Migration control station Oir

The migratory fish monitoring station is located downstream of the Oir, approximately 1.5 km from its confluence with the Sélune. The upstream and downstream traps are situated at the site of the Cerisel mill, in the municipality of Ducey. Operational all year round, they enable the monitoring of the migrations of salmon, trout, lampreys and eels.  

Trapping
The migratory fish monitoring station has been in operation since 1983 (Extended Lambert II GPS coordinates: E 333834 – N 2409347). It is situated on the inlet channel of the Cerisel mill and comprises two trapping systems for upstream and downstream flow. Devices have been installed to guide the fish towards the station, but they only allow for partial capture, which means the effectiveness of the traps must be estimated.

The various parameters recorded during trapping fall into three categories: environmental, technical and biological.

• Date, general weather conditions; rainfall (measured using the rain gauge at the station), water level (measured on the water level gauge downstream of the traps), whether or not spawning fish were observed jumping at the waterfall at the Moulin des Geins downstream, which indicates migration activity.
• Trap in which the fish was caught, time of recording.
• Species; type for migrating juveniles (smolt, silver, adult, etc.), sex (mainly for adults), size, weight; observations on the general condition of the fish, scale sampling depending on species and size (placed in a numbered paper bag), additional samples taken for experimental purposes, type of marking observed and applied (Upstream trap: salmon and trout are marked with Alcian blue on both pectoral fins. Downstream trap: trout are individually tagged with Pit tags).
   

The number of migrating fish, both upstream and downstream, is estimated using capture-mark-recapture surveys to assess the trap’s effectiveness.
Depending on the species, age can be estimated from size distribution histograms, via scalimetry or through individual marking (Pit Tag).

The downstream fish trap
Since 1984, it has been used to catch fish swimming downstream. The fish are first guided towards the mill’s intake by a barrier consisting of metal rods that swing in the water and are positioned across the main channel. Upon reaching the monitoring station, the fish encounter a fixed grid fitted with a leaf remover. They must take a side overflow, fall onto a sloping surface and are directed into a cage from which they cannot escape. A new downstream migration cage was installed in 2012. This has improved conditions for i) the fish (new stainless steel grids and materials to prevent injury, and a larger water capacity) and ii) the operators (better ergonomics for retrieving fish or cleaning the cage).

Technical specifications:
Cage: 1.46 m (L) x 1 m (W) x 1.40 m (H). Mesh size: 1 cm²
Screener: 3.84 m (L) x 1.95 m (W) x 1 m (H). Bar spacing: 1 cm. 70° incline
Inclined plane: 1.50 m (L) x 2 m (W). Bar spacing: 1 cm

The upstream trap
Since 1983, it has been used to capture fish migrating upstream. The fish are first guided towards the mill’s side channel, where the trap is located, by an electric barrier system. It consists of a generator that powers a deflection screen made up of bars dropping into the water every 50 cm (cathode), suspended by a cable stretched across the river, and a bar (anode) placed on the riverbed parallel to the cathode, spaced 1 metre apart.
The fish enter the cage through a funnel and cannot find their way out (fishing trap system).
Technical specifications:

Cage: 3.20 m (L) x 2 m (W) x 1 m (H). Mesh size 1 cm²
2 funnels: maximum opening 76.5 x 76.5 cm and minimum 31 x 31 cm. One with a 2 x 2 cm mesh when there is not too much leaf litter and debris (May to September) and another with a 4 x 4 cm mesh for the rest of the year.
Electric fence: 4 m (L) x 1 m (W) x 1 m (H). Spacing between bars: 50 cm

The lamprey trap
In addition to these two trapping devices, there is another system that uses the guide rail technique to capture young lampreys (of any species). The lampreys pass through an opening at the bottom of the weir system, which maintains the water level in the headrace, and are directed into a net (guide net). This system has been in operation since 2008.

The glass eel trap
In 2014, the first trials of trapping glass eels were carried out. A few eels were caught. The system was modified in 2015 to improve its effectiveness.

	Salmon Monitoring salmon migrations since 1984
	Trout Monitoring trout migrations since 1984
	Lampreys Monitoring lampreys migrations since 1984
	Eels Monitoring eels migrations since 1984