FISHERY BULLETIN: VOL 76, NO. 1 



1975 systematic observations of spawning be- 

 havior were conducted. 



MATERIALS AND METHODS 



For purposes of determining diel variation of 

 spawning activity, the daylight period (from sun- 

 rise to sunset) was divided into 16 equal periods. 

 As occasional checks during the morning indi- 

 cated that the spawning population was not pre- 

 sent at the spavwiing site and was not spawning 

 elsewhere, only the latter eight periods of the day 

 were included in this study. Since day length var- 

 ied considerably between January and June ob- 

 servations, the length of each period also varied by 

 the same factor. The change of day length during 

 each of the two series of observations was only a 

 few minutes. 



During the winter observations (12-28 Janu- 

 ary), the day length was 11 h 10 min with 42 min 

 for each period. During summer ( 19-29 June), the 

 day length was 13 h 10 min with 50 min for each 

 period, an increase of 17% in day length. Water 

 temperature at the study site varied between 26° 

 and 29°C seasonally. 



The number of spawning rushes, the upward 

 dash by groups of parrotfish culminating in the 

 release of eggs and sperm, occurring during 15 

 min within the observation period was counted by 

 an observer (wearing scuba equipment). This time 

 was chosen as the minimum for measurements of 

 spawning rush frequency due to the somewhat 

 irregular occurrence of the rushes on a minute by 

 minute basis. In the latter portion of the study, 

 data were recorded minute by minute for the full 



15-min period. The observers were tethered near 

 the spawning site by lines attached to the bottom 

 which caused them to float nearly motionless at 21 

 m depth, approximately 3 m above the substrate. 

 This allowed observations to be made from a con- 

 sistent location, minimized movement needed to 

 stay in position, and decreased the depth of the 

 observers slightly to allow more bottom time for 

 observations with no or short decompression at the 

 end of the dive. The presence of the observers did 

 not seem to interrupt or affect the spawning be- 

 havior as the population did not move away or 

 cease spawning after the observers' arrival. 



Color motion pictures (16 mm) were made of 

 spawning and feeding behavior of S. croicensis, 

 including some at two times normal film speed for 

 slow motion analysis of movement. Films were 

 analyzed on a frame by frame basis. 



GENERAL BEHAVIOR 



A general profile of the area near the spawning 

 site is presented in Figure 2. Sand channels run 

 between fingers of reef directed seaward which 

 gradually slope from a shallow reef crest to a zone 

 dominated by the branched coral 'Acropora cer- 

 vicornis at 10-13 m depth. At the seaward edge of 

 this A. cervicornis zone, the reef slopes steeply to a 

 sandy bottom at 24-25 m depth. Beyond this point 

 the sand bottom either slopes rapidly downward to 

 the near vertical dropoff or has an outer reef rising 

 above it, often resembling a rounded pinnacle and 

 somewhat trapping the sediment behind it. The 

 pinnacle of "Dancing Lady Reef" was the location 

 of the spawning observed in this study. On the 







10 



20 



30 



10 



.>0 







100 



200 



300 



ACROPORA CKRVKORMS ZONK 



BOTTOM OF 

 SAND (HANNKLS 



Figure 2. — Bottom profile of "Dancing Lady Reef" offshore from the Discovery Bay Marine Laboratory. Vertical exaggeration 



is2x. 



118 



