Consequences of unreliable age determination in the management of the European eel, Anguilla anguilla (Linnaeus, 1758)

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The study of growth in fish usually requires the determination of age. Although in temperate waters age is often obtained by counting the year rings on the hard parts of fish, accurate estimation of age requires validation. Several attempts have been made to validate age determination. Nevertheless, due to temporal, spatial and individual changes it has become a complex task especially when it involves anguillid species. Age determination in these species is hampered by a complex life cycle, an extensive geographic range and the colonization of a wide variety of habitat types. This study focuses on the discrepancies in age estimation for the European eel, Anguilla anguilla, throughout its distribution area considering both estuarine and freshwater habitats. Despite considerable effort, estimating age and growth of anguillids remains a difficult task mainly due to the recognized difficulty in interpreting otolith marks. As a consequence, big discrepancies in age estimation have been found not only among localities but also among readers. Although the inevitable influence of latitudinal differences, environmental conditions and individual variability cannot be underestimated, such discrepancies might reflect differences in the interpretation of ring periodicity as well as differences in absolute age. Due to many constraints, age validation methods have not been fully successful to clarify this situation.