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Spatial variation in increment otolith formation can affect precision of age estimation – example from the NSW sea mullet (Mugil cephalus) fishery.

Kimberley A. Smith *

Cronulla Fisheries Centre, New South Wales Fisheries, PO Box 21, Cronulla, 2230, NSW, Australia
^*Present address : Department of Fisheries, Western Australian Marine Research Laboratories, PO Box 20, North Beach, WA, 6020, Australia.
Email: ksmith@fish.wa.gov.au

Several methods were used to validate the annual periodicity and determine the timing of formation of opaque and translucent zones in the sagittal otoliths of sea mullet 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 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 earlier in the year, 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 0-11 y were examined. Otoliths were sampled from two locations on the NSW 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 was recommended that sampling of sea mullet otoliths 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 stage of completion of increments is most variable.