Mace and Rozas: Population dynamics of juvenile Litopenaeus setiferus 
85 
Table 6 
Estimates of secondary production (with 95% confi- 
dence interval and measured as kilograms per hectare), 
mean biomass (measured as kilograms per hectare), and 
production-to-biomass ratios for juvenile white shrimp 
{Litopenaeus setiferus) within intermediate, brackish, 
and saline zones of Sabine Lake. Production (P) and 
biomass (B) estimates are based on data collected over 
an 84-d period from July through October 2011 (trips 
1-6) and production was estimated by using the size- 
frequency method (Garman and Waters, 1983). 
Salinity zone 
Production 
Biomass 
P:B 
Intermediate 
115.7 (26.8 to 204.7) 
23.3 
5.0 
Brackish 
231.6(102.4 to 360.8) 
35.6 
6.5 
Saline 
382.0 (186.8 to 577.3) 
51.7 
7.4 
44 million kg in comparison with 113 million kg from 
the saline zone. 
Our study was conducted during drought conditions 
in southwest Louisiana, and the relative habitat value 
of the 3 salinity zones in our study area may differ 
during nondrought periods. During years of normal 
rainfall, the intermediate zone may provide less valu- 
able habitat for white shrimp. For example, the mean 
density of juvenile white shrimp within the marsh edge 
habitat of the intermediate zone during September in 
2012, a year of normal rainfall, was 80% less than the 
density we observed there in the same month in 2011 
and report here; whereas, shrimp densities in the same 
habitat type in the brackish and saline zones differed 
between these 2 years by <20% (senior author, unpubl. 
data). Juvenile penaeid shrimp abundance and com- 
mercial landings may be higher during warm, dry pe- 
riods with low freshwater inflows to estuaries (Moller 
et al., 2009; Piazza et al., 2010), although Gunter and 
Hildebrand (1954), Browder (1985), and Palmer and 
Montagna (2015) reported examples of a positive rela- 
tionship between rainfall or freshwater inflow and the 
abundance or commercial landings of penaeid shrimp. 
More comparisons of secondary production from other 
estuaries conducted over several years are needed be- 
fore definitive conclusions can be drawn about patterns 
of juvenile white shrimp production among salinity 
zones. 
Inferences from our results are limited to the lo- 
cations and time period we sampled. Our study was 
confined to a single year, and sampling sites were rep- 
licated in space but drawn from an area limited to 
100 ha in each salinity zone. Support for extending 
the inferences from our study more generally, however, 
comes from studies of other estuaries that corroborate 
our results. For example, the spatial distribution and 
growth rates of juvenile white shrimp among salinity 
zones that we observed are consistent with the pat- 
tern reported from studies of Barataria Bay, Louisiana 
(Rozas and Minello, 2010, 2011). In a meta-analysis of 
5149 samples collected from multiple locations in the 
northern Gulf of Mexico, mean densities of juvenile 
white shrimp were highest in mesohaline and polyha- 
line areas (equivalent to our brackish and saline salin- 
ity zones, respectively) (Minello, 1999) — densities that 
were consistent with our results. Additional compari- 
sons of demographic rates and secondary production 
from other estuaries of the Gulf of Mexico would allow 
broader inferences to be drawn in the future. 
Our estimates of density, biomass, growth, natural 
mortality, and secondary production of juvenile white 
shrimp were generally higher in the saline or brack- 
ish zones and lowest in the intermediate zone. To our 
knowledge, this study is the first attempt to simultane- 
ously estimate and compare population dynamics and 
secondary production of juvenile penaeid shrimps along 
an estuarine salinity gradient. Although inference from 
our work is limited in scope to the 3 locations and the 
time period we sampled, the saline and brackish zones 
provided more important nursery habitat for juvenile 
white shrimp than such habitat in the intermediate 
zone. The total amount of production from the inter- 
mediate zone, however, was not trivial when the area 
that this zone covers in coastal Louisiana is consid- 
ered. Nursery habitats that provide a small contribu- 
tion on a per-area basis, such as the intermediate zone 
in our study area, may still have a large effect at the 
population level because the habitat covers a relative- 
ly large total area (Dahlgren et al., 2006). Moreover, 
the relative value of nursery habitats can be dynamic, 
with variation occurring both spatially (e.g., within as 
well as among estuaries) and temporally, from year to 
year (Kraus and Secor, 2005). We documented within- 
estuary (i.e., among salinity zones) differences in habi- 
tat value for white shrimp in Sabine Lake and expect 
this value, especially in the intermediate zone, to vary 
interannually. For example, the habitat value of the 
intermediate zone in our study area likely would be 
less in a year in which rainfall was greater than or 
equal to average levels (i.e., in a year of lower salinity) 
than in the year we documented, 2011, a year of se- 
vere drought and relatively high salinity. This dynamic 
nature of habitat value should be considered when as- 
sessing estuarine nursery areas. 
Acknowledgments 
We thank S. Hillen and J. Salas of the NOAA South- 
east Fisheries Science Center’s (SEFSC) Galveston 
Laboratory; L. Broussard, S. Beck, and D. O’Malley of 
Louisiana State University; A. Cummings of the SEF- 
SC Estuarine Habitats and Coastal Fisheries Center 
in Lafayette, Louisiana; and J. C. Robichaux and J. 
Thompson of the University of Louisiana at Lafayette 
for help collecting and processing samples. We would 
also like to thank D. Richard at Stream Wetland Ser- 
vices, Lake Charles, Louisiana, for helping to locate 
sites and for providing lodging, the staff at Murphree 
