Fl: lERY BULLETIN; VOL. 79. NO. 3 



ferred to aquaria and used in the experiments. 

 Water temperature was maintained at27±0.5° C. 



Experiments under 24-h Photoperiod 



The juveniles were transferred to four 60 1 aer- 

 ated aquaria, each kept under a different photo- 

 regime: 12L-12D (Hght phase, 0800-2000 h), 12L- 

 12D (light phase, 2000-0800 h), 18L-6D (light 

 phase, 0800-0200 h), and 6L-18D (light phase, 

 0800-1400 h). For lighting, a daylight fluorescent 

 lamp (20 W) was fixed 15 cm above the water sur- 

 face. In all groups, fish were fed to satiation on fish 

 food pellets supplied throughout the 24-h photo- 

 periods. After acclimation to each condition for at 

 least 12 d, 6-10 fish were killed every 3 h and their 

 otoliths removed for measurement of completion of 

 the newest increment. In addition, 10 fish in the 

 first group (12L-12D, light phase, 0800-2000 h) 

 were killed 19 and 28 d after hatching to compare 

 the number of otolith rings with the fish's age in 

 days. 



Feeding Experiments 



For experiments on effects of feeding we used 

 two groups of juvenile fish of the same brood. Five 

 days after leaving their mother's mouth, one 

 group was fed for 3 h after each lights-on; and the 

 other group was fed for 3 h before lights-off under 

 12L-12D cycle (light phase, 1200-2400 h). The ex- 

 periment lasted 15 d. Five fish were killed from 

 each group every 3 h during the last 2 d of the 

 experiment and otoliths were examined for com- 

 pleteness of the newest increment. 



Otolith Preparation for 

 Scanning Electron Microscopy 



Otoliths (sagittae) were removed from the fish 

 under a dissecting microscope, washed in water, 

 embedded in a few drops of epoxy resin (Bond E,^ 

 Konishi Co.), and placed on a glass microscope 

 slide. After hardening, both otoliths from each fish 

 were ground by hand with a whetstone to the mid- 

 transverse plane, parallel to the long or short axis, 

 and then polished with a compound whetting 

 paste for sharpening microtome knives. The 

 specimens were cleaned in xylene, etched with 



0.5% HCl for about 20 s, and coated v,^ith gold in 

 vacuum. The specimens were examined with a 

 JSM-25 scanning electron microscope (SEM) at 15 

 kV for number of growth rings or for determina- 

 tion of completeness of the newest increment. 



For the observation of the internal structure, 

 otoliths were placed in a drop of water and broken 

 into several pieces by a razor blade. The broken 

 pieces were air-dried, coated with gold, and the 

 nonetched fractured surface was observed with 

 the SEM. 



Measurement of 

 Daily Growth Rhythm 



In order to determine the exact hour when a new 

 increment is formed in the otolith, we calculated 

 the index of completion for current increment (C) 

 by the following formula: 



C = 



w. 



X 100 



W 



n-i 



where 



W. 



W 



n -1 



width of current increment 

 width of previous complete 

 increment. 



Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Measurements were made on SEM photographs 

 of specimens collected every 3 h. Following the 

 change of C in time sequence, one can estimate the 

 time of completion or start of zone formation when 

 C suddenly drops from approximately 100% to a 

 lower level. The area of the measurement of C was 

 at the distal surface near the anterior or the pos- 

 terior edge of the otolith, where increments are 

 thick, discontinuous, and easily observed. 



RESULTS 



The sagitta of a juvenile T. nilotica takes the 

 form of an oval disc. It lies vertically in the sac- 

 culus of the inner ear with its long axis parallel to 

 the anteroposterior axis of the fish. The inner sur- 

 face of the otolith facing the macula region of the 

 sacculus is somewhat concave, whereas the other 

 surface is slightly convex. 



When the otolith is ground parallel to its long or 

 short axis down to the core region, or the growth 

 center, and then etched, it exhibits a concentric 

 ring pattern (Figure lA), with each ring composed 

 of two alternate layers, each different in structure. 

 One layer usually measures 2-8 /ttm wide and has 

 needlelike crystals of the aragonite form CaCOa 



460 



