Eels and many other fish species must migrate up (and down) river systems at different points in their life cycle. Some spawn in the sea and live in rivers whilst others spawn in rivers but live in the sea.
Eels arrive from the sea as tiny 55mm long transparent glass eels. They invade the mouths of rivers and most of them instinctively swim upstream to find a suitable place to live. Some even reach high alpine streams and lakes.
But, what happens when they run into this?
What about this natural obstacle. What happens then?
Whilst glass eels and elvers are very good climbers, some barriers are simply impossible to get past.
Many hydro schemes now have ‘trap and carry’ operations based at the lowest dam. Elvers are caught and then transferred above the various dams in the scheme.
EECo have operated the Karapiro trap and transfer scheme since 1992 in conjunction with Mercury Energy. Other operations include North Power’s Wairua Falls along with Trust Power’s Matahina and Patea Dams
Maori have operated ‘trap and transfer schemes at natural barriers and waterfalls for centuries. They are now also heavily invested in improving fish passage past barriers at Matahina, Wairua Falls, Karapiro and other dams around the country. Wai Maori Trustee have produced a detailed publication, "Report on Hydroelectric Dams in New Zealand and Fish Passage". To read this publication CLICK HERE
Perched culverts – elvers cannot climb a vertical waterspout. Perched culverts are being replaced or re-designed all over the country – usually at the behest of local bodies.
Culverts are often now fitted with baffles to slow the water and create eddies thereby assisting migrant fish. To see a video of fitting baffles to culverts CLICK HERE
Culverts are being designed with easier entry to the downstream end of the pipe by placing a sloping rock face with plenty of cover and slow water down.
Mussel growing ropes trailed down an obstacle assist elvers and other small migrating fish to use the loose fibres as leverage points to better climb. They also offer resting points and some protection from birds or predatory fish.
After spending their lifetimes somewhere up-river, large female eels will mature and look to migrate downstream to the sea. Of course, they now have to negotiate all of the above obstacles from the reverse direction.
The dynamics of downstream migration are quite different but it is also fraught with danger. This time the migrating female eels are large and powerful. They are also packed with those valuable eggs which will provide for the next generation. On their way downstream:
Large females can fall off weirs and waterfalls often landing on rocks
If they enter the intakes of hydro turbines – dead – end of story
Going over the spillways, if water is being spilled – usually safe
Fishing weirs and nets. Note: North Island commercial eel fishermen no longer target downstream migrant female eels.
Things got pretty bad for a while but they are improving.
These days, a large amount of good work goes on behind the scenes to mitigate the risks of downstream migration.
The New Zealand Fish Passage Advisory Group comprises ecologists, engineers and environmental advisors representing various agencies involved in fish passage management in New Zealand.
They are doing some really good work on identifying barriers around the country, raising awareness of these issues and encouraging / developing mitigation tools. CLICK HERE to discover detailed information on freshwater fish passage.
Niwa has developed protocols for water reservoirs and other weirs, Example: Morrinsville water supply dam. To read a statement, from Niwa, which demonstrates the prudent care with which fish passage is approached in building new dams and weir CLICK HERE
Niwa researchers are also developing downstream by passes and protocols for larger structures such as hydro dams. To read NIWA's report "Tuna - solutions: downstream passage for adult migrants at large barriers" please CLICK HERE.
Some dam operators now spill water at critical migration times to allow the big females to safely go down the spillway
Meridian Energy employs fishers to trap and carry migrant females in Lakes Te Anau and Manapouri to replace them to the river downstream of the lowest dam.
Northpower have made their spillway at Wairua Falls less dangerous for downstream migrators.
After EECo lobbying, commercial fishermen when they inadvertently catch large females are now allowed to transport them below the lowest barrier to passage.
Or, in the process of getting over the barrier, they become totally vulnerable to predation, exhaustion or dessication.
Most of these barriers have hordes of larger eels and fish at the bottom waiting for an easy meal of glass eels trying to make their way past.
If they are lucky enough to make it to the top they run a second gauntlet. Just watch these big eels in the next link – gorging on glass eels and elvers at the base of a weir.
The dynamics of climbing over obstacles vary between different fish species. The DoC website have videos that beautifully demonstrate some of the difficulties faced by different species. To watch these videos CLICK HERE. The videos also show us how easy it can be to mitigate some of these problems!
Fortunately, some eels decide to remain in coastal areas even in full salt water for life. This reservoir of salt or brackish water residents provides a back-up source of eels for breeding.
So, we humans (and nature) have excluded a great deal of habitat from eels and other migrating fish by throwing up barriers to their passage. How do we now give them a helping hand to get back into these waterways?
“Flood Pumps - the silent killer”
To protect communities downstream, some river stop banks have floodgates that can be opened to deliberately spill flood water onto less densely settled farmland. New Zealand’s large dams were built for power generation, irrigation and water supply, but they have at times buffered major floods.
In a number of large low lying flood prone areas large flood pumps are used to move the water back over stop banks into the river course.
Unfortunately, many of these systems were built many years ago before fish friendly pumps were readily available. These old systems chop up and kill huge numbers of eels.
What is left of migrant eels passing pumps Ruptured eel
EECo strongly advocates that, when pumps are replaced, they are replaced with fish friendly systems.