An Evaluation of Restocking Practice and Demographic Stock Assessment Methods for Cryptic Juvenile European Eel in Upland Rivers

Restocking of the critically endangered European eel Anguilla anguilla is widespread, but it
is rarely scientifically evaluated. Methods used to assess its associated performance by estimating
the survival rate and implement restocking for maximum recruitment in rivers have not yet been
investigated. Based on two glass eel restocking events using a single release site/point and multiple
sites per river performed in upland rivers (>340 km from the North Sea), the recruitment success of
stocked eels was scientifically evaluated during a 3-year study using multiple capture-mark-recapture
methods and mobile telemetry. We compared the observed data with the data estimated from the
Telemetry, De Lury and Jolly-Seber stock assessment methods. For recruitment data, Telemetry was
very close to Jolly-Seber, an appropriate stock assessment method for open populations. Using
the best model of Jolly-Seber, survival probability was higher (>95%) in both restocking practices,
but recruitment yields were higher and densities of stocked eels were lower in multiple sites compared
to a single site. Our results suggest that Telemetry can help to rapidly assess cryptic juvenile eel stocks
with good accuracy under a limited number of capture-mark-recapture sessions. Artificial dispersal
of glass eels on several productive habitats/sites per river appears to be the better-suited practice
for restocking.