ANSWER 1

Written response:

This is a complex question, but the life cycle of Australian freshwater eels is equally complex, and many factors can influence migration, survival, biology, and behaviour of eels. Australian Shortfinned and Longfinned Eels are thought to spawn/hatch in the Coral Sea region, likely between New Caledonia and the Solomon Islands, although the exact locations remain uncertain. Eels begin life in the ocean as leptocephali (eel larvae), which spend extended periods at sea before metamorphosing into glass eels and recruiting to coastal and freshwater systems. Leptocephali are thought to semi-passively drift in ocean currents (such as the South Equatorial Current and East Australian Current) during their development. Ocean circulation therefore plays a central role during this phase, as currents determine transport pathways, arrival locations, and the timing of recruitment. Changes in ocean circulation associated with climate change, including for example the strengthening and southward extension of the East Australian Current (as highlighted by Dr Arani Chandrapavan in another question) may alter these pathways and increase variability in recruitment of glass eels.

Marine and terrestrial heatwaves may further affect early life stages by modifying coastal and river water conditions, which then influence food availability from the early recruiting stage (glass eel) through to later life stages. With potential consequences for survival and behaviour, although direct evidence for Australian eels remains limited.

Eels then spend many years growing in freshwater as elvers, yellow eels, and silver eels before undertaking a single, long return migration to the ocean to spawn. While recent research has highlited that changes at sea may influence adult migration routes and travel times, some of the most significant and immediate pressures occur in freshwater. Pollution of catchments, habitat loss, diseases/parasites, and migration barriers reduce growth, body condition and survival during the freshwater phase, which can impair swimming performance and overall fitness before ocean migration. These freshwater stressors can carry over into the marine phase and potentially interact with changing ocean conditions. Overall, eel populations are likely to be affected by climate change at sea, and may already be so, but maintaining healthy freshwater habitats remains one of the main factor in supporting successful ocean migration and reproduction. Eel scientists around the pacific use a range of analytical tools, such as stable isotopes, environmental DNA, tagging, morphometric monitoring, traditionnal knowledge…etc, to try to understand the fundamental aspects of eel’s life cycle. And a lot remains un-answered.

Scientists such as Wayne Koster and Alexis Farr may be able to add some information to that question.

Attached are two photos and links to papers that address these issues from a broader perspective, including global threats to freshwater species (including eels) and a representation of the threats that affect eels’ life cycle at different moments.

Nature 2025 (Threats on freshwater species)
Global Ecology and Conservation (Threats on eels’ life cycle)

Supporting gallery: