EFFECTS OF PHOTOPERIOD AND FEEDING ON DAILY GROWTH 

 PATTERNS IN OTOLITHS OF JUVENILE TILAPIA NILOTICA 



KuNiAKi Tanaka, Yasuo Mugiya, and Juro Yamada' 



ABSTRACT 



Effects of varying photoperiods and feeding times on the formation of daily rmgs in otoliths of juvenile 

 Tilapia lulotica were studied with a scanning electron microscope. Two alternate concentric zones, each 

 different in structure, were distinguished in the acid-etched ground plane: one thin and appearing as a 

 groove (discontinuous zone) and the other thick and with well -developed, needlelike crystals (incre- 

 mental zone). The number of incremental zones showed a good correlation with the chronological age in 

 days after hatching for at least 28 days. The otolith showed a daily rhythm of growth under a 

 photoperiod of 24 hours (12 light-12 dark). Growth of incremental zones started a few hours after 

 light,s-on and stopped or slowed down within a few hours of the following lights-on, during which time 

 the discontinuous zone seemed to be formed. When the light and dark cycle was reversed, the cycle of 

 otolith growth began to change on the second day and accommodated to the new photocondition 6 days 

 after the reversal. A change in length of light and dark phases 1I8L-6D or 6L-18D) or a shift of feeding 

 time did not affect the time of the discontinuous and incremental zone formation. 



Otoliths have been widely used for aging fishes 

 along with other hard tissues such as scales and 

 vertebrae. Determinations of ages of larvae and 

 juvenile fish will yield information on their early 

 life history which is important for analyses of 

 their population dynamics. However, accurate age 

 determinations in terms of months or days have 

 not been possible for young fish up to 1-yr-old until 

 Pannella ( 1971, 1974) showed the presence of daily 

 rings in otoliths (sagittae) of some temperate as 

 well as tropical species. Brothers et al. (1976), 

 Struhsaker and Uchiyama (1976), Taubert and 

 Coble (1977), Timola (1977), Barkman (1978), and 

 Radtke and Dean (in press) have studied the use of 

 such rings, or daily increments, for determining 

 the age in days of some larval and adult fish in 

 temperate or tropical species. Their results 

 showed good correlations between the number of 

 rings and the ages in days after hatching in larval 

 fish. However, the number of rings was often 

 greater than the age in days in larval fish and less 

 in adult fish, and the difference seemed to vary by 

 species. This shows that otolith daily rings may 

 be formed in the embryonic stage in some fish 

 (Brothers et al. 1976; Radtke and Dean in press) 

 and that the increments may not be formed in old 

 fish by cessation of growth under some environ- 

 mental conditions (Taubert and Coble 1977). 



'Laboratory of Physiology and Ecology, Faculty of Fisheries. 

 Hokkaido University, Hakodate, Hokkaido 041, Japan. 



Manuscript accepted -January 1981. 

 FISHERY BULLETIN: VOL. 79, NO. 3, 1981, 



In order to accurately interpret otolith daily 

 rings for age readings, it is necessary to un- 

 derstand the mechanisms which induce formation 

 of individual rings. Taubert and Coble (1977) 

 suggested that a 24-h light-dark cycle is essential 

 for the formation of the ring pattern in the otolith 

 o{ Tilapia mossambica. The morphological studies 

 by Degens et al. (1969) and Pannella (1971, 1974) 

 showed that the ring pattern of fish otoliths is 

 composed of increments of two alternate light and 

 dark bands: a thick band of well-developed arago- 

 nite crystals with their long axis roughly perpen- 

 dicular to the outer margin of the otoliths and a 

 relatively thin band intersecting the aragonite 

 crystals. 



Our study was undertaken to verify the pres- 

 ence of a daily growth rhythm in the otolith of T. 

 nilotica and to determine the time of a day at 

 which a new increment is formed. We investigated 

 the effects of various photoperiods and the feeding 

 time on the zone formation cycle of the otolith. 



MATERIALS AND METHODS 



Juveniles of T. nilotica (12-24 mm standard 

 length) were used. They were obtained by natural 

 fertilization and rearing by eight females in our 

 laboratory. Eggs hatched about 4 d after fertiliza- 

 tion and the larvae remained in their mothers' 

 mouths for about 10 d. Immediately after leaving 

 the mothers' mouths, the juveniles were trans- 



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