Spawning salmon spotting in Shropshire

The River Onny is an important river for migratory Salmon, with numerous fish running the river to get to suitable spawning ground in the upper reaches.

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To spawn salmon cut redds, which are shallow excavations in gravel beds, and lay their eggs at the tail end of the redds where the fertilised eggs can grow into alevins in the relative sanctuary of the deep gravel beds.

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The simplest way to determine the amount of salmon that have run the river to spawn is to count the amount of redds that are cut over a set area and time period. This gives us a good idea of how strong the salmon run has been in any one year and therefore how this will relate to year class strength within the existing population.

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On the stretch of the River Onny, from Stokesay to Onibury, that we surveyed last week 10 salmon redds were seen as well as a pair of salmon guarding a recently cut redd. Four dead cock fish and one dead hen fish were seen. This is all part of the natural life cycle as not all salmon make it back downstream due to exhaustion.

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Counting fish – from my desk!

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Our fish pass next to Carreghofa weir on the Afon Tanat near Llanymynech allows fish to pass upstream, bypassing the weir.

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We have an automatic fish counter at Carreghofa. It is a resistivity counter and measures the changes in conductivity caused by an object passing through the counter. The signal can be calibrated to recognise the signature of a salmon passing through.
In order to check the validity of our data, we have installed a video camera above the fish counter.

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To enable a clear image to be seen by the camera we have carefully cleaned the white base of the counter. This has allowed us to have recorded some great images of the salmon swimming upstream through the pass to reach their spawning grounds throughout November and into December.

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We then have to laboriously cross reference images of salmon passing through the counter with counts recorded by the counter. This gives us much greater confidence in the data we use to report on our salmon numbers.