Testing three stock assessment methods to evaluate the reintroduction success of the cryptic european eel anguilla Anguilla L.

Glass eel stocking in rivers as conservation measure is usual, but methods to assess the associate performance by estimating precisely the survival rate are not investigated yet. From 2013 to 2019, scientific stocking tests have been implemented in Belgian rivers (>300 km from the estuary). Two release events, cohort 2013 (density 1-1.5 kg released at a single release point) and cohort 2017 (density 2.4 kg/ha dispersed over sites spaced from 250 m) were conducted in eight typologically different rivers. One reference site per release campaign, with the best recruitment level was selected to test different stock assessment methods. We used both multiple capture-mark-recapture sessions (CMR; cohort 2013: n = 4155 glass eels, 5 sessions in 2014-2019; cohort 2017: n = 1586 glass eels, 4 sessions in 2017-2019) in combination with mobile RFID-Telemetry investigations, over 200-m long river stretches. We compared the observed data with the data corrected using: (i) ratio between RFID detection and electrofishing (DCM), (ii) number of eels captured in a capture session using the DeLury model (DLM) and, (iii) multiple CMR occasions for open-population Jolly-Seber model (JSM). Eel density estimation was close between release techniques. JSM data were close to DCM data and estimated time-varying capture probability, survival probability between sessions and super-population abundances. Our results suggest multiple CMR campaigns with applying JSM were better-suited methodology to assess juvenile eel cryptic stocks, but DCM helped to rapidly assessing stock under limited CMR occasions. Glass eel release on several sites seems to be the best stocking technique.