Niwa freshwater ecologist Dr Paul Franklin recently completed the first nationwide assessment of river fragmentation, looking for structures that could act as barriers to fish.
He said the structures were often human-made and included dams, culverts and weirs, and 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 fish, 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."
However, ORC principal compliance specialist Rachel Brennan said new measures were now in place to ensure native and sports fish could move up and downstream unimpeded, to complete their life cycle.
She said culverts, weirs, flap gates, fords and dams installed after September 3, 2020, must now meet minimum reporting requirements such as height, width and location under the Essential Freshwater regulations and the National Environmental Standards for Freshwater.
"These requirements apply to all rivers — a broad term describing any natural water course, with water from natural sources, such as springs, peat bog, lakes or ground water which may or may not have water running through it all year.
"It’s important that the in-stream structures, such as culverts and weirs, are designed to allow for fish passage, so they can move upstream and downstream between different river and stream habitats."
The new rules also allowed for barriers which prevented the passage of undesirable fish.
"That’s why ORC needs to be advised of work, including installing and maintaining in-stream structures which may affect fish passage, as we work with the communities to promote desirable fish and limit the undesirable fish."
While the new regulations do not apply to structures installed before September 3, 2020, those older structures, including weirs and dams, must still provide fish passage, and there were options for remediating or retrofitting those structures to help fish passage, she said.
