Methods used in the collection, preparation and interpretation of narrow-barred Spanish mackerel (Scomberomorus commerson) otoliths for a study of age and growth in Western Australia

..................................................................................................... 1 1.0 Introduction .................................................................................................... 2 2.0 Collection of otoliths ........................................................................................ 3 3.0 Otolith preparation and sectioning ................................................................ 5 3.1 Sectioning of otoliths for microincrement counts ................................ 5 3.2 Sectioning of otoliths for annuli counts ................................................ 7 4.0 Interpretation of otolith structure .................................................................. 8 4.1 Microincrements .................................................................................... 8 4.2 Annuli .................................................................................................... 9 4.3 Whole adult otoliths .............................................................................. 12 5.0 Conclusions ...................................................................................................... 14 6.0 Acknowledgements .......................................................................................... 14 7.0 References ...................................................................................................... 15 8.0 Figures ...................................................................................................... 16 Figure 1. Distal view of right sagittal otolith from S. commerson (Male, fork length (FL): 1032 mm) showing orientation, location of core and sectioning axis. .................... 16 Figure 2. Illustrations of methods used to remove the sagittal otoliths from S. commerson, a) location of cut at base of skull for method 1 of otolith removal, b) base of skull removed, arrows indicating exposed otolith cavities, c) location of first cut in method 2, d) location of second cut for method 2, arrow indicates the nerve pores. 16 Figure 3. Photomicrographs of transverse sections of sagittal otoliths from juvenile S. commerson a) central region of otolith section showing location of primordium (black arrow), the core region with clear microincrements, and the initial opaque zones containing numerous secondary increments, b) from a juvenile, FL 325 mm, with a microincrement count of 86, line indicating the axis of the count and pointing out some of the radial striae (triangles). .................................................... 17 Figure 4. Distal-proximal thickness of S. commerson sagittal otoliths with age (microincrement count), bold line indicating one year (365 days), shaded area indicates the change in otolith deposition at the completion of translucent zone with radial striae in the transverse section containing the core and corresponding lines from fitted line (logarithmic regression line) indicate otolith thicknesses at these times. ............................................................................................................ 18 Figure 5. Transverse sections of S. commerson otoliths showing annuli (black dots) and secondary opaque zones (white dots); a) complete section from a 1204mm FL male showing orientation, 16 annuli either side of the sulcus, the first radial striae zone (black triangles), the bump at the end of this zone (white arrow), b) ventral portion of section from a 1189mm FL female showing 12 annuli and the secondary opaque zone at the completion of the radial striae, c) complete section from 1246mm FL female showing 8 annuli on both dorsal and ventral portions, indicating abnormal outgrowths (white arrows). ........................................................ 19