Mace and Rozas; Population dynamics of juvenile Litopenaeus setiferus 
77 
Table 1 
Distribution of samples used to estimate density, natural mortality, growth, and secondary production 
of juvenile white shrimp (Litopenaeus setiferus) in Sabine Lake. Samples were collected on 6 sampling 
trips in 2011 in 3 salinity zones (intermediate, brackish, and saline) and 4 habitat types (1 — marsh 
edge, or marsh vegetation <1 m from the interface of marsh and open water [ME]; 2 — shallow water <1 
m from the marsh edge [SWlj; 3 — shallow water 1-5 m from the marsh edge [SWl-5]; and 4 — shallow 
water >5 m from the marsh edge [SW>5]). 
Zone 
Trip 
Date 
ME 
SWl 
Habitat 
SWl-5 SW>5 
Total 
Intermediate 
1 
07/12/11 
0 
5 
5 
5 
15 
2 
07/26/11 
0 
5 
5 
5 
15 
3 
08/9/11 
0 
10 
5 
5 
20 
4 
09/9/11 
5 
5 
5 
5 
20 
5 
09/20/11 
5 
5 
5 
5 
20 
6 
10/4/11 
5 
5 
5 
5 
20 
Brackish 
1 
07/13/11 
0 
5 
5 
5 
15 
2 
07/27/11 
0 
5 
5 
5 
15 
3 
08/10/11 
0 
10 
5 
5 
20 
4 
09/7/11 
5 
5 
5 
5 
20 
5 
09/21/11 
5 
5 
5 
5 
20 
6 
10/5/11 
5 
5 
5 
5 
20 
Saline 
1 
07/14/11 
0 
5 
5 
5 
15 
2 
07/28/11 
5 
5 
5 
5 
20 
3 
08/11/11 
0 
10 
5 
5 
20 
4 
09/8/11 
5 
5 
5 
5 
20 
5 
09/22/11 
5 
5 
5 
5 
20 
6 
10/6/11 
5 
5 
5 
5 
20 
maximum, and mean size, were also computed for sa- 
linity zones and sample dates. 
Density 
We restricted our analysis of shrimp density to the 
last 3 sampling trips when sample sizes were equal 
both among habitats types and salinity zones (Table 
1). Before analysis, these density data were ln(x+l) 
transformed to remove the relationship between the 
mean and variance present in untransformed data 
(Milliken and Johnson, 1992). We used an analysis of 
variance (ANOVA) with 3 factors: 1) salinity zone (in- 
termediate, brackish, saline), 2) habitat type (marsh 
edge, SWl, SWl-5, SW>5), and 3) sampling trip (4 to 
6). The ANOVA model included a 3-way interaction, all 
2-way interactions, and all main effects. After a signifi- 
cant main effect of habitat type, 3 a priori contrasts 
were made: 1) marsh edge versus SWl, 2) SWl versus 
i SWl-5+SW>5, and 3) marsh edge versus SWl+SWl- 
! 5+SW>5. The first and third contrasts allowed us to 
I compare marsh edge with adjacent shallow water and 
i all shallow-water sites combined, respectively. We com- 
pared shallow-water sites near (<1 m) and those sites 
farther away from the marsh edge with the second con- 
trast. Results were considered significant at F<0.05. 
We also estimated omega squared (co^) for each factor 
in the ANOVA, using the formulas for measured factors 
given in Olejnik and Algina (2003). Omega squared is 
an estimate of effect size that can be interpreted as the 
proportion of variance explained by a given factor in an 
ANOVA. All density data presented in the text, tables, 
and figures are untransformed means. 
Mortality 
We converted length-frequency distributions to age-fre- 
quency distributions to estimate mortality. Before con- 
verting to age-frequency distributions, length-frequency 
distributions were weighted to account for differences 
in the relative amount of habitat types within each sa- 
linity zone (Suppl. Table 1, avail, online). Weighting was 
accomplished by first constructing length-frequency 
distributions for each habitat type within a salinity 
zone and then multiplying the number of individuals 
in each size class by the total area of a given habitat 
type in that zone, resulting in 4 different length-fre- 
quency distributions for each zone on each sample date. 
Length-frequency data from samples collected in marsh 
edge habitat were applied to the entire marsh within 5 
m of the shoreline, although densities of juvenile white 
shrimp may decline as distance from the shoreline in- 
creases from 1 to 5 m into marsh vegetation (Minello 
et al., 2008). Because we were interested in the entire 
