TANAKA ET AL : EFFECTS OF PHOTOPERIOD AND FEEDING ON TILAPIA NILOTICA 



h, the otolith showed two phases, growth and rest, 

 in a 24-h period (Figure 3A). The growth phase 

 started between 0900 h and 1200 h and continued 

 until 0500-0800 h next morning. The growth 

 seemed to stop or slow down about the time of 

 lights-on, resulting in the formation of a discon- 

 tinuous zone. After several hours, the otolith 

 started the next cycle of growth. This growth pat- 

 tern occurred regularly for a consecutive 3-d 

 period (Figure 3A). Accordingly, one incremental 

 and one discontinuous zone were formed in the 

 otolith in 24 h. When the photoperiod was reversed 

 to lights-on at 2000 h and lights-off at 0800 h, the 

 time of zone formation was also shifted about 12 h 

 (Figure 3B), i.e., the growth phase started at 

 sometime between 2100 h and 2400 h and lasted 

 until about 2100 h. It should be noted that the 

 formation of the incremental zone started a few 

 hours after lights-on in both the experiments. 



To see how the time of incremental zone forma- 

 tion shifts following a sudden reversal of light and 

 dark phases (12-h shift), the index of completion 

 for current increment at 2000 h was traced 

 through the reversal (Figure 4). Under the ordi- 

 nary photoperiod before the change, the index had 

 been maintained at about 509f and persisted in the 

 first day after the shift to the new photoperiod. 

 However, the index at 2000 h became variable in 

 the second day, and then gradually changed to a 

 level of about 100% by the sixth day. 



100 



50 





' i "S =f i 



I2L-I2D ; I2L-12D 



(L 0800 - 2000h) ; ( L,2000 - 0800 h ) 



n LIGHT PERIOD 



 dark period 



Figure 4. — Change in the index of completion for current in- 

 crement at 2000 h in Tilapia nilotica otoliths when the fish were 

 kept under a photoperiod of 12L-12D ( light phase. 0800-2000 h) 

 for 10 d and then transferred to the reversed light condition ( light 

 phase, 2000-0800 hi. Each dot represents one otolith. 



6D) and in the other with lights-off at 1400 h 

 (6L-18D) despite the 12-h shift in the time of 

 lights-off (Figure 5). The growth phase started 

 about 3 h after lights-on and continued until about 

 0800 h the following morning. Thus, the discon- 

 tinuous zones were formed sometime between 

 0800 h and 1100 h in both groups. It was also 

 observed in both groups that the growth slowed 

 down several hours before lights-on, as seen in 

 12L-12D experiments. 



Formation of Otolith Rings Under 

 18L-6D and 6L-18D Photoperiods 



Feeding Time and 

 the Formation of Otolith Rings 



The daily growth pattern was found to be the 

 same in the group acclimated to photoperiods of 

 lights-on at 0800 h and lights-off at 0200 h (18L- 



Two groups of fish were acclimated to different 

 feeding schedules under the same photocondition 

 (12L-12D, light phase, 1200-2400 h). One group 



Figure 3.— Daily growth of Tilapia nilotica 

 otoliths as represented by changes in the index of 

 completion for current increment every 3 h for 72 

 h. The fish were maintained under two contrast- 

 ing photoperiods: A) 12L-12D (light phase, 0800- 

 2000 h); B) 12L-12D (light phase, 2000-0800 hi. 

 Each circle represents mean ± SE for six fish. 

 START = start of new incremental zone. 



24 



12  24 



LIGHT PERIOD 



12 24 



 DARK PERIOD 



24 



TIME 



463 



