Lake Hawea eels get help­ing hand

Re­searchers in­ves­ti­gate long-range migration

Central Otago Mirror - - FEATURES - By MAR­JORIE COOK

When in an Up­per Clutha au­tumn, a Lake Hawea tuna (eel) gets ‘‘that look’’ in its eye there’s no stop­ping it.

A large, sil­very eye sig­nals the in­ten­tion to make the long jour­ney from Lake Hawea to deep Pa­cific Ocean wa­ters around Tonga, where the eels spawn and then die.

Their tiny, translu­cent descen­dants then find their own way back to New Zealand and Lake Hawea. Some eels could be a ven­er­a­ble 80 to 100 years old when, come Fe­bru­ary to June, they es­chew fresh­wa­ter alpine wa­ter for the sea nurs­ery of their in­fancy. Up un­til 1956, the eels would have had a good run at the 338 kilo­me­tre trip down the Clutha River. But dams at Roxburgh (1956), Lake Hawea (1958) and Clyde (1992), now ob­struct an eel’s life pur­pose.

The hu­man changes mean aging eels need a help­ing hand from power com­pany Con­tact En­ergy to get past the dams to the sea.

En­ter eel track­ers Ben Ludgate and Dr Ruth Gold­smith of Dunedin com­pany Ry­der Con­sul­tants, which is con­tracted to run Con­tact’s ecol­ogy pro­gramme.

Armed with mar­mite (an eel at­trac­tant), the track­ers travel the catch­ment in au­tumn, work­ing from dusk to dawn to catch and mea­sure eels, be­fore re­leas­ing the mi­grants down­stream of Roxburgh Dam. From there, the fish still have an­other 120km to swim to the sea.

The Otago Uni­ver­sity zo­ol­ogy grad­u­ates are fresh­wa­ter fish­eries spe­cial­ists and worked at Lake Hawea and on the Clutha River dur­ing April. They’ve also re­cently com­pleted work in South­land, Otago, West Coast, Can­ter­bury, Nel­son, Taranaki and the Bay of Plenty.

The sci­en­tists be­came familiar with shortfin and longfin eels while work­ing on post-grad­u­ate re­search projects.

‘‘While th­ese fish weren’t the fo­cus of our re­search, they cer­tainly were an in­ter­est­ing part of the projects. With our work at Ry­der Con­sult­ing, we com­plete sur­veys for wa­ter qual­ity, pe­ri­phy­ton [al­gae that grows on the beds of rivers], in­ver­te­brates [the small bugs that live amongst the stones in rivers] and fish. We cap­ture large num­bers of na­tive fresh­wa­ter fish, in­clud­ing bul­lies, galaxi­ids and eels.

‘‘We’re al­ways fas­ci­nated when we cap­ture eels, as they are a mys­te­ri­ous mem­ber of New Zealand’s fresh­wa­ter fish­eries and a lot of peo­ple don’t un­der­stand much about them,’’ Ludgate said.

The sci­en­tists usu­ally put an elec­tric cur­rent through the wa­ter to stun the fish, then scoop them up in nets and put them in buck­ets for iden­ti­fi­ca­tion and mea­sur­ing.

‘‘The fish very quickly re­cover from be­ing stunned, and it is a use­ful method for cap­tur­ing fish. While it sounds cruel to the fish, when done prop­erly it does not cause any harm,’’ Ludgate said.

‘‘In larger bod­ies of wa­ter, and when we are tar­get­ing eels, we use fyke nets. Th­ese are long nets with mul­ti­ple hoops that the eels swim through but can’t swim out again. We set the nets in the early evening and col­lect them the next morn­ing. Any fish are mea­sured and re­leased.’’

Ludgate and Gold­smith love their work, par­tic­u­larly be­cause they also en­counter fish few other peo­ple see or even know about, such as short­jaw kokopu and tor­rent fish. Eels are the most com­mon fresh­wa­ter fish in the coun­try.

Even so, there are con­cerns about the scarcity of large fish and elver re­cruit­ment. Var­i­ous rea­sons are cited, in­clud­ing changes in oceanic cur­rents, habi­tat losses, over-ex­ploita­tion of adult stocks, along with the mor­tal­ity and migration de­lays ex­pe­ri­enced by up­stream and down­stream mi­grants.

An­other is­sue is that Lake Hawea has low nu­tri­ent lev­els and low amounts of fish food. Eels, trout and bul­lies must com­pete.

Ludgate says cap­tur­ing eels in any large body of wa­ter – such as Lake Hawea – is dif­fi­cult, so they tar­get their sur­veys near the lake out­let.

They also don’t aim to catch all mi­grants.

Rather, they want to un­der­stand where the fish go as they swim down­stream. At other dams in New Zealand, eels are found up­stream of the dam but in the Clutha, which has very large dams, no sim­i­lar ob­ser­va­tions have been found.

Signs of migration are not con­fined to in­creased eye size and not all mi­grants dis­play ob­vi­ous changes. A mi­grant eel’s stom­ach will shrink be­cause it won’t feed dur­ing migration. Sex­ual or­gans grow, the belly changes from yel­low to sil­ver, fins darken and the head changes shape too.

‘‘Stud­ies have found a re­la­tion­ship be­tween eel length and eye size – the eyes get con­sid­er­ably larger dur­ing migration.

‘‘As we can’t look at stom­ach size and sex­ual or­gans with­out cut­ting into the eels, we’re us­ing the ex­ter­nal changes to help us de­ter­mine if the eels are mi­grat­ing. So we look at colour, fin size, and we mea­sure eye size and body length, as th­ese are all things we can as­sess with­out hav­ing to hurt the eels,’’ Ludgate says.

The track­ers love their work, not the least be­cause they get to learn more about New Zealand’s fish­eries and get to work out­doors in ev­ery­thing from small for­est streams to large lakes.

‘‘We also spend a lot of time work­ing in the of­fice, ei­ther study­ing small an­i­mals down the mi­cro­scope, or analysing data and writ­ing re­ports.

‘‘So we have a lot of va­ri­ety in our work, with new ques­tions al­ways to be an­swered, which keeps it very in­ter­est­ing. It’s also re­ward­ing pro­vid­ing in­for­ma­tion to peo­ple about fresh­wa­ter com­mu­ni­ties and en­vi­ron­ments, and to help them un­der­stand any po­ten­tial ef­fects of their ac­tiv­i­ties,’’ Ludgate says.

They rec­om­mend peo­ple who want to do fresh­wa­ter fish­ing for a job com­plete a science di­ploma or de­gree at a uni­ver­sity or polytech­nic and ask other re­searchers if they can help with their projects, ei­ther as a vol­un­teer or in paid work.

As­sist­ing other re­searchers was a great way to ex­plore the op­tions and pro­vided use­ful ex­pe­ri­ence that em­ploy­ers looked on favourably, Ludgate and Gold­smith said.

‘‘An en­quir­ing mind, love of the out­doors, and also a good level of fit­ness is es­sen­tial, as the work can in­volve car­ry­ing heavy loads of gear through steep and dif­fi­cult ter­rain in some­times chal­leng­ing con­di­tions,’’ Ludgate said.

Dunedin fresh wa­ter sci­en­tist Ben Ludgate mea­sures an eel eye dur­ing a sur­vey of the Clutha catch­ment in April. Pho­tos: Ry­der Con­sul­tants

An eel with an eye for travel.

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