LIKE other native fish species, the migration of whitebait suffers from the construction of dams, culverts and weirs. PHOTO SUPPLIED
NEARLY half of the New Zealand river network is partially or fully inaccessible to migratory fish, a new study shows, and the problem applies to Waikato rivers.
Dr Michael Pingram, Waikato Regional Council’s water quality and ecology team leader, said Waikato rivers were no exception to the problem of dams, culverts and weirs restricting passage of native fish.
“Yes, there is a problem with blocked rivers limiting movement of native fish species in the Waikato region, and rivers like the Mōkau are very important for our native fish,” Michael said.
“The importance of native fish whitebait migration is well known by whitebaiters. A 2008 study by the regional council found 60% of structures were a potential barrier for fish migration across our region. That’s similar to the NIWA national numbers.
“While we don’t have numbers specific to the King Country, this area was included in our study.
“To mitigate the issue, we have rules in our Waikato Regional Plan relating to construction of new structures like culverts and crossings.
“Our staff were also involved in developing national guidelines for how to build these structures in a way that does not stop fish migration. But that still leaves a legacy of older structures that were built long before modern guidelines were put in place. That will be a challenge for our council and the community going forward,” Michael said.
NIWA conducted the first nationwide assessment of river fragmentation, looking for structures that can act as barriers to fish. These are often human-made and include dams, culverts, and weirs. It found that New Zealand has some of the highest densities of barriers to fish passage in the world, with approximately 0.16 barriers/km.
This works out at a minimum of 48% of the river network being at least partially inaccessible to migratory fish, and a further 36% that could be restricted but has not yet been assessed for risk to fish passage.
NIWA freshwater ecologist Paul Franklin said the fragmentation of river systems was a key driver of freshwater biodiversity loss.
‘There are just over 50 native New Zealand fish species, with around 85% of those not existing anywhere else in the world. Many of our iconic fishes, such as eels and the whitebait species, migrate between the sea and our rivers to complete their life cycle.
‘However, they often encounter different structures along the way that act as a barrier, stopping them reaching key habitats. This ultimately leads to lower abundances and potentially loss of these species,’ Paul said.
To address this challenge, NIWA is studying the swimming abilities of fish and coming up with solutions to help them migrate.
‘We put fish into the equivalent of a water treadmill to see what speed they can maintain – we want fish to be able to move upstream casually, not having to fight the whole way. We can then assess what adjustments need to be made to structures in our river networks to help fish pass through these environments more easily,’ Paul said.
Culverts, for example, are put in streams and rivers to help people cross safely, while allowing an adequate passage of water. However, they can alter the flow of the stream or river, and therefore the ability of fish to swim through them.
‘The good news is that many of the solutions are extremely easy to implement,” he said. “You can install baffles into culverts, which are basically panels placed inside the tube that help to slow water down and create resting areas, so that fish can pass through into the stream above.
‘It’s small changes like this that can make a big difference. If we don’t start addressing the threats that we’ve placed into their environment, these precious fish will one day be gone.’
