Webb and Kneib: Individual growth rates and movement of Litopenaeus setiferus in a tidal marsh nursery 



377 



shrimp, in contrast to other penaeid species, are found 

 across a wider range of environmental conditions and 

 habitats (Kutkuhn, 1966) and often make tidal excursions 

 between subtidal and vegetated intertidal habitats to for- 

 age (Mayer, 1985; Kneib. 1995; 2000). However, relatively 

 little is known about movements within subtidal creeks of 

 the primary nursery areas, and the degree to which indi- 

 viduals exhibit fidelity to a particular tidal creek drainage 

 system is unknown. 



Direct measurement of juvenile shrimp growth rates 

 within the nursery have also been rare. Most growth 

 estimates for free-ranging juvenile shrimp are based on 

 analyses of size-frequency data, which can be misleading 

 (Loesch, 1965). Shrimp grow rapidly while in the estua- 

 rine nursery throughout the summer and early fall, and 

 juveniles approach adult, or commercially harvestable size 

 within 2-4 months after immigration to the estuary 

 (Kutkuhn, 1966; Williams, 1984). Mean absolute growth 

 rates of 0.7-1.1 mm/d are commonly reported for many 

 penaeids (Dall et al., 1990). However, growth studies are 

 difficult to compare because the rate of growth may vary 

 between years and among seasons, as well as with size, 

 age, and sex of individuals (Perez-Farfante, 1969). Growth 

 estimates for Litopenaeus setiferus range widely, from 10 to 

 65 mm/month (Williams, 1984). Previous estimates were 

 based on a variety of approaches including experimental 

 studies in aquaria and ponds (Pearson, 1939; Johnson and 

 Fielding, 1956), size distributions from tagging studies of 

 adults (Lindner and Anderson, 1956), length-frequency 

 distributions of juveniles in field samples (Gunter, 1950; 

 Williams, 1955; Loesch, 1965; Harris, 1974; Mayer, 1985), 

 mark-recapture of uniform size ranges of subadults and 

 adults (Klima, 1974), and mark-recapture of shrimp in 

 marsh ponds (Knudsen et al., 1996). Many estimates of 

 growth for small (<80 mm TL) juvenile L. setiferus have 

 been extrapolated from mark-recapture studies of larger 

 (>100 mm TL) individuals (Lindner and Anderson, 1956; 

 Harris, 1974; Klima, 1974). However, there is a paucity 

 of empirical data on growth rates of small, free-ranging 

 juvenile white shrimp within natural estuarine nursery 

 habitats. The purpose of the present study was to provide 

 reliable data on growth and movements of individual ju- 

 venile white shrimp within a natural estuarine nursery 

 environment and to initiate an assessment of spatial 

 variation in habitat quality in relation to tidal marsh 

 landscape structure. 



Recent innovations in tagging techniques have pro- 

 duced an effective way to obtain information on individual 

 organisms through the use of sequentially numbered bi- 

 nary-coded microwire tags (Northwest Marine Technol- 

 ogy. Inc. Shaw Island, WA). Microwire tags were first used 

 in tagging experiments by Jefferts et al. (1963) and have 

 since been used successfully to tag a variety of crustaceans 

 including prawns (Prentice and Rensel, 1977), crayfish 

 (Isely and Eversole, 1998), blue crabs (van Montfrans et 

 al., 1986; Fitz and Weigert, 1991), and lobsters (Krouse 

 and Nutting, 1990; Uglem and Grimsen, 1995). Results 

 of these studies and others generally show that tag reten- 

 tion rates are high and tagging has little effect on the 

 growth or survival of the fishes and crustaceans in which 



microwire tags have been used. In a laboratory study 

 involving 240 juvenile white shrimp, Kneib and Huggler 

 (2001) confirmed that tag retention was high (-98%), 

 growth rates between tagged and control individuals were 

 not significantly different, and the best location (based 

 on tag retention and survival) for tag injection was in the 

 muscle tissue of the abdomen. This type of tag allows for 

 identification of individuals because each tag is etched 

 with a unique number encoded in binary form. In addi- 

 tion, the tag is completely internal and inconspicuous, thus 

 eliminating problems associated with external tags (e.g., 

 streamer-type tags) that might interfere with molting or 

 increase predation risk (Garcia and LeReste, 1981; van 

 Montfrans et al., 1986; Isely and Eversole, 1998). 



Materials and methods 



Study area 



All samples were collected from four tidal creek subsys- 

 tems associated with the Duplin River on the west side of 

 Sapelo Island, Georgia. The Duplin River tidal drainage 

 ( -11 km 2 ) includes almost 10 km- of tidal salt marsh that is 

 inundated twice daily by unequal tides with a mean range 

 of 2.1 m (Wadsworth, 1980). Smooth cordgrass iSpartina 

 alterniflora) is the dominant vegetation in the intertidal 

 marshes of this area. Seasonal water temperatures aver- 

 age between 10°C and 30°C, and salinity is characteristi- 

 cally polyhaline, ranging from 15 to 30 ppt (Kneib, 1995). 

 Freshwater flow into the system is intermittent and 

 originates largely from local upland runoff and indirect 

 flows by several interconnected tidal channels from the 

 Altamaha River about 8 km to the southwest (Ragotzkie 

 and Bryson. 1955). 



Tidal creeks included in this study were Post Office 

 Creek (PO) and Stacey Creek (SO in 1998, and the East 

 and West forks (EF. WF, respectively) of the upper Duplin 

 River in 1999 (Fig. 1). Logistical constraints precluded 

 sampling shrimp populations from more than two creek 

 systems within the same year, and different pairs of 

 creeks were chosen in each of the two years to broaden the 

 spatial coverage of the study. High-resolution black and 

 white photographs (1:16000 scale) from an aerial survey of 

 the region in December 1989 were used to measure broad- 

 scale structural characteristics of the creek systems, in- 

 cluding areal extent of the intertidal and subtidal portions 

 of each drainage. The metrics and methods of extracting 

 the information from the photographs are fully described 

 elsewhere (see Webb and Kneib, 2002). 



Field sampling 



Shrimp were collected by cast net along the shallow (<1 m 

 depth) edges of the subtidal portion of each creek system 

 during low tide. Preliminary studies showed that 1.52-m 

 diameter nets with ca. 1-cm mesh size collected the range 

 of juvenile shrimp sizes (40-80 mm) targeted for mark- 

 ing in this study. All samples were collected within 

 2-3 hours of low tide to ensure that the shrimp popula- 



