Eel fear has be­come fas­ci­na­tion

The Northland Age - - Local News -

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.

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