2 TIME 



14 20 2 



ILIGHT PERIOD 



14 20 2 TIME 

 DARK PERIOD 



Figure 5.— Effects of long and short light periods on the daily 

 growth of Tilapia nilotica otoliths. A) 18L-6D, light phase, 

 0800-0200 h; B) 6L-18D, light phase, 0800-1400 h. Each circle 

 represents mean ± SE for five or six fish. START = start of new 

 incremental zone. 



was fed during the first 3 h of light phases while 

 the other was fed during the last 3 h of light 

 phases, giving a 9-h difference in the beginning 

 feeding time. However, no significant difference 

 was found in the time of discontinuous zone forma- 

 tion in both groups (Figure 6). The zone was com- 

 pleted 1 or 2 h after lights-on in both groups, show- 

 ing that the feeding time had no apparent effect on 

 the formation of otolith rings under the 12L-12D 

 photoperiod. 



ILIGHT PERIOD 



IDARK PERIOD 



Figure 6. — Effects of feeding time on daily growth of Tilapia 

 nilotica otoliths when the fish were reared under 12L-12D (light 

 phase, 1200-2400 h). A) fed for 3 h after the onset of light phase; 

 B) fed for 3 h before the end of light phase. Each circle represents 

 mean ± SE for five fish. START = start of new incremental zone. 



FISHERY BULLETIN: VOL. 79, NO. 3 



DISCUSSION 



The ground and etched plane of otoliths of T. 

 nilotica exhibited a concentric ring pattern of 

 thick incremental and thin discontinuous zones. 

 In other teleost otoliths, a unit increment has been 

 described as composed of light and dark bands in 

 light microscope observations of replicas of ground 

 and etched planes (Pannella 1971, 1974) and light 

 microscope (Struhsaker and Uchiyama 1976; 

 Taubert and Coble 1977; Barkman 1978) or SEM 

 (Pannella 1974; Brothers et al. 1976; Timola 1977) 

 observations of ground and etched specimens. The 

 thickness of each band was variable but not as 

 different as observed with the discontinuous and 

 incremental zones in our results. This may be due 

 to different ways of preparation and observation 

 with different species of different ages. In particu- 

 lar, the thickness of discontinuous zones may vary 

 depending on the concentration of acid and the 

 etching time. 



It has been considered that the band easily 

 etched by acid contains more calcium and less 

 organic materials (Brothers et al. 1976; Timola 

 1977). However, our observations of nonetched 

 fracture planes in comparison with ground and 

 etched planes indicated that acid effects thin in- 

 terstitial layers between thick crystalline lamel- 

 lae to form grooves, which are wider than the in- 

 terstitial layers. This may be interpreted as 

 caused by dissolution of calcium from the intersti- 

 tial layers, and further, from the crystallized 

 lamellae. The presence of organic materials inter- 

 secting growing crystals was reported in otoliths 

 of various fishes (Degens et al. 1969; Pannella 

 1971). Dunkelberger et al. (1980) showed in a 

 transmission electron microscope study of the 

 Fundulus otolith that an interlamellar organic 

 matrix interrupts each growth layer. Therefore, 

 we consider that etching by acid discloses a layer 

 containing more organic materials and less cal- 

 cium and that acid infiltrated into the layer would 

 affect crystalline layers at both sides. The discon- 

 tinuous zones in the area along the anteropos- 

 terior axis of the otolith were not clear. Also, a few 

 weakly etched discontinuous zones at wide inter- 

 vals were found near the core region. This shows a 

 possibility that clearness of a discontinuous zone 

 reflects the growth rate of the otolith in relation to 

 the ratio in amount of organic versus inorganic 

 materials. 



Results of our experiments in which the com- 

 pleteness of the outermost incremental zone was 



464 



