The importance of monitoring: mark-recapture study, Sabden

Wildlife monitoring is a vital part of conservation work. It means that we can target our work towards the areas of the catchment that are most in need of our help and ensure the work we have done has proved beneficial in terms of river health and the wildlife habitat.

Our research showed that prior to the creation the fish pass, Sabden weir was proving to be a major barrier to fish migration. This lack of upstream fish also meant that species further up the food chain, such as kingfishers, herons, and otters were deterred from inhabiting the areas upstream of the weir.

The fish passage the Trust has created has already had a hugely positive response from the human residen  ts of Sabden, but to find out how the fish feel about this we’ve been revisiting the area to look at fish numbers!

A mark- recapture study was carried out which involved capturing fish that were upstream of the fish pass with rod and line and electric fishing over a one day period. The fish were then displaced below the fish pass with the hope that the fish’s natural homing instinct would prompt them to ascend the fish pass.

A total of 41 fish were marked and displaced, of these 41 fish 13 were found to have returned upstream- including adult fish up to 31cm and this years’ fry down to 10.8cm. Another 8 of our marked fish were found in the weir pool. This means that over one day period we could only account for 21 of the 41 fish we moved downstream.

As with all studies of wildlife inefficiencies must be considered. So, it is assumed that whilst some of the caught fish may have moved even further above the fish passage, beyond where we were recapturing the fish, some may have moved further downstream. Nature is unpredictable and we simply cannot say where the remaining fish chose to go!

Overall the study shows that the work has proved successful, and that the majority of recaptured fish had managed to ascend the fish passage. During the study we also found sea trout sitting in the weir pool downstream waiting to move upstream which shows that the migratory fish are ready to use the fish pass!