Formation and annual periodicity of opaque zones in sagittal otoliths of Mugil cephalus (Pisces: Mugilidae)

The present study used a variety of methods to validate the annual periodicity and to determine the timing of formation of opaque and translucent zones in the sagittal otoliths of sea mullet (Mugil cephalus) from south-eastern Australia. Otoliths of captive juvenile fish were sampled regularly to determine timing of formation of the first opaque zone and ‘marginal increment analysis’ was used to infer timing of formation of subsequent zones. Otoliths of tagged (oxytetracycline injected) and recaptured fish also provided direct observations about the timing and periodicity of otolith zone formation. An ‘increment’ was defined as a translucent zone followed by an opaque zone. The first opaque zone was typically completed in February–March, at an age of 22–23 months. Subsequent opaque zones were completed between November and March, but typically in December. Opaque zones were associated with periods of slow otolith growth during winter and spring. Otoliths of fish aged between 0 and 11 years were examined during the study. Otoliths were sampled from two locations on the south-eastern Australian coast, separated by 5.5° latitude. The transition between opaque and translucent zones appeared more distinct in otoliths from the higher latitude, resulting in greater precision of age estimates at this location. Precision of age estimation was also found to vary with time of year. It is recommended that sampling of otoliths for age determination of Mugil cephalus in south-eastern Australia occur from April to October. Sampling at these times would avoid months in which otolith increments are most difficult to interpret and when the stage of completion of increments is most variable.