Davis and West Reproductive biology of Lutjanus vittus from North West Shelf of Australia 



229 



caught during days of major spawning activity (lu- 

 nar days 2-10 and 18-29) were considered. Two 

 measures that proved useful in detecting changes 

 in spawning activity on temporal scales of less than 

 a day were the proportion of mature fish with ripe- 

 stage ovaries based on whole-oocyte staging, and 

 maximum oocyte diameter (MOD). 



A clear diel cycle of spawning was evident 

 (Fig. 8). Proportions of ripe-stage fish in samples 

 taken at different times throughout the day were 

 significantly different (likelihood ratio x 2 =69, df 7, 

 p<0.001). Proportions of ripe fish were highest be- 

 tween 08:00 and 14:00 h, and no ripe fish were 

 present by 16:00 h. The mixture of ripe and unripe 

 ovaries between 11:00 and 15:00 h could indicate 

 that spawning for that day had already begun and 

 that some of the fish were spent or that only a 

 portion of fish spawned each day. The temporal dis- 

 tribution of MOD showed a similar pattern. Fish 

 about to spawn that day were clearly separable from 

 other fish by MOD at ll:00h. The MOD's of all but 

 one fish sampled after 15:00 h were the same as 

 nonspawning fish, suggesting that most spawning 

 occurred between 11:00 and 15:00 h on these days, 

 which more or less coincided with rising daytime 

 tides. 



Postovulatory follicle data from the same subset 

 offish also showed a diel pattern (Fig. 9, page 232). 



1.0 



0.8 



CD 



i_ 



| 0.6 



o 



1 0.4 



O 



0.2 



0.0 J r-r 



.IfTTTTTTTlJ 



i  i i  i  ' ' ' i  i 1 1 1 ' i i ' 1 1 i ' 1 1 



100 150 200 250 300 350 



Length (mm) 



1.0 



0.8 

 S 



D 



« 0.6 



E 

 c 

 o 



t 0.4 

 o 



Q- 

 O 



°- 0.2 



0.0 



12 3 4 5 6 



Age (years) 



Figure 5 



Proportion of total female Lutjanus vittus that are mature by 

 10 mm length-classes and by age-class during the height of the 

 spawning season. Shown are 95% binomial confidence limits. 

 Age data from Davis & West ( 19921. 



The proportion of fish with early- or late-stage post- 

 ovulatory follicles differed significantly with time of 

 sampling (early-stage likelihood ratio .r 2 =130, df 7, 

 p<0.001; late-stage likelihood ratio .r 2 =131, df 7, 

 p<0.001). Fish with early-stage postovulatory follicles 

 were first detected at 12:00 h. By 17:00 h the propor- 

 tion of fish with early-stage postovulatory follicles had 

 reached its highest level (91%). Thereafter the propor- 

 tion declined, until by 04:00 h none were present in 

 fish sampled. Late-stage postovulatory follicles followed 

 a similar temporal pattern, with a peak that lagged 

 the early stage by 12-14h. The very few late-stage 

 postovulatory follicles in samples at 17:00 h may have 

 resulted from spawning early that day or late spawn- 



