Otago Fish & Game has produced a report on its drift dives on the Greenstone River, which will be presented at a council meeting today.
Drift dives, where divers drift parts of the river counting fish, were inherently variable, factors such as water clarity, diver efficiency, river flow and fish behaviour all influencing fish counts, Fish & Game officer Mason Court said.
But the dives remain one of the most effective methods of obtaining estimates of trout abundance.
The dive survey, done on the Greenstone River late last year, recorded the lowest trout numbers since records began. Ninety-one trout were recorded across the three surveyed sections of the river, 18 fewer than last year.
There was a timing difference between the surveys over the past couple of years, the latter being delayed by a couple of months because of poor weather in spring.
Mr Court said the results continued the concerning downward trend observed in recent years.
A range of environmental pressures might be contributing to the decline in trout abundance within the Greenstone catchment, he said. The establishment of didymo and Lindavia intermedia had likely altered habitat quality and aquatic food-web dynamics, potentially reducing the productivity of the river.
Increasingly frequent and severe flood events during key spawning and juvenile rearing periods might also have had an impact.
‘‘These flood patterns are consistent with wider climate change projections for alpine river systems.’’
Further expansion of Otago’s drift-dive monitoring programme was also recommended, particularly focusing on designated or pressure-sensitive fisheries.
Other rivers proposed to be included are the Young, Wilkin, Nevis and Makarora Rivers. The Caples River was also surveyed and had better results than the Greenstone River.
The Greenstone River operates under the designated waters licence system, where anglers are allocated separate beats along the river.
The Greenstone River, located in the Otago back country near Lake Wakatipu, is a clear, fast-flowing alpine river renowned for its high scenic values and, historically, quality trout fishing.
