Mace and Rozas: Population dynamics of juvenile Litopenaeus setiferus 
83 
Figure § 
Comparison of mean densities of juvenile white shrimp 
{Litopenaeus setiferus) among 4 habitat types and 3 sa- 
linity zones (intermediate, brackish, and saline) within 
Sabine Lake, based on data from the last 3 sampling 
trips (trips 4-6) in 2011. Habitat types are 1 — marsh 
edge (ME), or marsh vegetation <1 m from the interface 
of marsh and open water; 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). Means and standard er- 
rors were computed from 15 replicate samples. Figure 
represents the significant interaction of habitat type 
and salinity zone. 
tion of marsh >1 m from the marsh edge because this 
short duration limited access to the vegetated marsh 
surface and concentrated individuals at the marsh 
edge sites. 
Nekton density patterns in shallow water also can 
be influenced by the distribution of submerged aquatic 
vegetation (SAV) or other structure (Rozas and Minello, 
2010). In our study area in 2011, SAV was absent be- 
cause of an ongoing drought; however, SAV can be pres- 
ent, especially in the intermediate and brackish zones 
of estuaries during periods of normal rainfall (Chab- 
reck, 1971). Because densities of juvenile white shrimp 
are similar in marsh vegetation and SAV (Howe and 
Wallace, 2000 ; Rozas and Minello, 2006 ), we expect 
that the distribution pattern of shrimp in our inter- 
mediate and brackish zones could be quite different in 
years of normal rainfall, when SAV beds in these loca- 
tions may be extensive. 
The pattern we observed in growth rates of juve- 
nile white shrimp among salinity zones was similar to 
that reported for the Cape Fear River estuary in North 
Carolina (Laney and Copeland^), but the growth rates 
we observed in our study area were overall higher than 
^ Laney, R. W., and B. J. Copeland. 1981. Population dynam- 
ics of penaeid shrimp in two North Carolina tidal creeks. 
Rep. 81-1, 161 p. Carolina Power & Light Co., Raleigh, NC. 
Trip 
Figure 6 
Comparison of mean densities of juvenile white shrimp 
{Litopenaeus setiferus) among 3 sampling trips (trips 
4-6) made in 2011 and among 3 salinity zones (in- 
termediate, brackish, and saline) within Sabine Lake. 
Means and standard errors were computed from 20 
replicate samples. Figure represents the significant in- 
teraction between sample date and salinity zone. 
those reported in Barataria Bay (Rozas and Minello, 
2011). In laboratory experiments, extremes in salinity 
have been shown to reduce growth rates of juvenile 
white shrimp (Zein-Eldin and Griffith, 1969). Salinity 
may directly influence growth rates through increased 
metabolic costs for shrimp in low-salinity (e.g., oligo- 
haline) areas (Rozas and Minello, 2011), although this 
explanation did not likely pertain to our study because 
salinities in the intermediate zone were elevated by the 
ongoing drought and more typical of the brackish zone. 
Although comparisons of natural mortality rates 
among salinity zones are rare, results of available stud- 
ies indicate that mortality may increase with salinity 
for a variety of species. Mortality of juvenile white 
shrimp within marsh tidal creeks of the Cape Fear 
River estuary was lower in low-salinity areas than in 
high-salinity areas (Laney and Copeland^). The mortal- 
ity rate for juvenile spot {Leiostomus xanthurus) in the 
Cape Fear estuary was lower at a low-salinity site than 
at a high-salinity site during the first year, although 
it was similar between the 2 sites in the second year 
(Weinstein and Walters, 1981). Mortality rates for this 
species and Atlantic croaker {Micropogonias undula- 
tus) were also lower at low-salinity, up-estuary sites 
than at high-salinity sites near tidal inlets in both the 
Cape Fear River and Pamlico Sound estuaries of North 
Carolina (Ross, 2003). Possible explanations given by 
Weinstein and Walters (1981) and Ross (2003) for this 
pattern include the following: 1) higher density of pred- 
ators at high-salinity sites, 2) higher stress on juvenile 
fish induced by high salinity indirectly increasing mor- 
tality, and 3) higher migration rates from saline sites 
biasing mortality rates upwards in those areas. 
