84 
Fishery Bulletin 115(1) 
Table 4 
Daily instantaneous natural mortality estimates (and standard errors) 
for juvenile white shrimp (Litopenaeus setiferus) within intermediate, 
brackish, and saline zones of Sabine Lake. Mortality rates were es- 
timated with 2 different catch-curve analyses, linear regression and 
the Chapman-Robson estimator, by using data from trips 4-6 in 2011. 
Analysis Intermediate Brackish Saline 
Linear regression 0.05 (0.006) 0.08 (0.005) 0.09 (0.007) 
Chapman-Robson 0.07 (0.008) 0.09 (0.006) 0.08 (0.006) 
Table 5 
Absolute growth rate estimates (Gabsoiute) standard 
errors calculated by following individual cohorts of Ju- 
venile white shrimp {Litopenaeus setiferus) collected 
during 3 sampling trips (trips 4-6) in 2011 within in- 
termediate, brackish, and saline zones of Sabine Lake. 
Growth rates 
per day. 
are given 
in total length in 
millimeters 
Zone 
Start 
End 
^absolute 
Intermediate 
Trip 4 
Trip 5 
1.22 (0.13) 
Brackish 
Trip 4 
Trip 5 
0.77 (0.45) 
Trip 4 
Trip 5 
0.74 (0.28) 
Trip 5 
Trip 6 
1.72 (0.50) 
Trip 5 
Trip 6 
0.72 (0.28) 
mean 
0.99 (0.19) 
Saline 
Trip 2 
Trip 3 
1.42 (0.07) 
Trip 2 
Trip 3 
1.83 (0.23) 
Mean 
1.62 (0.12) 
Although we did not directly address these possible 
explanations for our results, we know that juvenile 
white shrimp can survive in a wide range of salini- 
ties (Zein-Eldin and Griffith, 1969), and we believe it is 
unlikely that environmental conditions would be more 
stressful for juvenile shrimp in a saline zone than in 
an intermediate zone (Rozas and Minello, 2011). We 
also tried to minimize any bias due to migration by 
considering only shrimp <60 mm TL in our mortality 
estimates. White shrimp do not begin to migrate from 
estuaries into the northern Gulf of Mexico until they 
reach 60 mm TL (Pullen and Trent, 1969). We, there- 
fore, conclude that increased predation may be respon- 
sible for the relatively higher mortality rates we esti- 
mated in the saline and brackish zones, compared with 
the rates in the intermediate zone. We did not quantify 
the density of all potential predators of juvenile white 
shrimp in each salinity zone to test this possibility, but 
previous studies indicate that juvenile penaeid shrimps 
are more abundant in the diets of fish predators from 
high-salinity areas of estuaries than in 
the diets of predators in areas with low 
salinity (Minello et al., 1989) and in the 
diets of predators at estuarine sites near 
the sea than at sites farther up an estu- 
ary (Salini et ah, 1990). Alternatively, this 
higher shrimp mortality may have been 
the result of less access to marsh edge 
habitat in the brackish and saline zones 
than in the intermediate zone. In the in- 
termediate zone, marsh edge habitat was 
flooded for longer periods and, therefore, 
may have provided more protection from 
predators than that provided by this type 
of habitat in the other salinity zones. 
Our secondary production estimates for juvenile 
white shrimp in shallow marsh habitats of Sabine Lake 
are similar to, and within the range of, production esti- 
mates for this species reported elsewhere. Comparisons 
among published values should be made cautiously, 
however, because of differences in definitions of the 
term secondary production and the methods used to 
estimate it. We know of no other comparisons of white 
shrimp production among salinity zones, but produc- 
tion (technically yield) of white shrimp in aquaculture 
ponds of different salinities was approximately 3.5 
times greater in 4 ponds with high salinity (2 ponds at 
15 and 2 ponds at 21) than in 2 low-salinity (7) ponds 
(Hysmith and Colura, 1976). Our estimate of 382 kg/ha 
for production during the 84-d sampling period in the 
saline zone is higher than the estimate of annual pro- 
duction for juvenile white shrimp (109 kg/ha) reported 
for saline marshes in Galveston Bay (Minello et ah, 
2008; Table 4). The difference in these 2 estimates of 
production of juvenile white shrimp is most likely due 
to the higher densities we documented in our study 
area than those reported for Galveston Bay. Using 
shrimp landings data and area of emergent wetlands 
in the U.S. states bordering the Gulf of Mexico, Engle 
(2011) estimated mean annual production (technically 
yield) of penaeid shrimps (all species combined) as 241 
kg/ha (range: 57-1660 kg/ha). 
The intermediate zone contained less valuable habi- 
tat than the saline zone on a per-hectare basis during 
the 84-d sampling period, but the estimated produc- 
tion from the intermediate zone, which occupies a large 
proportion of coastal marshes within Louisiana, was 
not trivial (116 kg/ha). Therefore, the production from 
these low-salinity areas would contribute substantially 
to the total production of white shrimp in Louisiana 
estuaries. Multiplying our production estimates in 
the saline and intermediate zones by the total area of 
these 2 zones determined for coastal Louisiana in 2013 
(Sasser et al.®), the total production over the 84-d sam- 
pling period from the intermediate zone would equal 
® Sasser, C. E., J. M. Visser, E. Mouton, J. Linscombe, and S. 
B. Hartley. 2014. Vegetation types in coastal Louisiana in 
2013: U.S. Geological Survey Scientific Investigations Map 
3290, 1 sheet, scale 1:550,000. [Available at website.] 
