Mace and Rozas: Population dynamics of juvenile Litopenaeus setiferus 
75 
stocks (Diop et aL, 2007; NMFS, 2010; Baker et al., 
2014). Abundance of early juvenile white shrimp is, 
for example, a good predictor of late juvenile shrimp 
abundance, which is itself a good predictor of the num- 
ber of shrimp landed by the fishery (Diop et al., 2007). 
The white shrimp is harvested as an annual fishery 
crop (Nance et al., 2010), and survival during the ju- 
venile life stage may have a larger effect on the adult 
population than adult mortality or fecundity (Baker et 
al., 2014). Mortality may decrease as the size of juve- 
nile white shrimp increases (Baker and Minello, 2010); 
therefore, growth also could be an important factor 
that influences the recruitment success of white shrimp 
to the fishery. 
Salinity is a major environmental factor thought to 
influence the use of estuarine nursery areas by white 
shrimp and other nekton at the landscape scale (Wein- 
stein et al., 1980; Rakocinski et al., 1992; Wagner and 
Austin, 1999; Rozas and Minello, 2010). For example, 
juvenile white shrimp are reported to be most abun- 
dant in mesohaline and polyhaline environments of es- 
tuaries (Gunter et al., 1964; Howe et al., 1999; Minello, 
1999; Rozas and Minello, 2010), although they range 
much more widely in salinities <1 and >35 (Gunter 
et al., 1964). These observations are based mainly on 
comparisons of shrimp density, abundance, or catch per 
unit of effort from samples collected within estuarine 
habitats of differing salinity. Density alone, however, 
may be an insufficient indicator of habitat quality (Van 
Horne, 1983); additional measures, such as growth, 
mortality, or secondary production, can provide a more 
comprehensive picture of habitat quality (Van Horne, 
1983; Beck et al., 2001; NMFS, 2010; Dolbeth et al., 
2012 ). 
We are unaware of any other study in which this 
full suite of metrics (density, growth, mortality, and 
secondary production) has been incorporated to com- 
pare nursery areas of white shrimp along an estuarine 
salinity gradient. Few studies have examined the ef- 
fect of salinity on growth and mortality of young white 
shrimp (Zein-Eldin and Griffith, 1969; Rozas and Mi- 
nello, 2011), and available estimates indicate that sec- 
ondary production of white shrimp is higher in estu- 
aries of the northern Gulf of Mexico than in those of 
the U.S. Atlantic coast (Zimmerman et al., 2000; Kneib, 
2003; Minello et al., 2008). Habitat-specific vital rates 
(growth and mortality) and estimates of secondary pro- 
duction are needed to fully assess and compare nursery 
areas among different salinity regimes. This informa- 
tion can be used 1) to identify important nursery areas 
and essential habitats (Beck et al., 2001), 2) to develop 
detailed population models and improve stock assess- 
I ment models for white shrimp (Baker et al., 2014), and 
I 3) to calibrate ecosystem models used to predict effects 
I of human activities on this and other estuarine species 
I (Rose et al., 2014). 
j The purpose of our study was to measure and com- 
I pare the value of nursery habitat for white shrimp in 
3 salinity zones in the Sabine Lake estuary along an 
estuarine salinity gradient. We collected quantitative 
density data from each salinity zone in summer and 
fall, when white shrimp were most abundant in the 
estuary. Length-frequency data from these samples 
were used to examine size distributions and to esti- 
mate growth and mortality rates among the 3 salin- 
ity zones. We used these data and the size-frequency 
method (Garman and Waters, 1983) to estimate and 
compare secondary production of white shrimp among 
salinity zones. 
Materials and methods 
Study site 
Our study was conducted in the Sabine Lake estuary 
(hereafter referred to as Sabine Lake) which is located 
in southwest Louisiana within the area of the coast 
known as the Chenier Plain (Fig. 1). Sabine Lake en- 
compasses an area of 375,979 ha; approximately half 
(49%) of that area is composed of marshes (Gosselink 
et al-^). Marshes in coastal Louisiana are classified into 
salinity zones based on dominant vegetation (Chab- 
reck, 1970; Visser et al., 1998, 2000). We selected 3 sa- 
linity zones (intermediate, brackish, and saline) using 
this vegetation classification rather than water salin- 
ity measured at sampling sites because conditions in 
estuaries are in constant flux. Vegetation (e.g., marsh 
type) represents average environmental conditions (e.g. 
salinity regime) integrated over time better than a sin- 
gle salinity measurement. The salinity ranges for these 
vegetation-based zones tend to be 0. 5-5.0 for interme- 
diate, 5.0-18.0 for brackish, and 18.0-36.0 for saline. 
Salinity in any zone, however, occasionally may extend 
outside the typical range. These 3 salinity zones are 
comparable to the oligohaline, mesohaline, and polyha- 
line zones, respectively, of the Venice system (Anony- 
mous, 1958; Visser et al., 1998). 
Sampling procedure 
White shrimp were sampled during 6 sampling trips in 
2011 (Table 1) that commenced on July 12 (trip 1) and 
26 (trip 2), August 9 (trip 3), September 7 (trip 4) and 
20 (trip 5), and October 4 (trip 6). A l-m^ drop sampler 
(Zimmerman et al., 1984) was used to collect 45-60 
samples during each of these sampling trips, which 
each required 3 days to complete (with one salinity 
zone completed each day). Details of our sampling de- 
sign are given in Mace and Rozas (2015). Briefly, we se- 
lected an area of 1 kmxl km within each salinity zone, 
divided it into 16 squares of equal size (0.25 kmxO.25 
km), and, before each sampling trip, we randomly se- 
lected 5 of these squares for sampling. To obtain a rep- 
^ Gosselink, J. G., C. L. Cordes, and J. W. Parsons. 1979. An 
ecological characterization study of the Chenier Plain coastal 
ecosystem of Louisiana and Texas. 3 vols., FWS/OBS-78/9, 
78/10, and 78/11. Natl. Coast. Ecosyst. Team, Off. Biol. 
Serv., U.S. Fish Wildl. Serv., Slidell, LA. 
