Aalbers: Spawning periodicities of Atractosaon nobihs 
145 
35 % 
0 % 
February March April 
May June 
July August 
Figure 1 
Monthly frequency distribution and photoperiods (♦) of spawning 
events for 297 white seabass (Atractoscion nobilis) spawning events 
from 2001 through 2003. 
The most intense sounds, as recorded, of each 
type were analyzed against a 1-kHz reference 
tone on each 3-min recording. Sound inten- 
sities were scaled with values consistent to 
one another, but not as absolute sound pres- 
sure level (dB re IpPa) because the entire 
hydrophone system had not been calibrated 
recently. The terms “level” and “intensity” 
are used synonymously to convey the relative 
data presented here. 
Statistical analyses 
A one-way analysis of variance (ANOVA) was 
used to examine whether diel spawning times 
were consistent throughout the spawning 
season both in relation to sunset and the 
24-h clock. The numbers of spawning events 
observed at each phase of the lunar cycle 
were analyzed against expected occurrences 
by using a chi-square test. Sound rates from 
24-h recordings during the spawning season 
(March- July) were compared to those when 
there was no spawning (August-January) using a two- 
sample £-test. Two-sample f-tests were also used to 
compare the rates and relative levels of sounds from 
evening recordings made on days that spawning did 
and did not occur. 
Results 
sunset. The inverse seasonal trend was observed when 
spawning times were analyzed in relation to sunrise. 
No equivalent seasonal shift in the time of spawning 
occurred when events were analyzed in relation to the 
24-hour clock (ANOVA, F=2.41, df=4, P= 0.06). Spawn- 
ing peaked between 19:00 and 20:00 h PST in all three 
seasons and was not confirmed before 17:30 or after 
22:00 h (Fig. 2B). 
Seasonal spawning 
Over the three-year study, 297 spawning events were 
documented; 53, 146, and 98 events were recognized in 
2001, 2002, and 2003, respectively. The longest spawn- 
ing season extended between 22 February and 15 August 
2002, whereas the other spawning seasons ranged from 
17 March to 27 June 2001 and from 3 March until 
16 July 2003. Of the documented spawning events, 
76% occurred from April through June, and 98% from 
March through July over the three-year period. Spawn- 
ing peaked in May as photoperiod increased to 14 hours 
(Fig. 1). 
Diel spawning 
Spawning occurred from two hours before sunset until 
4.5 hours after sunset. Sixty-five percent of spawning 
activity occurred over the two-hour period following 
sunset and 91% occurred within the four-hour period 
spanning from one hour before sunset to 3 hours after 
sunset (Fig. 2A). A significant shift in the diel spawn- 
ing periodicity in relation to sunset as the spawning 
season progressed occurred in all three years (ANOVA, 
F= 8.78, df=4, P>0.001). For instance, in March 2002 
the average spawning time was between two and three 
hours after sunset; however, by July 2002 the average 
spawning event occurred within the first hour after 
Lunar spawning 
Spawning occurred at every phase of the lunar cycle, 
but not uniformly (Fig. 3, % 2 = 49.57, df=29, P=0.01). 
The greatest amount of spawning occurred from the 
new moon until four days after the new moon; 63% of 
documented mass spawning events (dates with at least 
four successive spawning events) took place during this 
five-day period. 
Successive spawning events were common and as 
many as eight events occurred in a single night, pre- 
sumably from different females. The time between suc- 
cessive spawning on 21 sampling dates ranged from 1 to 
98 minutes (mean = 31 min, n = 66). A female individual 
was identified with the PIT reader before spawning on 
12 June 2002 and again on 16 June 2002. 
Environmental factors 
Ambient water temperatures ranged seasonally from 
11.3° to 21.7°C over the study period. Spawning occurred 
between 12.5° and 20.8°C, and bimodal peaks in the 
percent occurrence of spawning were apparent at 15° 
and 18°C. Seventy-six percent of spawning occurred 
in water temperatures from 15° to 18°C. Salinity at 
the study site ranged from 34.2° to 34.8 ppt. Spawn- 
ing occurred throughout the range of tidal cycles with 
no apparent tendency towards ebb or flow. The ratio of 
