Abstract
Visual evaluations of “opaque” and “hyaline” zones in fish otoliths do not dependdirectly on the absolute optical density of a certain zone or on the organic matter contentr on the rate of calcium deposition in the otolith at the period of zone formation. When observing zone formation at the edge of an otolith we say “the opaque zone is laid down” when calcium deposition slows down, and “the hyaline zone is laid down” when calciumdeposition speeds up. It is obvious that we can only observe that a new zone has been laid down when the change in calcium deposition is great enough to produce a change in the optical densityof the otolith substance that can be detected visually.

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