sampling. In 1976 and 1977, the collector was op- 

 erated only during the spawning periods, which 

 were predictable on the basis of data collected dur- 

 ing the previous years. Eggs of P. sexfilis were 

 distinguished from occasional eggs of other species 

 by their diameter (800-825 ^im) and by comparing 

 hatched larvae with larvae obtained from 

 hormone-induced spawnings; the identification 

 was further corroborated by rearing larvae to the 

 juvenile stage on several occasions. Since P. 

 sexfilis undergoes a male-female sex reversal, ad- 

 ditional males were added to the population each 

 year to maintain a female to male sex ratio be- 

 tween 1:1 and 1.5:1 during the spawning season. 

 In 1976, additional males were not added until the 

 third spawning' month, so data on spawning were 

 not available for the first two spawning months of 

 that year. At the end of this study in December 

 1977, the population of spawners numbered 57. 



In order to determine the exact time of day when 

 spawning occurred, water was sampled from the 

 holding net continuously with a centrifugal pump 

 at 10 1/min and passed through a small collecting 

 basket (500-/i,m nylon mesh) on a barge anchored 

 next to the net. The collecting basket was moni- 

 tored visually, and the time when eggs first ap- 

 peared was noted. The superstructure of the barge 

 provided a barrier between the observer and the 

 holding net, so that activities associated with 

 monitoring the basket did not disturb the spawn- 

 ers in the net. 



Results 



Lunar Spawning Rhythm 



Eggs of P. sexfilis were first observed in the 

 collector from 23 to 25 July 1973. Subsequently 

 the fish were found to spawn at night over a period 

 of 3-7 (in one instance, 10) days once each month, 

 always in proximity to the last quarter phase of 

 the moon (Figure 1). and in a spawning season 

 that extended from May or June to October ( Table 

 1). Because the lunar month is 29.5 days long and 

 does not coincide exactly with the calendar month, 

 the calendar dates of spawning (Table 1) were 

 generally 1-3 days earlier in each succeeding 

 month. The first spawning of each monthly series 

 was usually preceded by 1 or 2 days on which the 

 fish fed less actively than normal. Counts of P. 

 sexfilis eggs from the collector were made in Au- 

 gust 1973, and thereafter. Judging from the sam- 

 ples obtained by the collector, relatively few eggs 



Figure l. — Numbers of Polydactylus sexfilis eggs obtained by 

 the airlift egg collector between August 1973 and October 1975. 

 In July 1973, eggs were noted on 3 days but were not counted. 

 Carets indicate the time of the last quarter phase of the moon. 



Table l. — Dates on which eggs were produced by captive 

 Polydactylus sexfilis during study period. For each spawning 

 month, the upper date is the first observed day of spawning, the 

 lower date the last day. 



Spawning monttn 



Year 



III 



IV 



VI 



'Data not available 



^Collector malfunctioned on previous day: initial spawning day possibly ear- 



were produced on the first and last days of spawn- 

 ing, with a peak usually on the second or third day 

 (Figure 1). However, because the eggs collected 

 represented only a sample of the total number of 

 eggs produced, it is possible that the peaks 

 reflected some sampling variability. 



The time of spawning relative to the lunar cycle 

 appeared to change as the spawning season pro- 

 gressed. The first spawning series of the year al- 

 ways began (Figure 2) on the day of the last quar- 

 ter (in one case the egg collector malfunctioned 

 prior to the last quarter, and it is possible that the 

 first day of spawning occurred earlier). In sub- 

 sequent months the series began 1-4 days prior to 



901 



