Aalbers: Spawning periodicities of Atractosaon nob/lis 
147 
most commonly detected within 30 minutes before a 
spawning or in between successive spawning events. 
Spawning chants were detected only during the re- 
lease of gametes and consisted of an elongate series of 
intense, overlapping drum-roll and thud sounds at an 
average rate of 7 sounds/sec that continued for up to 55 
seconds. The intensity of spawning chants was second 
only to multiple pulse-trains recorded after spawning. 
Hydrodynamic booms were caused by sudden pressure 
waves generated underwater by rapid body movement 
and occurred throughout the day and night over the 
entire year. Their average rate and intensity remained 
relatively constant throughout the day and throughout 
the year, but increased considerably during feeding or 
in the presence of a predator around the net pens. 
Discussion 
Seasonal spawning 
White seabass maintain a protracted 5-month spawning 
season from March through July, and 98% of spawning 
events are documented during this period. Allen and 
Franklin (1992) identified a peak in the catch per unit 
of effort of newly settled young-of-the-year white seabass 
off Southern California in July 1988 and June 1989, 
supporting the seasonal spawning data presented here. 
A March-July spawning season is a month earlier than 
the previously estimated April-August spawning season 
for white seabass (Skogsberg, 1939). Annual California 
Cooperative Oceanic Fisheries Investigation (CalCOFI) 
surveys revealed that white seabass larvae were 
most abundant off central Baja California from 
May through August and abundance peaked in 
July (Moser et al., 1983), which is later than 
the May peak in spawning described here. Lati- 
tudinal differences in photoperiod and water 
temperature throughout the southern extent 
of the population range may explain this dis- 
crepancy. 
One-third of white seabass spawning activity 
occurred outside the current March 15 -June 15 
restricted commercial fishing period and time 
of reduced catch allowable under recreational 
bag limits. Under present regulations, spawn- 
ing aggregations off California are exposed to 
heightened recreational and commercial fishing 
effort for approximately 60 days each season. 
Delaying the opening of commercial fishing 
season in addition to maintaining a one-fish 
recreational bag limit throughout the spawn- 
ing season would considerably benefit white 
seabass spawning stocks. 
Diel spawning 
A dusk and nighttime spawning period, as 
documented here for white seabass is common 
in other temperate sciaenids (Holt et al., 1985; 
3% 3% 3% 
Figure 3 
Lunar spawning periodicity of white seabass iAtrac- 
toscion nobilis) for 297 spawning events documented 
from 2001 through 2003. Values correspond to the % 
occurrence of spawning events on each day of a synodic 
month. Increased shading represents the lunar cycle 
as it moves from full (light) to new (dark). Spawning 
activity was not uniform over the lunar cycle (x 2 = 49.57, 
df=29, P=0.01). 
Figure 4 
Mean monthly rate (±1 standard error) of all white seabass ( Atrac - 
toscion nobilis) sounds for audio recordings (n = 366) made from 
March 2003 to January 2004. A significant increase in sound 
production occurred during the spawning season (t=4.88, df=118, 
P>0.001). Hydrodynamic booms, which are generated by rapid 
movement through the water, accounted for a majority of the 
sounds recorded during the nonspawning season. 
