First Tracking of the Oceanic Spawning Migrations of Australasian Short- Finned Eels (Anguilla Australis)

Abstract Across their near-global range, anguillid eel populations have declined dramatically over the last fifty years and many species are now threatened. A critical life history phase is migration from freshwater to distant oceans, culminating in their single life-time spawning event. For many anguillids, especially those in the southern hemisphere, mystery still shrouds their oceanic spawning migrations. We investigated the oceanic spawning migrations of the Australasian short-finned eel ( Anguilla australis ) using pop-up satellite archival tags. Eels were collected from river estuaries (38°S, 142°E) in south-eastern temperate Australia. In 2019, sixteen eels were tracked for up to about 5 months, ~2620 km from release, and as far north as the tropical Coral Sea (22°S, 155°E) off the north-east coast of Australia. Eels from southern Australia appeared to access deep water off the Australian coast via two main routes: (i) directly east via Bass Strait, or (ii) south-east around Tasmania, which is the shortest route to deep water. Tagged eels exhibited strong diel vertical migrations, alternating between the warm euphotic zone (~100 to 300 m, 15 to 20°C) at night, and the mesopelagic zone (~700 m to 900 m, 6 to 8°C) during the day. Marine predators, probably lamnid sharks, tuna, or marine mammals, ended many eel migrations (at least ~30%), with many eels lost before leaving the Australian continental shelf. The long and risky marine migrations of Australasian eels highlight the need for better information on the processes contributing to eel mortality across the life cycle, including the impacts of future changes to oceanic currents, predator abundance and direct anthropogenic disturbances.