Eel fear has become fascination
As a young child growing up on an Irish farm, one of Eimear Egan’s chores was to regularly clean out the well that supplied her family’s drinking water. In the well lived a large eel that, no matter how many times it was shifted, just kept coming back.
“I was traumatised by it for years. It bit me several times,” she said.
Now Dr Egan, a Niwa freshwater ecologist based in Hamilton, is studying eels and what makes them tick, leading a team that recently secured a grant from the Ministry of Business, Innovation and Employment Endeavour Fund for a threeyear project that will enable her to do “some very exciting research” on both longfin and shortfin eels.
Longfin and shortfin eels spend a portion of their lives in marine and coastal environments. Once the adults reach sexual maturity in freshwater they migrate to the western Pacific Ocean to reproduce.
The resulting larvae are transported by ocean currents back to New Zealand’s coastline, where they develop into glass eels, up to 7cm long. The marine phase of their life cycle is mysterious and poorly understood, and Dr Egan and her team will be studying the ear bones of glass eels to learn more about their spawning locations and larval oceanic movements.
“Longfin eels are only found in New Zealand, and according to the New Zealand threat rankings, they are at risk and declining,” she said.
“At the moment we don’t really have a good understanding of their early life history, including the locations of spawning grounds and larval migration routes.
“This information is important, because we don’t know if the numbers of glass eels making it to New Zealand are affected by processes happening during their marine life. We are going to use some quite novel methods to help fill in these knowledge gaps.”
It was glass eel ear bones that were expected to provide some answers.
“Ear bones can tell us so much about the environment a fish experiences on a daily basis. Each day they add a layer of calcium carbonate, which is almost like keeping a diary of their lives,” Dr Egan said.
“They tell us about their growth, their spawning and hatching dates, about the chemistry of the environment they live in, about their diet and movement. And because of that daily layer we can potentially figure out what routes the larvae took to get to the New Zealand coastline.”
One technique the research team would use, microstructure, involved 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, and we can count the rings to estimate age and the date the larvae hatched,” she said.
A second technique would involve using chemistry to estimate the temperatures the larvae experienced at sea, Dr Egan hoping analysis of the eel tissue, using compound-specific stable isotopes of amino acids, would help pinpoint where in the western Pacific Ocean the larvae might have come from.
“We’re using these techniques in the hope we can understand if they are coming from different places within the western Pacific Ocean, and whether long and shortfin eels share spawning grounds and larval migration routes,” she added.
Glass eels arrived at the mouths of rivers in pulses, waiting for suitable conditions to enter. During the early part of the season, they tended to migrate into rivers on the incoming tide, especially on the spring tides of new and full moons, or after heavy rainfall.
Once in fresh water, they took several days to adjust before migrating further upstream.
There is evidence of a worldwide decline in recruitment of glass eels, particularly for the European and Japanese species. Factors contributing to that decline are believed to include climate change, loss of habitat, pollutants, overfishing and obstacles to migration.
In April Niwa scientists will undertake a project that will complement Dr Egan’s work, tagging female eels so their location can be tracked by satellites to help pinpoint their spawning grounds.
Niwa ecologist Dr Eimear Egan showing fine eel-catching technique for research that might save the species.