Implications of individual growth status on the future sex of the European eel

Sex‐related differences in growth status was demonstrated in eels Anguilla anguilla reared indoors at 17, 20 or 26° C, from the elver stage. Growth status was defined as length increase, weight increase and length–weight relationship. Eels attaining at least 10 g body weight (180–220 mm body length) were tagged with Passive Integrated Transponders (PIT). Length and weight were measured at 6‐week intervals, until individuals stopped growing or had attained 150 g weight (380–450 mm). Sex‐specific data from potentially undifferentiated eels were provided by retrospective classification of sex. Comparisions between sexes were made within groups graded by length or weight data from the beginning of each 6‐week period. There was no consistent difference in absolute length increase between small males and females, but below 40–60 g initial body weight, males displayed on average a higher weight increase than females. Males also had lower length at weight than females, even in the smallest weight groups. Early growth status may influence the future sex of undifferentiated eels, but other approaches are needed for distinction between cause and effect.