Counting Through Fish Passes

Fish passes are often installed in rivers to aid migration past obstructions. Most passes are associated with constructed weirs and dams but can be built to allow fish access past natural obstacles such as waterfalls. Passes provide an excellent opportunity to count fish as they are guided through a narrow counting zone.

River Erriff, Co. Mayo
A series of pools with notches in the downstream wall allows salmon and sea-trout to circumvent a seven meter waterfall. To pass upstream, fish must swim through an open tube. Three electrodes on the inner surface of the tube are connected to a resistivity counter. When fish swim through the tube, and across the electrodes, the direction of passage is logged.

Ballysadare, Co. Sligo
With a ten meter waterfall at the mouth of the river, historically, no salmon could enter the Ballysadare River. This fish pass, built in the 19th Century allowed salmon access to the river for the first time. A series of screens and an infra-red counter, fitted within the pass, ensures all fish swimming upstream must move through the counting zone.

River Feale, Co. Kerry
Across the River Feale a weir, for water abstraction, was built. To allow salmon and sea-trout passage upstream, a Denil pass was built through the weir. This pass has been fitted with an underwater tube which is connected to a nearby resistivity counter. Screens over the fish pass can be opened to allow access to the counter tube and CCTV system.

River Slaney, Co. Wexford
To supply water to a hydro-electric scheme a weir was constructed across the river. Fish passage across the weir is facilitated by two passes, a Denil and a Pool Pass, built through the weir. To supply water to a hydro-electric scheme a weir was constructed across the river. Fish passage across the weir is facilitated by two passes, a Denil and a Pool Pass, built through the weir.

Dungloe, Co. Donegal
An infra-red counter is fitted to the fish pass. The frame for the counter unit has been designed to ensure that any debris traveling downstream will be washed clear of the screens. These same screens ensure fish swim through the counting zone, past an underwater camera.