380 



Fishery Bulletin 102(2) 



(SPSS, Inc. Chicago, ID. When parametric tests were 

 performed, residuals were analyzed to determine whether 

 the data met the required assumptions (Sokal and Rohlf, 

 1995). Levene's test was used to evaluate conformity to 

 the assumption of variance homogeneity among groups. 

 When this assumption was violated, the data were trans- 

 formed and retested. If the assumptions were still not met, 

 then an appropriate nonparametric test was applied (e.g., 

 Kruskal-Wallis one-way analysis of variance ). Two sample 

 /-tests were used to compare spatial and temporal varia- 

 tion in water temperature, salinity, and dissolved oxygen 

 between creeks within a year and between years. August 

 was omitted in comparisons of data between creeks in 

 1998, and between years because sampling in SC was 

 suspended during August 1998. Regression analyses were 

 performed to determine whether there were significant 

 linear relationships between initial shrimp length and 

 growth rates within each tidal creek. One-way ANOVA 

 (controlling for the covariate initial length) was used to 

 test for differences in growth rates between creeks within 

 each year. A similar approach (controlling for initial size) 

 was used to test for monthly (seasonal) differences in 

 growth rate within years. If growth rates did not differ 

 significantly between creeks, the data were pooled within 

 year, otherwise creeks were treated separately. Only indi- 

 viduals at large for a month or less (to ensure that growth 

 was representative of individual months) were included 

 in the analyses. 



Shrimp at large for fewer than 3 days were excluded 

 from the statistical analyses to reduce certain antici- 

 pated biases associated with estimating growth rates. 

 These included 1) measurement error (assumed to be at 

 least 1 mm), which would likely represent a substantial 

 proportion of the growth rate estimate when absolute 

 change in size was small; 2) increments of growth associ- 

 ated with molting (Dall et al., 1990), which could either 

 underestimate growth for shrimp that had been at large 

 for a short time or had not molted since they were tagged 

 or overestimate growth if shrimp were recaptured shortly 

 after the first molt following marking; and 3) size-specific 

 growth, where shrimp marked at a relatively small size 

 and smaller shrimp exhibit a higher relative growth, so 

 that early recaptures could represent larger than average 

 growth rates. 



Results 



Physical parameters 



Average water temperature, salinity, and dissolved oxygen 

 (measured at the mouth of each tidal creek) were similar 

 between creeks within years (see Table 1 in Webb and 

 Kneib, 2002). In 1998, temperature ranged from 18.9 to 

 33.4°C, salinity from 18.2 to 28.0 ppt, and dissolved oxygen 

 from 1.4 to 11.3 mg/L. In 1999, temperature ranged from 

 15.0 to 33.4°C, salinity from 23.9 to 32.5 ppt, and dis- 

 solved oxygen from 0.8 to 7.1 mg/L. Temperature followed 

 expected seasonal patterns each year; mean values were 

 highest in summer and declined toward autumn. Results 



of /-tests with separate variance estimates showed no 

 significant differences between years in either mean tem- 

 perature ( = 0.14, df=134.0, P=0.80) or dissolved oxygen 

 (£=1.82, df=115.9, P=0.07) but mean salinity was sig- 

 nificantly (£=11.63, df=122.7, P<0.01) higher in 1999 (28.1 

 ppt) than in 1998 (24.8 ppt). Cumulative rainfall was 83.9 

 cm/yr in 1998 and 82.9 cm/yr in 1999 (Garbisch 4 ). These 

 values were indicative of drought conditions because they 

 were well below the long-term mean annual precipitation 

 value of ca. 132 cm/yr reported for Sapelo Island between 

 May 1957 and March 2003 (Southeast Regional Climate 

 Center 5 ). 



Growth 



Shrimp collections during recapture efforts ranged from 

 20,077 to 78,724 individuals, but the proportion of marked 

 individuals recaptured was low in both years, averaging 

 just over 1% (Table 1). However, the recaptures included 

 184 individuals for which growth rates and net movements 

 within the nursery were known precisely. 



Daily absolute growth rates of individuals, which 

 ranged from 0.25 to 2.5 mm, averaged 0.86, 0.78, 0.84, and 

 0.61 mm at PO, SC, WF and EF, respectively. The mean 

 values are on the low end of the range reported in previ- 

 ous studies of juvenile Litopenaeus setiferus with other 

 methods and conducted in different locations (Table 2). 

 Daily specific growth rates were size-dependent in both 

 years. Negative linear relationships between growth rate 

 and initial size (i.e., smaller shrimp grew relatively faster) 

 was the prevalent trend in all creeks (Fig. 3). No sig- 

 nificant difference (£=1.19, df=74, P=0.237) in growth was 

 detected between PO and SC, where mean (±SD) specific 

 (instantaneous) daily growth rates were 0.014 ±0.006 and 

 0.012 ±0.007, respectively. In 1999, shrimp exhibited sig- 

 nificantly U=2.12, df=56, P=0.038) higher mean specific 

 growth rates in the WF ( 0.014 ±0.008) compared to the EF 

 (0.010 ±0.006) of the Duplin River. The physical environ- 

 ment was similar at these sites (Webb and Kneib, 2002), 

 and there was no significant difference (£=1.43, df=81, 

 P=0.156) in the mean final sizes of shrimp recaptured 

 from these sites. However, the mean (±SD) initial size of 

 marked shrimp at EF (61.3 ±8.3) was significantly U=2.20, 

 df=81, P=0.031) larger than at WF (56.0 ±10.9); therefore 

 a lower specific growth rate was to be expected at EF. 



On a finer temporal scale, seasonal variation in growth 

 rates occurred in both years, more rapid growth early in 

 the season, and a general increase in the mean size of in- 

 dividuals as the season progressed were evident i Fig. 4). 

 The earlier observation that specific growth rate declined 

 with size (Fig. 3) opens the possibility that seasonal 

 variation in growth rates could be explained simply by 



4 Garbisch, -J. Unpubl. data. Univ. Georgia Marine Institute 

 Flume Dock Monitoring Station, NOAA, Sapelo Island Nat ional 

 Est ua vine Research Reserve. Univ. Georgia Marine Institute. 

 Sapelo Island, GA 31327. 



5 Southeast Regional Climate Center. Unpubl. data. Website: 

 http://water.dnr.state.sc.us/water/climate/sercc/elimateinfo/ 

 Instmical/historicaLga.html. [Accessed 21 November 20031. 



