FIGURE 2.— Tetracycline-marked otolith from H. tropicalis (17.6 

 mm SL), photographed with a combination of fluorescent and trans- 

 mitted polarized light. Arrows indicate the fluorescent band pro- 

 duced by the marking technique. This individual is from experiment 

 IIB, and shows six discontinuous zones between the innermost 

 fluorescent growth zone and the edge. The edge appears to be a 

 growth zone. 



In summary, tetracycline can be administered by 

 three techniques: feeding, injection, and immersion. 

 Feeding has apparently not been used in otolith 

 studies. The immersion technique presented here 

 has advantages over injection in some situations. It 

 can be used on fish which are too small or fragile for 

 inj ection. The fluorescent mark obtained is relatively 

 narrow, covering only two increments, compared 

 with the wider mark resulting from injection 

 (Kobayashi et al. 1964; Campana and Neilson 1982). 

 Therefore, it is distinguishable from edge autofluores- 

 cence after a shorter period of time, and allows finer 

 resolution of increment formation, which may be use- 

 ful in some experimental situations. Also, immersion 

 requires minimum equipment, facilities, and han- 

 dling of fish. 



Rate of Increment Formation 



In interpreting the results of my experiments, the 

 number of discontinuous zones between the inner- 

 most fluorescent growth zone and the edge was com- 

 pared with the number predicted if one dis- 

 continuous zone formed every day from ca. 0700 to 

 1000 h. Tanaka et al. (1981) found that growth zones 

 in juvenile Tilapia nilotica held under various 

 photoperiods started forming a few hours after 

 lights-on, continued through the dark period, and 

 stopped or slowed down about the time of the follow- 

 ing lights-on. The discontinuous zone was formed in 

 the few hours after lights-on. Mugiya et al. (1981) 

 demonstrated that the deposition of calcium in 

 goldfish, Carassius auratus, slowed down or stopped 



at sunrise and resumed in 3 h. Since otoliths are made 

 of a matrix of organic fibers, which are calcified in the 

 growth zones and not calcified in the discontinuous 

 zones (Panella 1980; Watabe et al. 1982), the find- 

 ings of Mugiya et al. (1981) support Tanaka et al. 

 (1981). Whether this rhythm of increment formation 

 is found in most fish remains to be investigated. 



The results for all experiments are presented in 

 Table 1. For fish that were killed between 0545 and 

 0830 h, the predicted number includes an additional 

 discontinuous zone that should have been forming at 

 the time of death, although this ring was probably not 

 always sufficiently formed to be counted. In these 

 cases, an otolith was considered to show daily incre- 

 ment formation even if the number of discontinuous 

 zones was one less than predicted. 



One growth increment was formed each day in 85% 

 of H. tropicalis (n — 20); the rest had one more than 

 the predicted number of increments. In S. 

 delicatulus, 46% (n = 26) showed daily formation of 

 growth increments; 27% showed one less, and 27% 

 showed one more, than expected if increments form 

 daily. Thus, the variability in rate of increment for- 

 mation was greater in S. delicatulus than in H. 

 tropicalis, but the average rate for S. delicatulus was 

 still 1 increment/d. 



This apparent difference in the rate of increment 

 formation between species may be partially due to a 

 difference between larvae and juveniles. Almost all 

 (93%) of the S. delicatulus treated were juveniles, but 

 only about half (52%) of the H. tropicalis were 

 juveniles. However, no conclusion can be drawn from 

 these data because the experiments were not 

 designed to examine this factor, and the numbers are 

 too small to compare larvae with juveniles. 



It is possible that tetracycline may affect the rate of 

 increment formation. Some workers have reported 

 that tetracycline inhibits mineralization in scales and 

 bone (Harris 1960; Kobayashi et al. 1964), although 

 others note neither growth promotion, retardation, 

 nor structural weakness in bone as a result of tet- 

 racycline administration (Weber and Ridgway 1967). 

 The possibility that the tetracycline treatment inter- 

 feres with growth of otoliths or fish was not con- 

 sidered in this study, but should be examined before 

 further use is made of this technique. 



In conclusion, the rate of increment formation has 

 been examined in only a small number of species 

 under a limited range of conditions. Recent evidence 

 suggests that increment formation may be affected in 

 some species by temperature, food availability and 

 feeding frequency, photoperiod, and developmental 

 stage (Taubert and Coble 1976; Brothers 1978; Pan- 

 nella 1980; Wild and Foreman 1980; Geffen 1982; 



240 



