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Fishery Bulletin 95(4). 1997 
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Figure 9 
Estimated linear growth rates of wild white seabass (n=50) and three 
groups of juveniles spawned on 8 May, 24 June, and 25 September 1989 
and reared at 17-20°C. Symbols represent the mean length (± 1 SD) of a 
subsample of 16-76 reared fish. For clarity, symbols are not shown for 
wild fish. Slopes, adjusted least squares (ALS) means, and coefficients of 
determination (r 2 ) are shown. 
(Fig. 3). Other species of mysids were 
much less abundant. Density of M. 
elongata within the nursery can be quite 
high. Clutter’s data indicate that the 
mean density of mysids at 6 m was over 
4,000/m 3 , whereas Roberts et al. (1982) 
estimated that the mean density of 
mysids at 6 m near the San Onofre site 
was over 100/m 3 . Mysids are also about 
an order of magnitude more abundant 
during spring and summer, when white 
seabass are in the nursery (Clutter, 1967). 
Mysids are not only abundant within 
the nursery; their broad size distribu- 
tion makes them suitable prey for both 
recently settled larvae and much larger 
juveniles. Mature M. elongata brood and 
release relatively large young that re- 
main in shallow water (Clutter, 1967). 
This reproductive strategy results in a 
population of mysids in the nursery that 
ranges over 100-fold in individual 
weight (Fig. 6). Although larger fish eat 
larger mysids, juveniles >40 mm SL can 
apparently feed on the largest mysids 
available (Fig. 6). At about this size, a 
transition from mysids to larger prey 
such as fish and shrimp also begins. The diets of other 
closely related sciaenids show a similar shift. Small 
(10-40 mm SL) sand seatrout, Cynoscion arenarius, 
small (15-30 mm SL) spotted seatrout, C. nebulosus, 
and juvenile (50-129 mm SL) weakfish, C. regalis, 
all feed extensively on mysids and at larger sizes shift 
to eating fish (Stickney et al., 1975; Sheridan, 1979; 
McMichael and Peters, 1989). Large juvenile, sub- 
adult, and adult white seabass feed principally on 
fish (Quast, 1968; Thomas, 1968). 
A second feature of the shallow nursery that may 
enhance growth and survival of settled white seabass 
is warm water. Temperatures in the 4-8 m stratum 
during the summer ranged from 16 to 20°C, an aver- 
age of 1.2 and 1.8°C warmer than the two deeper 
strata. The effect of a 1-2°C increase on growth has 
not been calculated for juveniles, but Orhun (1989) 
observed that a temperature increase from 15 to 17°C 
resulted in an increase in growth rate (dry weight 
gain) from 13.8%/d to 16.7%/d in 4-21 d old larvae. 
Temperature may have less influence on growth of 
juveniles, but any increase in growth rate will accu- 
mulate over the 2-3 months that juveniles are in the 
shallow nursery. Houde ( 1987) has demonstated that 
a small increase in growth rate acting over a moder- 
ate time interval can reduce stage duration and theo- 
retically result in substantial increases in survival 
and cohort size. 
Correlations between abundances of drift macro- 
phytes and white seabass suggest that macrophytes 
are also an important feature of the nursery. Al- 
though the correlation among abundances in single 
tows was weak (r=0.29), the sensitivity of this analy- 
sis was poor because the catch of white seabass was 
low; a maximum of only two white seabass was 
caught in a single tow at these stations. The correla- 
tion among mean abundances in the four tows at each 
site — the mathematical equivalent of making longer 
tows — was stronger (r=0.52) and does suggest that 
white seabass are more common near drift macro- 
phytes. Allen and Franklin (1992) also noted that 
white seabass were rarely caught unless drift algae 
was present. It is possible that white seabass are sim- 
ply more vulnerable to the trawl when drift macro- 
phytes are present, but numerous studies with drop 
nets and purse seines have demonstrated that many 
juvenile fishes associate with drift macrophytes 
(Kulczycki et al., 1981; Robertson and Lenanton, 1984; 
Kingsford and Choat, 1986). Small white seabass may 
associate with drift macrophytes because they harbor 
suitable prey or serve as a refuge from predation. 
In addition, the nursery area may be preferred by 
white seabass because the risk of predation could be 
lower than at adjacent depths. Unfortunately this 
hypothesis is difficult to evaluate. Surveys of the 
nearshore areas of southern California show that 
