Fish Counters
Fish Counters are a vital way of assessing fish populations, particularly of migratory fish like salmon and sea trout. Both salmon and sea trout spend 1-3 years as juveniles in freshwater before migrating to sea as smolts. They then spend 1-2 years at sea feeding before returning to their native river as adults to spawn. Fish counters placed upstream of the river mouth can count every adult fish that passes, although accurate counting of small juvenile fish is not yet possible.
There are several types of fish pass, using either acoustic, resistivity or infra-red technology.
Resistivity counters
These are the most common type in Ireland, and the WRFB operates several counters of this type. Typically placed at a weir, the counter consists of 3 electrode strips across the weir, such that when a fish crosses the electrodes it causes a disturbance in the electrical field passing through them, which is recorded on computer. Each passage of a fish is recorded in a database as an “event”, but other items, such as trees or debris being washed downstream can also cause events to be registered. To avoid over-counting fish, therefore, and to verify the computer count, a video clip of each event is saved on file, and checked to ensure the event was a fish.
Most of the resistivity counters in the West are placed on a weir, such as the one at Ballynahinch, but the one in the fish pass on the River Corrib at Galway consists of electrodes circling the tube that fish must pass through. See the video below for a video clip of a salmon passing through the Corrib pass.
Download video here (478KB, WMV)
Infra-Red
The infra-red fish counter produced by VAKI Riverwatch uses an infra-red beam across a small area that fish must pass through. Each fish is recorded, along with a computer-generated outline shape and size, see output screen below. The counter on the Erriff River at Aasleagh Falls is a VAKI infra-red counter and uses this technology to count fish passing through the fish pass here. There is also a trapping facility here, and all fish passing through can be intercepted for survey purposes, such as tagging or removing hatchery fish or farm escaped fish.
Fish Counter Data
The data from the fish counters is used to inform scientific estimates of the number of salmon returning to a river. This is then used by scientists on the National Salmon Commission to assess if a river is meeting its Conservation Limit (the number of salmon required to spawn to maintain a healthy population).
Rivers where numbers of returning salmon exceed the Conservation Limit may then be declared open for fishing, with a quota of the number of salmon that may be taken by anglers in that river catchment. Rivers that do not meet the conservation limit may be closed, or open on a catch-and-release only basis. These rivers can also be targeted for habitat improvement works to improve juvenile survival and smolt production, in order to improve runs of adult fish in future years.