Scanning electron microscopic analysis of annulus microstructure in otolith of European eel, Anguilla anguilla

To validate the yearly periodicity of annulus formation in the otolith of the eel, the structure of annuli in otoliths of the European eel, Anguilla anguilla , stocked for 7 and 12 years in Lake Ommen on the east coast of southern Sweden, was examined. The population was stocked from elvers imported both from France (Bay of Biscay) in April 1979 and England (River Severn) in March–April 1984. The microstructure of an annulus consisted of single, double and/or composite tings depending on the location in the otolith. The counts of annuli in otoliths of these eels were approximately consistent with the expected age. However, supernumerary false annuli and/or annulus underestimation frequently occurred. The methodology for annulus discrimination with light and scanning electron microscopes is described.