BROTHERS ET AL.: DAILY GROWTH INCREMENTS IN OTOLITHS 



Figure l. — Light microscope photographs of 

 otoliths from laboratory-reared northern an- 

 chovy: a) 8-day-old larval otolith showing two 

 daily growth rings; b) 18-day-old larval otolith 

 showing 12 daily growth rings. 



Table l. — Chronological age (days from hatching) and numbers 

 of growth increments in otoliths of northern anchovy. 



cyclical units are 1 to 2 ^tm thick in this part of the 

 anchovy otolith and that they do not appear to 

 contain any smaller units. It is these units that 

 are counted and appear in the data in Table 1. The 

 daily increment would therefore appear to be the 

 smallest unit of growth that is formed at the 

 supra-molecular level and, as such, is in principle 

 the most natural unit to use for age estimation. 



Fertilized eggs of the California grunion, 

 Leuresthes tenuis, were obtained and reared in the 

 laboratory. The larvae were maintained in a 

 natural light cycle at 17° to 20°C with food 

 { Artemia nauplii) continuously available. Larvae 

 were sacrificed at intervals and their otoliths were 

 examined. Table 2 shows the results obtained and 

 Figure 3 shows a photograph of a grunion otolith. 



Table 2 shows that there is a close relation be- 

 tween the number of growth increments and the 

 chronological age of the larvae. Although the 

 agreement between age and daily increments is 

 not as good as it is for the anchovy, the results are 

 still very good. Table 2 also shows that in L. 

 tenuis, daily increments appear at hatching, 

 rather than at yolk absorption. Prehatching 

 marks also occur, although they were not tallied 

 in Table 2. Clearly the exact timing of the initi- 

 ation of daily increment formation varies from 



