Serendipitous insight into pre-hatch microstructure formation in larval lake whitefish (Coregonus clupeaformis) otoliths

Otoliths are useful tools to estimate daily age, growth, and ontogenetic shifts in the early life stages of fish, providing information to support management efforts. Little effort, however, has gone into the examination of pre-hatch increment development. To assess variation in pre-hatch otolith size and increment formation, lake whitefish (Coregonus clupeaformis) sagittal otoliths were examined at the time of hatch from the control group of a previous toxicology study. The otolith surface area, nucleus surface area, and increment counts were measured, displaying variability in pre-hatch size and increment formation. While the nucleus surface area did not vary based on incubation time, the number of pre-hatch increments and total surface area increased for delayed-hatched individuals despite being reared under near-identical conditions. Increment deposition, however, did not appear to be based on an endogenous daily circadian rhythm. These results refine our understanding of otolith microstructure during embryonic development and confound the interpretation of otoliths from wild-caught larvae.