It is especially useful in interpreting the outer zones on the otoliths close to the margin, which may be thin, split, or poorly defined. This zone is typically overgrown with calcium on the otoliths of older fish. Therefore, the hyaline core area is formed over a period of approximately 6 months, assuming that herring are spawned during early autumn. Few, if any, otoliths of herring from the Gulf of Maine or Georges Bank exhibit a tiny hyaline core area surrounded by opaque material that would indicate that they were spring-spawned fish. Time of annulus formation can vary in some years, however.
Checks can also cause confusion with edge interpretation.
Therefore, correct evaluation of the type of edge formed on the otolith, and its meaning in terms of the growth of the fish, is an important aspect of herring age interpretation. False annuli, formed during the first summer of the first full year of growth, are typical anomalies encountered on herring otoliths and may be especially characteristic of particular year-classes. These zones can be readily confused with annuli, and therefore the resulting age interpretation should be compared with that of fish of similar sizes. Subsequent to the second annulus, checks are normally formed during the second, third, and fourth summers corresponding to the prespawning period. Therefore, the rostrum is very useful in discriminating between checks and annuli.