Dirk C. Welsford

Validation of age and growth estimates using thin otolith sections from the purple wrasse, Notolabrus fucicola

The age of Notolabrus fucicola from the east and southeast coasts of Tasmania was estimated from counts of opaque (transmitted light) growth zones in thin transverse sections of sagittal otoliths. The position of the first annulus was validated through the otolith radii of known-age juveniles. Marginal increment analysis was performed by examining the growing edge of otoliths over 2 years. This showed that opaque zones were formed annually and were consistently deposited from late September to early December (spring season), which is coincident with annual increases in somatic growth and water temperature during this season. A criterion was developed to ensure accurate age estimation for individuals sampled during the time of year when deposition of opaque material was incomplete or not apparent. The maximum age estimated was 20 years and the relationship between fork length and age was described by the von Bertalanffy growth function (L∞ = 368 mm, K = 0.116 and t0 = –1.87 for males and L∞ = 385.7 mm, K = 0.109 and t0 = –1.96 for females). The clarity of increment structure and rigorous validation protocol that was adopted minimised the risks associated with using marginal increment analysis to validate the periodicity of increment formation and it provided robust age and growth estimates for N. fucicola in Tasmanian waters.

Interpretation of otolith microstructure in the early life history stages of two temperate reef wrasses (Labridae)

Notolabrus fucicola and N. tetricus larvae were reared in the laboratory at a constant temperature of 11°C; both species hatched after 4 days. Mortality was high throughout the post-hatching stage, with no N. tetricus surviving beyond 24 hours post-hatching. Notolabrus fucicola late yolk sac larvae showed no clear incremental structure in their sagittal otoliths up to 9 days post-hatch. The radius of the sagittae of these late yolk sac larvae corresponded with the radius of a non-incremental region around the primordium of sagittae taken from post-settlement juveniles of both species captured in the wild. Therefore, it is likely that the first increment is formed at or near yolk sac absorption in these species. Post-settlement juveniles of both species were exposed to oxytetracycline (OTC) and held in aquaria for up to 33 days. Post-OTC increment counts showed that increments were formed daily in the sagittae of both species.