The Unseen Voyage: Tracing the Elusive Migration of Tuna (Eels) to and from Aotearoa

After years or even decades living in freshwater streams (awa), mature tuna embark on a remarkable journey to the ocean, where they spawn and die. Baby tuna must then return to the awa of Aotearoa to begin the cycle anew. Despite intensive research, no-one has identified where tuna go to spawn, or which routes their larvae take to return to Aotearoa.

Previous attempts to identify the migration routes of tuna have relied on satellite tracking, which is often hindered by tag detachment, failed satellite transmission, and predation. Dr Armandine Sabadel and her team are hoping to reconstruct the journeys of these enigmatic taonga using an alternative method, based on biochemical markers known as stable isotopes.

Isotopes are different forms of an element. For example, carbon has three naturally occurring isotopes, all with 6 protons and electrons but varying numbers of neutrons: carbon-12 (with 6 neutrons), carbon-13 (with 7 neutrons), and carbon-14 (with 8 neutrons).

However, only carbon-12 and carbon-13 are stable, as carbon-14 is a radioactive isotope that eventually decays into another element. Among the stable isotopes, carbon-13 is slightly heavier than carbon-12 due to its extra neutron.

This small difference significantly impacts the ratio of these stable isotopes during natural processes, such as photosynthesis and respiration, creating geographical variations.

In aquatic systems where tuna live, factors like temperature, salinity and ocean currents also influence these ratios, contributing to a distinct isotopic landscape or signature.

Marine animals like tuna in their first life stages incorporate and store in their tissues these specific isotope ratios from their environment as they feed.

Therefore, analysing stable isotope ratios in an animal’s continuously growing tissues, such as eye lenses and vertebrae, can provide scientists with information about their whereabouts as they developed.

These ratios can then be matched to an isotope map (or isoscape) to trace their historical movements.

To kick-start this project, Dr Sabadel and her team embarked on their own voyage in October 2024.

They collected environmental samples from the South Pacific to measure spatial variation in stable isotope abundance and have now started building isoscape maps.

Later, they will compare these maps to the isotope ratios found inside the eye lenses and vertebrae of tuna from our awa to reconstruct their historical migratory routes.

The team will also be net-sampling and collecting samples of environmental DNA and RNA across the South-Pacific to confirm the presence of tuna in specific areas.

This unique study will provide critical understanding regarding the life history of tuna, and will inform the future protection of these taonga; for example, by shedding light on their vulnerability to climate change.

This project may also pave the way for similar studies on other migratory marine species.

_{Marsden team at our first voyage planning workshop. From left to right: Dr Moira Décima (Scripps, USA), Therese Miller (PhD student at Cawthron/University of Auckland), Associate Professor Xavier Pochon (Cawthron/University of Auckland), Alex Che (PhD student at Auckland University of Technology), Dr Mike Miller (former-University of Tokyo, Japan), Dr Erik Behrens (NIWA) and Dr Amandine Sabadel (Auckland University of Technology/NIWA).}