Individual Growth of European Eel ( <i>Anguilla Anguilla</i> ) Assessed by Mark‐Recapture With Passive Integrated Transponders (PIT Tags) in Four Isolated Water Systems in the Netherlands

Growth rates are generally obtained from back‐calculation of increments of otolith readings and based on the assumption that mean annual growth can be scaled directly to the distance between age rings. Although otoliths continue to grow even when somatic growth ceased. To analyse growth rates of yellow eel, a passive integrated transponder (PIT) mark‐recapture study was performed in four water bodies (Delfland, Markiezaatsmeer, Süderpolder and Vinkeveense Plassen) in the Netherlands. PIT‐tags were used as an alternative method instead of the generally applied back‐calculation of annual increments from otolith readings. During 2017–2020, 2209 eels were tagged individually and in the subsequent years, 541 eels were recaptured. The mean annual growth rate decreased with increasing length of eel, with growth rates of 2.2–8.8 cm/year for undifferentiated (sex undetermined) eel between 30 and 50 cm and 1.3 and 5.7 cm/year for female eel between 50 and 80 cm. Differences in growth rate were also observed between the four water bodies, which having consequences to population demographics, such as age at first maturity.