Validation of otolith daily increments for larval and juvenile Japanese halfbeak Hyporhamphus sajori

Daily ring formation was verified by examining the growth of the marginal increment on sagittal otoliths of larval and juvenile Japanese halfbeak Hyporhamphus sajori. The relationship between age (x) and number of increments (y) is y = 1.0x + 2.0. The first daily increment was formed during the 2 days before hatching. The relationship between the number of rings deposited after the alizarin complexone (ALC) mark (x) and the number of increments (y) is given by: y = 1.0x − 0.2. The index of completion of the marginal increment was 99 ± 4.1% (mean ± S.D.) at 04:00, and from 24 ± 4.4% at 08:00 it increased with time of day and reached 98 ± 3.6% until the next day at 04:00. Growth of the incremental zone started a few hours after sunrise prior to which the discontinuous zone seemed to be formed. Light rhythms tend to be one of the most important factors for the formation of the marginal increment on otoliths. Based on the relationships between time of day and the marginal increment on otoliths, it would be possible to estimate the predation time for specimens retrieved from stomach contents, and also clearer analysis of the growth history immediately prior to the sampling time.