Crystal-clear waters yield valuable fish data

Otago Fish & Game has wrapped up its drift-diving programme helping to monitor fish populations in high-country fisheries for the 2024-25 season.

Last week staff drift-dived the Makarora River, the third river in Otago to be surveyed by the method this season.

Previously drift-dived were the Upper Lochy and Upper Greenstone Rivers, which are classified as Designated Waters fisheries.

Pictured: Otago Fish & Game officers Ben Sowry (left) and Mason Court prepare to drift-dive the Upper Lochy River earlier this season. Photo: Jakub Kanok

The Caples River was also scheduled for assessment, but bad weather conditions had prevented that dive.

Drift-diving involves Fish & Game staff drifting side-by-side downriver to count and classify fish.

Otago Fish & Game officer Mason Court said staff encountered ideal conditions for fish counts last week with low flows and crystal-clear water on the upper Makarora River.

“However, these same conditions made drifting challenging, requiring staff to navigate several shallow sections on foot.”

The survey is part of Otago Fish & Game’s ongoing initiative to expand its drift-dive programme and establish baselines for long-term monitoring of key fisheries. 

Mr Court said the Makarora survey covered two 3.5km stretches, providing interesting insights, as significant numbers of large rainbow and brown trout were seen.

“Rainbow trout were notably found in exceptionally shallow, fast-moving riffles, holding their positions until divers approached closely.

“In contrast, the larger brown trout were mostly located in deeper, slower-moving pools and runs, often using in-stream structures, overhanging banks, and submerged branches for cover.”

The surveyed sections of the Makarora recorded the highest number of fish per kilometre this season, with an impressive 23, compared with 5.8 in the Upper Greenstone and 0.5 in the Upper Lochy.

Mr Court said prolonged dry weather and low flows in tributaries may have contributed to fish congregating in larger rivers, seeking more stable and cooler conditions.

“Overall, the average fish observed appeared to be in excellent condition, with well-proportioned rainbow and brown trout recorded.

“In addition to strong fish populations, staff observed a healthy abundance of invertebrates, including stoneflies, caddisflies, and mayflies — key indicators of a thriving river ecosystem.” 

Otago Fish & Game actively monitors waterways and wetlands, including the native and introduced species that live in them, to ensure their health and sustainability.

“As well as drift-dives, our fisheries monitoring includes spawning surveys, electric fishing, environmental DNA testing, acoustic monitoring, habitat modelling, and analysis of angler success, those being creel surveys and competition data,” Mr Court said.

“With the upcoming triannual Angler Notice review next season, this data will play a crucial role in informing future fisheries management decisions and potential regulation updates.” 

The survey is part of Otago Fish & Game’s ongoing initiative to expand its drift-dive programme and establish long-term monitoring baselines for key fisheries. 

This marks the first year of drift-diving on the Makarora River, with plans for a follow-up survey next year to build a comprehensive baseline dataset.

Unlike the Lochy and Greenstone Rivers, which have historical data for comparison, the Makarora River study provides a new baseline point for future monitoring.