Mace and Rozas: Population dynamics of juvenile Litopenaeus setiferus 
81 
the intermediate zone (0.05 [SE 0.006] and 0.07 [SE 
0.008]). Only the linear- regression estimates indicated 
significantly greater mortality in the saline zone than 
in the intermediate zone. 
Growth 
We were unable to consistently track individual co- 
horts of shrimp over sampling trips in each salinity 
zone. Identifying individual cohorts on some sample 
dates was difficult because there were no clear modes 
that indicated separate cohorts, and the mixdist pack- 
age could not adequately fit any of the 3 distributions 
to the data. On most sampling dates when we were 
able to track individual cohorts, more than 1 model 
had some support from AIC values; for these dates, 
we used model-averaged estimates for mean carapace 
length from models that had a combined AIC weight 
>0.95 (Suppl. Tables 2 and 3, avail, online). 
Mean growth rate estimates among all salin- 
ity zones and cohorts combined ranged from 0.72 mm 
TL/d (SE 0.28) to 1.83 mm TL/d (SE 0.23) (Table 5). 
Among salinity zones, mean growth rates were 1.22 
mm TL/d (SE 0.13), 0.99 mm TL/d (SE 0.19), and 1.62 
mm TL/d (SE 0.12) in the intermediate, brackish, and 
saline zones, respectively (Table 5). Mean growth rates 
were significantly higher in the saline zone than in the 
brackish and intermediate zones. No significant differ- 
ence in mean growth rates was detected between the 
brackish and intermediate zones. 
Secondary production 
Secondary production of juvenile white shrimp esti- 
mated with the conservative approach (i.e., the one 
based on the same growth rate of 1 mm TL/d in 
each salinity zone) varied by salinity zone and was 
significantly higher in the saline than in the inter- 
mediate zone (Table 6). Secondary production in the 
saline zone during the 84-d sampling period was es- 
timated as 382 kg/ha (95% Cl, 187 to 577), which 
was approximately 3 times the value of 116 kg/ha 
(95% Cl, 27 to 205) for the intermediate zone. In the 
brackish zone, production was estimated as 232 kg/ 
ha (95% Cl, 102 to 361), approximately twice that of 
the intermediate zone. Production-to-biomass ratios 
over the 84-d sampling period were estimated as 5.0, 
6.5, and 7.4 in the intermediate, brackish, and sa- 
line zones, respectively (Table 6). When we used the 
mean growth rates estimated for each salinity zone 
(Table 5) to compute shrimp age and CPIs, the over- 
all pattern of secondary production during the 84-d 
sampling period was the same among salinity zones, 
although some estimates of production were higher 
(intermediate=142 kg/ha, brackish=227 kg/ha, and sa- 
line=614 kg/ha) when compared with results from the 
0.007] and 0.08 [SE 0.006]) and brackish (0.08 [SE conservative approach (Table 6). Differences in pro- 
0.005] and 0.09 [SE 0.006]) zones, but estimates that duction estimates appear to have been driven mainly 
resulted from the use of both methods showed signifi- by differences in shrimp density among salinity zones 
cantly higher mortality in the brackish zone than in (Table 2). 
140 - C 
120 - 
100 - 
80 
60 
40 
20 •[ 
0 
1210 
344 
75 
702 
X 
1 2 3 4 5 6 
Trip 
Figure 4 
Box plots of the size, measured as total length in 
millimeters, of juvenile white shrimp {Litopenaeus 
setiferus) collected on 6 sampling trips in 3 salinity 
zones, (A) intermediate, (B) brackish, and (C) sa- 
line zones, in Sabine Lake in 2011. The black line 
inside boxes represents the mean size, the boxes 
extend to the 25^^ and 75*^ percentiles, and the 
whiskers extend to the minimum and maximum 
size. Numbers above each box-and-whisker plot are 
the sample size for each sampling trip. 
