A school of brightly coloured whitebait are leading the way to help our native fish.
Dyed pink and orange, they've been sent swimming up a town culvert near Nelson to see if recent improvements are helping.
"They are really easy to spot in the culvert when they're bright pink or brown," said NIWA freshwater ecology technician Peter Williams.
Around 200 unmarked clear whitebait, 200 pink (Rhodamine B stained) whitebait, and 200 orange (Bismarck Brown stained) whitebait were released into the Reservoir Creek culvert in Richmond to battle upstream.
"Seventy-four per cent of New Zealand freshwater fish species are in decline and upstream barriers are stopping them from getting up to their habitat that they need to complete their life cycle," explained Mr Williams.
In April, the Tasman District Council stepped in to help, installing flexible weir baffles.
"Water goes from A to B very quickly in a natural culvert, that's what they're designed to do," said Fish and Wildlife Services' Tim Olley. "What we're looking to do is create resting pools in the culvert, low velocity areas for the fish to burst swim and rest, burst swim and rest, more or less like a stepladder."
The whitebait released have a 136-metre journey up the culvert while being monitored by NIWA, Tasman District Council and F&WS specialists over 48 hours. It's hoped the majority will make it out the other end.
NIWA and the Department of Conservation recently released national fish passage guidelines for keeping waterways swimmable. But freshwater ecologist Mike Joy says tougher rules are needed.
"A lot of these things (have) been put in and very, very little if any measurement of actually if they work or not," Mr Joy said.
"Without a doubt, the solution would be to not allow them to happen in the first place. Under the Freshwater Fisheries Act you're not allowed to impede the passage of native fish, so if you just said, 'No you couldn't do it', you wouldn't have to retrofit these things afterward."
Tasman District Council resource scientist Trevor James, who is also a member of the country's Fish Passage Advisory Group, agrees that "the best culvert is actually a bridge".
"So that's correct, but in the world of reality bridges are expensive - they have to be certified to take a lot of load all that sort of thing," he said.
He'd like to see all councils step up monitoring of fish passages after installation.
"Roading engineers contract out every year, every second year to monitor the culverts from an engineering point of view," he explained. "It would only be a small add-on to actually assess for fish passage."
NIWA says the guidelines have been well received by councils so far and the monitoring at the Reservoir Creek culvert will help other councils find cost-effective solutions for the future.