Use of an acoustic camera to monitor seaward migrating silver-phase eels (Anguilla anguilla) in a regulated river

Downstream migration dynamics of silver-phase eels have traditionally been documented using fisheries catch data. However, improved monitoring protocols and more comprehensive analyses are needed because of widespread declines in Anguillid eel stocks. Therefore, daily catches of silver-phase European eel (Anguilla anguilla) were recorded at an eel fishing weir on the lower River Erne in northwest Ireland. In parallel, eels detected downstream with an acoustic (DIDSON) camera were surveyed and the results were compared with catch data obtained at the eel weir. The principal objective was to evaluate the use of the acoustic camera to monitor migration patterns and eel swimming behaviour in a regulated river. A highly significant relationship between the acoustic camera eel counts and fishing weir catches was observed (p < 0.001). This indicates that fisheries-independent estimates of eel spawner biomass escapement will be possible in the future using acoustic cameras. Acoustic camera observations on eel swimming behaviour indicated significantly more eels swimming downstream when discharge was continuous through the night (p < 0.05). On nights with no discharge more eels were observed swimming upstream (p < 0.05). The swimming speeds of eels differed significantly (p < 0.05) between continuous and low discharge conditions. European rivers are increasingly affected by anthropogenic control of discharge to facilitate hydropower generation, flood control and navigation, and this affects natural patterns of eel migration. The results of this study demonstrate the potential use of acoustic cameras for monitoring eel migration dynamics and for behavioural analyses of eel migration in such regulated rivers.