Construction of a Weir Fish Counter

To count adult salmon and sea trout migrating upstream, existing structures such as fish passes built into weirs or dams can be modified to accommodate a resistivity or infra-red fish counter and CCTV verification system. On rivers where no suitable sites already exist a custom designed counter installation can be built.

Completed Site
The finished site comprised of three counting zones, an open fish pass allowing salmon movement under low flow conditions and two adjacent Crump weirs providing a wide area of passage under normal flow conditions.

Site Before
This site, on the River Boyne in Ireland, was historically used as a salmon trap to supply ova to a nearby hatchery. The site had become derelict since use of the trap was discontinued.

Coffer Dam
Initially a coffer dam is erected around the site. The level of the dam is raised high enough to ensure that the site would not suffer from flooding under high water conditions.

Pumping Out
Once debris and redundant structures, such as old separating walls, are cleared from the site the area is pumped clear of water subsequently allowing machinery access to dig foundations for the new weir sections.

Foundations
Foundations for the new weirs were dug out from the bed of the river. Concrete can then be poured to form a new weir base.

Base Level of Weir
Once the base of the new weir is set in place the weir can be built up in stages. When the final profile of the weir is set, electrodes are embedded on the downstream side, these are then connected to a resistivity fish counter.

Separating Weir Walls
The higher the maximum conductivity of a river, the shorter the electrode span must be. Formers for a dividing wall between the two main weirs are put in place. This wall physically separates the counters electrodes, preventing cross-talk between channels and shortening the overall distance of each electrode set across the weir.

Coffer Dam Removed
Once the electrodes are fixed to the new weir surface the upstream coffer dam can be removed allowing the river free passage through the refitted fish pass and new open weir sections.