FRY: NATURAL STABLE CARBON ISOTOPE TAG 



generally progressing throughout the spring sam- 

 pling period. Both individual and composite bay 

 brown shrimp samples were examined by station 

 and station group (Figure 3) using graphic, linear, 

 and multinomial regression analysis for correla- 

 tions of shrimp length, weight, or date of collection 

 with shrimp 8^^C. These analyses were significant 

 only rarely, <1 case in 10, so that no consistent S'^C 

 relationships with size or date were evident. 



Offshore Samples 



In addition to migratory brown shrimp, three 

 other shrimp and stomatopod species were col- 

 lected offshore at stations 200, 204, and 208. These 

 species, Trachypenaeus similis, Sicyonia dorsalis, 

 and Squilla empusa, are normally resident 

 offshore and only occasionally enter estuaries 

 (Hoese et al. 1968). Composite samples of these 

 species fell within a narrow 8^^C range of - 16.2 to 

 -16.9% (Table 2). Forty-three individual rock 

 shrimp, Sicyonia dorsalis, also collected at these 

 stations showed a slightly larger 8^^C range of 

 1.91., - 15.5 to - 17.4 (Table 2). Mean S'^'C values of 

 migratory brown shrimp were very similar to val- 

 ues of the resident species, although some 5^^C 

 values of smaller brown shrimp differed signifi- 

 cantly from mean values of the offshore species 

 (Figure 4A, B). Several other samples of small 

 brown shrimp collected offshore or during out- 

 going tides 24 May and 21 June 1978 at Aransas 

 Pass Inlet also showed S^^C values significantly 

 different from the July offshore species (Figure 

 4 A, B). These individual and composite analyses 

 showed that small brown shrimp may enter the 

 offshore regions with widely differing 8^^C values 

 (-12.5 to - 19. Hi, Figure 4). But as brown shrimp 



Table 2. — S'^C values of shrimp species caught at three 

 offshore stations, July 1978. The standard deviation and the 

 number of samples (A^) are given after the means that contain 

 more than one sample. 



A. INDIVIDUALS 



613c 



B COMPOSITE SAMPLES 



A ° 



&13c 



-17j 



-15 

 -13 



t ' °' 



STATION 

 1978 



° ARANSAS 

 PASS 

 INLET 

 A 200 

 o 204 



• 208 

 197 7 



 208 



O 10 20 30 40 50 60 

 WET WEIGHT, GRAMS 



Figure 4. — Size dependence of brown shrimp 8"C values at 

 three offshore stations and from an outgoing tide at a major 

 migratory pass opposite these stations. Dashed lines indicate 

 95% confidence limits beyond which single brown shrimp sam- 

 ples differ significantly from the — 16.5X mean of resident 

 offshore shrimp species listed in Table 2 (Sokal and Rohlf 1969). 



feed and grow offshore, their 5^^C values rapidly 

 converge on the - 16 to - 17%,. 8^^C values charac- 

 teristic of offshore, nonmigratory species. 



Seasonality 



To supplement the late spring bay collections, a 

 longer term seasonal study was undertaken at sea 

 grass station 30. In addition to the migratory pink 

 and brown shrimp, three small, 20-50 mm TL, 

 nonmigratory shrimp species were also collected: 

 grass shrimp, Palaeomontes sp.; arrow shrimp, 

 Tozuema carolinense; and snapping shrimp, Al- 

 pheus heterochaelis. Mean values of these two 

 migratory and three nonmigratory species showed 

 a sigmoidal variation from — 15.6 in late April to 

 - 13.0 to - 13.2 in late August through late Oc- 

 tober to -14.2 in early December (Figure 5). Re- 

 gression analysis showed that mean 8^^C did not 

 correlate significantly with either temperature or 

 salinity. Collections offshore and at sea grass sta- 

 tion 50 showed a similar pattern in shrimp 6^^C 

 variation between early summer and late fall, al- 

 though collections at these stations were not made 

 regularly as at station 30. Late October offshore 

 composite samples averaged 1.21. less negative 

 than late July samples (N = 18 and 23, respec- 

 tively); at sea grass station 50, late September 

 composite samples averaged 2.2%o less negative 

 than late June samples (N = 4 and 2, respec- 



341 



