Back-calculation and time of hyaline ring formation in the otoliths of the pilchard off South West Africa

The frequency of hyaline and opaque edges, back-calculation, and examination of otoliths bearing a natural tag were used to determine the time of ring formation for the pilchard off South West Africa. The extent of the error in the back-calculation method caused by seasonal variation in the relationship between otolith radius and fish length is assessed. The times at which hyaline rings were laid down between 1970 and 1981 are documented. Formation of hyaline rings is related to sea surface temperature (p < 0,001), prominent rings forming when the temperature is colder than the annual average. Generally, a prominent hyaline ring is formed in September and secondary rings in May and December.