An eel mys­tery

Waipa Post - - Front Page -

As a young child grow­ing up on an Ir­ish farm, one of Eimear Egan’s chores was to reg­u­larly clean out the well where her fam­ily drew its 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,” Eimear says. “It bit me sev­eral times.”

The eel got the last laugh. Now Dr Eimear Egan, a NIWA fresh­wa­ter ecol­o­gist based in Hamil­ton, is study­ing eels and what makes them tick.

She is lead­ing a team that re­cently se­cured a grant from the Min­istry of Busi­ness, In­no­va­tion and Em­ploy­ment En­deav­our Fund for a three-year pro­ject which will en­able her to do “some very ex­cit­ing re­search” on both longfin and short­fin eels.

The eels spend a por­tion of their life in ma­rine and coastal en­vi­ron­ments. Once the adults reach sex­ual ma­tu­rity in fresh­wa­ter they mi­grate to the western Pa­cific Ocean to re­pro­duce.

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. Eimear 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. At the mo­ment, we don’t really have a good un­der­stand­ing of their early life his­tory in­clud­ing 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. That’s where ex­am­in­ing glass eel ear bones comes in.

“Ear bones can tell us so much in­for­ma­tion 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.

“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.”

They use mi­crostruc­ture — pol­ish­ing the ear bones by hand un­til the daily rings be­come 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 in­volves us­ing chem­istry to es­ti­mate the tem­per­a­tures the lar­vae have ex­pe­ri­enced at sea. Eimear is hop­ing anal­y­sis of the eel tis­sue will 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.”

Glass eels ar­rive at the mouths of rivers in pulses, wait­ing for suitable con­di­tions to en­ter. Dur­ing the early part of the sea­son, they tend 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, glass eels take sev­eral days to ad­just be­fore they mi­grate fur­ther up­stream.

There is ev­i­dence of a world­wide de­cline in re­cruit­ment of glass eels, par­tic­u­larly for Euro­pean and Ja­panese species­be­cause of cli­mate change, loss of habi­tat, pol­lu­tants, over-fish­ing and ob­sta­cles to mi­gra­tion.

In April NIWA sci­en­tists will un­der­take a pro­jectwhere mi­grat­ing fe­male eels will be tagged and their lo­ca­tion com­mu­ni­cated to satel­lites to help pin­point their spawn­ing grounds in the ocean.

Photo / Stuart Mackay

NIWA fresh­wa­ter ecol­o­gist Dr Eimear Egan is head­ing a three-year study into eels.

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