76 
Fishery Bulletin 115(1) « 
Intermediate. 
Sabine Lake 
Louisiana 
Brackish 
Gulf of Mexico 
Texas 
Saline 
0 10 Km 
1 I 
Figure 1 
Map of the study area in Sabine Lake (at the border of Texas and Loui- 
siana). Samples of juvenile white shrimp {Litopenaeus setiferus) were 
collected in 2011 to estimate density, natural mortality, growth, and 
secondary production at 3 locations along the estuarine salinity gradi- 
ent (zones=intermediate, brackish, and saline) that were each 100 ha in 
size. Black dots represent locations where individual drop samples were 
collected. 
resentative sample of the shrimp population from each 
selected square, a drop sample was taken from each of 
4 habitat types where shrimp are known to occur: 1) 
marsh vegetation <1 m from the interface of ro.arsh and 
open water (marsh edge), 2) shallow water <1 m from 
the marsh edge (SWl), 3) shallow water 1-5 m from 
the marsh edge (SWl-5), and 4) shallow water >5 m 
from the marsh edge (SW>5). 
Locations of SW>5 sampling sites were determined 
by randomly selecting distances between 6 m from the 
nearest shore and the middle of the water body. Only 
3 of the 4 habitat types were sampled on the first 3 
sample dates in most of the salinity zones when low 
water levels precluded sampling at the marsh edge 
Size 
(Table 1). Before collecting each drop 
sample, salinity, temperature, depth, 
and turbidity were measured according 
to the protocol described in Rozas et al. 
(2012). Each nekton sample was placed 
on ice, preserved in 10% formalin at the 
end of the day, and transported to the 
laboratory for processing. In the labora- 
tory, all organisms were separated from 
detritus, and juvenile penaeid shrimps 
were identified to species by using the 
characters from Perez Farfante^, Ditty 
(2011), and references therein. Cara- 
pace length was measured for all juve- 
nile white shrimp as described in Mace 
and Rozas (2015). 
The duration of flooding for each 
habitat was estimated by using water 
levels measured in the field and equa- 
tions fitted for each salinity zone (Mi- 
nello et al., 2012). We collected data 
on water levels at 20 randomly located 
transects within each salinity zone. 
Along each transect, water levels were 
measured at the marsh edge and at 0.5, 
1, 2, 3, 4, and 5 m on each side of the 
marsh edge. To derive equations for es- 
timating long-term water levels in each 
salinity zone, we regressed water-level 
data from temporary tide gauges placed 
in each salinity zone against data from 
nearby tide gauges of the NOAA Center 
for Operational Oceanographic Products 
and Services (CO-OPS: website) and 
the U.S. Geological Survey’s Coastwide 
Reference Monitoring System (CRMS; 
website): NOAA station 8770570 for 
the saline zone, NOAA station 8770475 
for the brackish zone, and CRMS site 
0662 for the intermediate zone. We es- 
timated the flooding durations (percent- 
age of time water depth was >5 cm) at 
habitats in each salinity zone from July 
through October 2011 by relating the 
water depth measured at each transect 
sampling site to concurrent tide data 
calculated from these fitted equations. 
We examined size, measured as total length in mil- 
limeters, of white shrimp collected in our samples by 
comparing box plots of size for each salinity zone and 
sample date. Descriptive statistics, such as minimum, 
3 Perez Farfante, I. 1970. Diagnostic characters of juve- 
niles of the shrimps Penaeus aztecus aztecus, P. duorarum 
duorarum, and P. brasiliensis (Crustacea, Decapoda, Penaei- 
dae). U.S. Fish Wiidl. Serv., Sped. Sci. Rep. Fish. 599, 26 p. 
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