Satellite tags reveal freshwater eel migration to the Coral Sea

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A tagged adult eel about to be released into the ocean. Image: ARI

GLOBALLY, anguillid (or freshwater) eels have declined dramatically over the last 50 years due to factors such as habitat loss and barriers to migration. Many species worldwide are now classified as threatened or endangered.

The Arthur Rylah Institue (ARI) says it's filling important knowledge gaps for the two species of anguillid eels found in Victoria; the long-finned eel (Anguilla reinhardtii) and short-finned eel (A. australis). These eels are an important part of Victoria’s biodiversity, support commercial and recreational fisheries and have significant cultural values.

Both eel species have an amazing lifecycle. Adults spend decades in freshwater habitats (such as rivers and wetlands) feeding and growing before metamorphosing into silver eels. The silver eels migrate downstream, often during high river flows, and into the sea to spawn. During these oceanic spawning migrations, adult eels travel several thousand kilometres to warm tropical waters. After spawning, the adults die and the newly hatched leaf shaped larvae (called leptocephali) commence a journey toward the coast, drifting on ocean currents and developing into glass eels before eventually entering rivers. The young eels, now known as elvers, migrate further upstream into freshwater, developing into yellow eels and eventually becoming adults. These adults will then return to the sea to spawn.

In 2018, ARI started a project using innovative technology to identify eel migration routes and spawning areas and describe the environmental conditions they experience as they migrate. Satellite tags were attached to adult migratory stage eels, enabling tracking over large distances. These tags also recorded environmental data such as temperature, depth and light.

Migrating adult eels were tracked for up to five months, and up to about 3,000 kilometres from where they were released. Tagged eels were tracked migrating to an area in the Coral Sea near New Caledonia, a presumed spawning area in the south Pacific Ocean. The eels moved up and down the water column; mainly between depths of 700–900 metres during the day and 100–300 metres during the night, likely related to avoiding predators and for thermoregulation. Marine predators such as whales and sharks ended many migrations, with many eels predated before leaving the Australian continental shelf.

The project is a collaboration between the Victorian Department of Environment, Land, Water and Planning, the Gunditjmara Traditional Owners, the Glenelg Hopkins Catchment Management Authority and Melbourne Water. ARI says, the information on migratory routes provides an important basis to inform future efforts to assess and mitigate interactions between eels and human activities in the marine environment.

For more information: www.ari.vic.gov.au

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