Publication Abstract

European eel are a high priority for conservation measures, with an EU-wide Eel Recovery Plan in place, as recruitment (the number of juvenile eels) has fallen by around 95% since the 1970s. The DIDSON (Dual frequency IDentification SONar) camera is a new system, increasingly being used to collect much-needed data on eel numbers. In this study, on free-swimming eels within an artificial lagoon, the ability of the system to provide images suitable for measuring an eel’s length and subsequently estimating biomass is assessed. Using mixed-effects models, we find that the two stages of the camera measurement process (pausing the image and making the on-screen measurement) contribute similar amounts to the total variance and observe linear relationships between true and camera measurements. For measurements taken by inexperienced users these relationships are of varying quality. In some cases applying the fitted calibration line is worse than using no calibration, while for others, a total biomass estimate with 95% CI width of around 20% of the estimate can be achieved. From this, we consider the benefits and disadvantages of using this approach to provide biomass data to eel stock assessments and the potential impacts of further developments in the technology.