Otolith Microstructore of Larval Herring (Clupea harengus): Image or Reality?

Abstract

When assessed with light microscopy, daily increment formation did not appear to occur in the otoliths of known-age larval herring (Clupea harengus). Increment counts underestimated age in all larvae. The age-increment discrepancy increased curvilinearly with age and appeared to stabilize after 50–60 d. Both the magnitude and rate of increase of the discrepancy were consistent with a hypothesis of resolution-limited increment visibility; models of daily otolith growth indicated that discrete daily growth increments would not be resolvable with a light microscope for the first 15–20 d after hatch. The hypothesis was also consistent with the observed effects of otolith polishing, a systematic difference in increment counts between different-sized sagittae in the same larvae, and other published reports of apparent nondaily increment formation in slow-growing pelagic larvae. Previous reports of growth rate limited increment formation appear to provide an empirical description of the same phenomenon. Otolith-based age, growth and mortality estimates can be expected to be biased if resolution effects are ignored. However, various procedures are available for the identification of potentially sensitive species and samples.