for modern fish protection screens at water diversions to mitigate this risk and help safeguard the future of freshwater catfish in the Murray–Darling Basin.