Fish & Game dive team counting trout in the Ruamahanga River. PHOTO/SUPPLIED

There’s been a major resurgence in the Ruamahanga River’s trout population and, after several lean years of fishing, recreational anglers are understandably ecstatic.

Wellington Fish & Game annually conducts surveys of the trout numbers in the Ruamahanga by drift diving – a technique in which trained divers, spaced across the river in a line, drift with the current down defined sections of the waterbody and count the fish they observe.

Surveys carried out found the most trout Fish & Game staff have seen in the river in years.

Wellington Fish & Game manager Phil Teal says the results are encouraging.

“The Ruamahanga is cherished by anglers throughout the lower North Island so it’s great to see the fishery bounce back after several years of frustratingly low fish numbers,” he says.

In 2017 the fish population had plummeted to around 90 trout across all five dive sites, totalling almost 7km of overall river length. This is relatively low by national standards at just 13 fish per km, especially considering the size and braided diversity of the river.

“It’s quite amazing that we’re now back at 45 trout per km,” Teal says. “It’s a remarkable turnaround.

“Some dive sections had fish numbers as high as 110 trout per km. These are large- to medium-sized fish too – we’re not talking yearling trout, they’re fish of between 1kg to 3kg.”

Teal suggests the higher counts can be put down to several factors, one of which is having several benign winters in a row with few major floods.

“This has made for ideal breeding conditions and survival of both young and mature fish.

“We also noticed that the trout were congregating in areas that clearly hadn’t been affected by the regional council’s in-stream bulldozing.

“This practice has disastrous impacts on all aquatic life – native and introduced. It was quite telling to see much greater aquatic biodiversity in areas left untouched.

“This shows trout and natives co-exist well, and that human impacts are by far the biggest limiting factor for all aquatic species.”

Teal says divers noticed a much more diverse range of species of fish and insects in the unmodified reaches of the river where the highest trout counts were obtained.

“We’ll be looking at the results in more detail to confirm the link between low biodiversity and regular instream river work.”