Abstract

The frequency of microincrement formation in sagittae otoliths of jack mackerel Trachurus symmetricus was validated using experiments on captive fish. Adult jack mackerel were injected with a dose of 100 mg of oxytetracycline/kg of fish. A second injection was performed 30 days later. The fish were then sacrificed and their sagittae otoliths were extracted. Thin sections of the otoliths were prepared and observed through an epifluorescent microscope using ultraviolet light. Two fluorescent marks corresponding to the two injections were clearly visible. The average number of microincrements between the two fluorescent marks was 29 (n=10; S.D.=1.63) and the median was 29.3. The Wilcoxon signed-rank test indicated that this value was not significantly different from 30. This result indicates that microincrements in otoliths of adult jack mackerel of between 28.4 and 37.7 cm fork length are formed with a daily frequency.