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



385 



Penaeids are most abundant in tidal marsh nurseries 

 when physical conditions (eg., temperature and salinity) 

 appear optimal for their growth and survival (Zein-Eldin 

 and Renaud, 1986), but environmental variability is 

 characteristic of most estuaries and therefore is an obvi- 

 ous starting point for explaining observed differences in 

 shrimp growth among sites or times. Salinity was the 

 only environmental factor we measured that showed a 

 significant difference between years but could not be as- 

 sociated with any interannual difference in mean growth 

 rates. 



Temperature may affect the growth and estuarine dis- 

 tribution of juvenile penaeids more than salinity (Vetter, 

 1983), and interactions between salinity and temperature 

 may have even greater effects than variation in either fac- 

 tor alone (Zein-Eldin and Renaud, 1986). Mean tempera- 

 tures throughout our study period (with the exception of 

 November) in both years were largely within the optimum 

 range for growth of white shrimp which, in the laboratory, 

 was reported to be between 25°and32.5°C (Zein-Eldin and 

 Griffith, 1969). Higher temperatures generally contrib- 

 ute to faster growth in young penaeids (Perez-Farfante. 

 1969; Muncy, 1984), and therefore it seems reasonable to 

 expect seasonal variation in temperature to be reflected 

 in growth rates. However, this interpretation is con- 

 founded by the fact that growth rates also are size depen- 

 dent (Fig. 3, Table 3) and that increases in mean size of 

 juvenile white shrimp (Fig. 4) occurred while tempera- 

 ture in the nursery habitat was decreasing from the July 

 maxima. It seems likely that growth rates of juvenile 

 white shrimp were robust over the relatively narrow range 

 of seasonal variation in temperature and salinity observed 

 in the present study. 



Alternatively, differences in growth between certain 

 sites could be the result of spatial variation in habitat 

 quality. This variation need not be a function of water 

 quality, but rather a function of some structural aspect 

 of the nursery habitat. There was a strong correlation be- 

 tween mean growth rates and the proportion of tidal creek 

 drainage area that was intertidal. Only four creek sub- 

 systems were examined in ours study, and we recognize 

 that this is an insufficient sample size to justify anything 

 more than a suggestive hypothesis. However, evidence of 

 relationships between the amount of intertidal habitat 

 and penaeid shrimp production (Turner, 1977, 1992), as 

 well as the amount of intertidal creek edge and juvenile 

 shrimp abundance in adjacent subtidal creeks (Webb and 

 Kneib, 2002), supports the contention that intertidal ac- 

 cessibility is an important component of nursery habitat 

 quality for juvenile white shrimp. We propose that the 

 ratio between intertidal and shallow subtidal habitat may 

 be a key feature of white shrimp nursery habitat quality. 

 When tidally inundated, the intertidal portion of marsh 

 creek drainage systems is used extensively by juvenile 

 white shrimp (Kneib, 1995, 2000), most likely as a rich 

 foraging area (Kneib, 1997), and the shallow subtidal 

 portion functions as a low tide refuge and corridor for the 

 seasonal migration of postlarvae and subadults between 

 the open estuary and coastal ocean spawning grounds 

 and the juvenile nursery (Kneib, 1997, 2000). 



Movement and residence time 



Understanding the causes of broad-scale migration of 

 penaeids has obvious implications for predicting com- 

 mercial catches and therefore these causes have been the 

 focus of research on shrimp movements for decades ( Perez- 

 Farfante, 1969; Muncy, 1984). However, finer-scale move- 

 ments, which may affect growth and survival of juvenile 

 shrimp within the estuary, are not as well known. Emigra- 

 tion of white shrimp from estuaries is determined by size, 

 maturity, and environmental conditions (Muncy, 1984), 

 and size plays a principal role (Dall et al., 1990). In the 

 South Atlantic Bight, larger white shrimp (>100 mm TL) 

 begin emigrating from the nursery to commercial fishing 

 areas in the nearshore coastal ocean in August (Lindner 

 and Anderson 1956, Shipman, 1983). We collected few 

 shrimp >100 mm in the tidal marsh creeks, which is con- 

 sistent with previous observations of ontogenetic migra- 

 tion to deeper waters. According to growth rates measured 

 in this study, a shrimp of 40 mm TL would become large 

 enough to emigrate from the estuary to the coastal ocean 

 in 2-3 months (i.e., a shrimp tagged at 40 mm TL could 

 reach 85-108 mm TL in 2.5 months). 



The presence of high densities of small juvenile white 

 shrimp in the upper reaches of Georgia's tidal marsh 

 creeks (Harris, 1974; Hackney and Burbanck, 1976; Webb 

 and Kneib, 2002) has supported the contention advanced 

 by Weinstein ( 1979) that these areas are primary nurser- 

 ies for juvenile fish and shellfish. However, it has been 

 unclear whether these aggregations represent stable res- 

 ident populations or are composed of tidal transients that 

 constantly migrate among creek subsystems within the 

 broader estuarine nursery. Young shrimp are known to 

 move short distances to avoid unfavorable physiochemical 

 conditions (Hackney and Burbanck, 1976; Dall etal., 1990) 

 and routinely make tidally mediated excursions between 

 subtidal and intertidal portions of the nursery to forage 

 or escape predators (Kneib, 1995, 1997). Our findings 

 showed that juvenile white shrimp also tended to remain 

 resident in the upper reaches of tidal creeks where they 

 were originally tagged until attaining a size ( 80-100 mm ) 

 at which they normally begin to emigrate from the nursery 

 (Perez-Farfante, 1969). 



Although there was some movement between tidal creek 

 subsystems, the high level of site fidelity demonstrated 

 by juvenile white shrimp was remarkable given the open- 

 ness and degree of tidal flux in the Duplin River system 

 (mean tide range=2.1 m). Data from the chemical analysis 

 of shrimp tissue composition also suggest limited move- 

 ments of juvenile penaeids within estuarine nurseries. 

 Using the stable isotopes of carbon and nitrogen from mus- 

 cle tissues of pink shrimp (Farfantepenaeus duorarum), 

 Fry et al. (1999) traced shrimp movements within and 

 between seagrass and mangrove habitats of southwestern 

 Florida. They found distinct differences among individu- 

 als sampled from similar inshore habitat types separated 

 by small (3-5 km) open water distances, indicating that 

 individuals remained "resident" in specific portions of the 

 estuary at least for several weeks. Noting a similar study 

 in coastal Louisiana, Fry et al. (2003) suggested that 



