examined, 1 (4.8%) was discarded. Of 29 S. 

 delicatulus, 3 (10.3%) were discarded (Table 1). 



Results and Discussion 

 Marking Technique 



In the experiment to determine an effective 

 tetracycline-marking concentration, all fish (n = 17) 

 in 400 mg/1 died during the 12-h immersion period. 

 Of 10 fish treated with 250 mg/1, 1 died during 

 treatment, and 1 died during the subsequent holding 

 period. Of 10 fish treated witlv50 mg/1, 1 died dur- 

 ing treatment. 



Otoliths of untreated specimens showed faint 

 fluorescence around the edge and occasionally along 

 cracks and surface irregularities (Fig. 1A); this is a 

 naturally occurring autofluorescence (Campana and 

 Neilson 1982). Otoliths of fish in 50 mg/1 were indis- 

 tinguishable from those of untreated specimens. 

 Otoliths of fish in 250 mg/1 showed a strong fluores- 

 cent band medial to the edge, in addition to the weak 

 fluorescence at the edge (Fig. IB, C). This strong 

 band consisted of two growth zones and one discon- 

 tinuous zone (Fig. 2). 



It is not known how long it takes for tetracycline to 

 be incorporated into the growing otoliths when 

 administered by immersion. Campana and Neilson 

 (1982) reported that after injection, 50% of fish 

 showed fluorescent otoliths after 10 h, and 100% 

 after 24 h. If one assumes similar or slightly longer 

 incorporation times in the present study, then the 

 inner fluorescent growth zone was probably formed 

 the day after the immersion period. The subsequent 

 discontinuous zone and growth zone were formed 

 while there was residual tetracycline in the water or 

 fish. Another possible explanation is that the 

 appearance of fluorescence in two growth zones is an 

 artifact of viewing whole otoliths. 



The results of this experiment indicate that immer- 

 sion in a concentration of 250 mg Achromycin/1 of 

 seawater for 12 h is adequate to mark one or more 

 growth increments in//, tropicalis and S. delicatulus 

 larvae and juveniles. The overall mortality rate in 

 experiments I, II, and III (total n = 37), was 5.4% 

 during treatment and 2.7% during the holding phase. 



To determine whether fluorescent marking would 

 occur if the tetracycline immersion period was during 

 daylight hours, an experiment was conducted using 

 S. delicatulus from 17.9 to 22.9 mm SL (experiment 

 IV). The fish were collected and divided between two 

 tanks. One tank received tetracycline from 1800 h to 

 0630 h, the other from 0600 to 1800 h. Mortality due 

 to treatment was not monitored. After 6 d, the fish 



FIGURE 1. — Flourescence photomicrographs of sagittae of larval 

 Hypaatherina tropicalis. A. Untreated otoliths, showing autofluores- 

 cence around the edge (10.1 mm SL). B. Tetracycline-marked oto- 

 lith, showing fluorescent band medial to the edge (16.2 mm SL). C. 

 Marked otolith under higher magnificaton (17.6 mm SL). 



were killed and examined. The fluorescent bands 

 medial to the edges were similar in width and inten- 

 sity to those in previous experiments, and showed no 

 difference between the two treatments. This 

 indicates that tetracycline is incorporated into grow- 

 ing otoliths and produces fluorescent increments 

 equally well during the day and night, regardless of 

 whether the solution is yellow or has oxidized to 

 pink. 



239 



