A method was developed to quantify the number and biomass of European eels Anguilla anguilla escaping to the ocean for breeding. The non-intrusive method, involving a fixed-position, high-frequency multi-beam sonar, permitted constant surveillance of A. anguilla movements throughout their 5 month escapement season (July to December). During this period, >1000 individuals were monitored escaping to the Atlantic Ocean from their freshwater habitat in the River Huntspill study site (Somerset, U.K.). The total length of each fish was measured using the sonar software. These measurements were then converted to an estimate of mass using a length:mass regression relationship derived from historical fyke-net data from this site, comprising c. 500 A. anguilla length:mass measurements collected over a 10 year period. The net biomass of escapement from the study site was equivalent to c. 6 kg ha⁻¹ year⁻¹, lower than the present European target which would require at least 7 kg ha⁻¹ year⁻¹ from this habitat. These findings demonstrate the capabilities of this monitoring technique and its usefulness both as a tool to assess the compliance with conservation targets and as a tool to evaluate the success of conservation measures for elusive aquatic species such as A. anguilla.