Lake Hawea eels get helping hand
Researchers investigate long-range migration
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.