Glass eel ear bones could reveal migration patterns - Kogonuso

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Jan 9, 2019

Glass eel ear bones could reveal migration patterns

The ear bones of tiny eels might hold clues to their largely secret life.

They might also reveal answers about their migratory habits from New Zealand rivers across vast areas of the ocean, and the effects of climate change in the ocean, a Niwa freshwater scientist says.

Eimear Egan, whose fascination with eels began as a child when bitten by one, has assembled a specialist team for the three-year research project, backed by a government grant.

The team will be studying the ear bones of glass eels which are up to 7cm long to find out more about where their marine life originated, and the oceanic routes they took before entering our freshwaters.

Longfin and shortfin eels migrate down rivers to breed thousands of kilometres away in the western Pacific ocean. The young then make their way back to the freshwater origins of their forebears.

Dr Egan said it was hoped the research would reveal some answers as to where they went, and why.

She said the ear bones of an eel told a story of their daily life and surroundings.

"The eel bones are formed at birth so when the eels migrate from the river, once they're 20 or 30 years of age, and out to the ocean to lay their eggs, this tiny structure made of calcium carbonate forms in the ears.

"A layer is deposited every day in this structure so basically we have a chronology or a time series or a diary, of their daily life in the ocean."

Dr Egan said the eggs hatched into larvae, which was then transported on ocean currents back to New Zealand's coastline where the larvae then transformed into glass eels. A daily record of their growth and of the chemistry of the water they inhabited, was embedded in their ear bones.

She said her fascination with eels began when as a young child growing up on an Irish farm. One of her chores was to regularly clean out the well from where her family drew its drinking water.

"In the well lived a large eel, and no matter how many times it was shifted, it just kept coming back.

"I was traumatised by it for years. It bit me several times," she said.

Dr Egan is now a Hamilton-based Niwa freshwater ecologist.

She said the marine phase of the life cycle of longfin and shortfin eels was mysterious and poorly understood.

"Longfin eels are only found in New Zealand and according to our threat rankings, they have been assessed as 'at risk and declining'.

"At the moment, we don't really have a good understanding of their early life history, including their spawning grounds and larval migration routes, but this information is important because climate change is having a large effect on the Western Pacific Ocean."

Dr Egan said it was not known if the numbers of glass eels making it to New Zealand were affected by processes happening during their marine life.

The team will use a couple of techniques to learn more about them, including polishing the ear bones by hand until the daily rings became visible.

"Then we can interpret distinct rings - or check marks - that coincide with when the larvae first feed and when they change from larvae into glass eels.

"We can measure the distance between rings to figure out daily growth rates, we can count the rings to estimate age and the date the larvae hatched."

The second technique involved using chemistry to work out what temperature the larvae had experienced at sea.

Niwa scientists will also start a project in April to tag migrating female eels, to see where they go in the Western Pacific Ocean to spawn.

Eels can live in rivers or lakes for up to 100 years.
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