Tuna – working with glass eels

While Erina Watene-Rawiri was working at NIWA, she was involved in the research of eels or tuna in the lower Waikato River. The two components to her research were:

• monitoring glass eels in the river with a view to comparing numbers with historical data

• investigating the possibility of growing eels quickly for commercial viability.

Glass eel

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Glass eel – the see-through stage before it becomes an elver.

Rights: Courtesy of the Vertebrate Zoology Division; YPM ICH.023326. Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA; peabody.yale.edu. Photography by Gregory Watkins-Colwell.

Glass eels

Glass eels are a stage in the life of a freshwater eel (Anguillidae). It comes after the larvae (leptocephalus) stage and before pigmentation (colouration) when the eel becomes an elver (juvenile or baby eel). As the name suggests, the eel is transparent. It is at this stage that the eels enter estuaries from the ocean and begin to swim upstream to live in freshwater. As they enter the freshwater, they begin to become pigmented and are referred to as elvers.

Researching glass eels in the river

The first part of Erina’s research involved investigating and monitoring glass eels, in particular, numbers of glass eels entering the Waikato Heads (the river mouth) over a 4-year period.

Erina and her colleagues discovered that glass eels arrive at the mouth of rivers in pulses, waiting for the right conditions before they swim up the rivers. They tend to migrate up the river on flood tides or spring tides (new or full moons). They take several days to adjust to freshwater before they continue to migrate upstream. They migrate (run) during the day as well as night at this stage, and they also develop the ability to swim strongly against the current. They tend to form large schools until they reach the limits of the tidal reaches – before continuing their journey as elvers.

A keen sense of smell

Glass eels have a highly developed sense of smell and are able to make specific choices about the type of river or stream to which they migrate. For example, the responses for longfin eels are different from shortfin eels. The reasons for the different responses are unknown at this stage. Small eels favour waterways where the substrate is coarse and the current is swift.

A worldwide decline in numbers

The goal of this research was to compare numbers of glass eels with numbers in the past. Historically, runs were reported as a metre wide and a metre thick. This research confirmed a decline in numbers. Runs were small – described as blips, certainly much less than suggested by local kaumātua of past runs.

This decline appears to be a worldwide phenomenon – related to such things as climate change, loss of habitat, parasite infestation, pollutants, overfishing and obstacles to migration.

Can eels be grown commercially?

The second part of Erina’s research investigated whether glass eels could be grown quickly to maturity and in a commercially viable way.

This involved catching glass eels when they entered freshwater and then growing them to a marketable size at an aquaculture facility at Bream Bay in Northland. Normally – in the wild – it would take 15 years for an eel to grow to a size that would be commercially viable. Erina and the team she was working with studied the growth rates of eels. They wanted to see if this time could be sped up.

The team fed the captured glass eels specially prepared food designed for optimum growth. They were able to grow some through to a marketable size in less than 2 years. Others took a bit longer.

Although this was successful, many people felt that the rivers and streams needed to be repopulated with eels before eels are commercialised through aquaculture. It was suggested that this process could be used to grow eels for release into the wild. Glass eels could be grown quickly and then released into the river to replenish declining stocks, contributing to river restoration. In the meantime, the commercial exploration has stopped, with a view to further investigation some time in the future.