Fewer sea-run trout in the Pomahaka River has prompted Otago Fish & Game to survey the river.
The major tributary to the Clutha River has been a popular fishing spot for generations.
The Otago Fish & Game Council commissioned a report by James Smith, who interviewed anglers who regularly used the river.
The council discussed the report at its meeting in Roxburgh last week.
Mr Smith interviewed eight anglers and one Pomahaka runholder.
The report found most of the anglers thought there had been fewer fish migrating to the river.
The interviewees debated the reasons for this but the report said they had noticed flow changes, declining water quality and changes to the river's in-stream habitat, which could be reasons for less migration.
Half of the participants thought bag limits were too high, mainly because of the decline in trout numbers.
Otago Fish & Game chief executive Niall Watson said a few options could help control the decline in fish numbers in the river.
It rises west of the Teviot Valley and flows south and east for 125km, entering the Clutha River north of Balclutha.
Regulations such as bag limits and designating the river as a backcountry fishery, so fish numbers would be recorded, could be options, Mr Watson said.
But a physical survey would also help.
Fish & Game could commission a drift dive survey, in which a diver would go into the river and count the number of fish.
However, he thought drones could be the most effective way to survey the river at a ``reasonable cost''.
``It will allow us to look at more fish monitoring and closely manage it.''
Mr Watson said the council would need to discuss these options at an upcoming meeting.
He could not say when surveys would begin.
