FRY: FISH AND SHRIMP MIGRATIONS 



When these patterns are examined by species, 

 several consistencies and contrasts are noteworthy. 

 Rapid convergence was characteristic for pink 

 shrimp (Figs. 3B, E, 4, 7B) and usually for brown 

 shrimp (Figs. 3A, C, D, 5B, C, D, 7A), although one 

 brown shrimp collection showed only a gradual con- 

 vergence (Fig. 5A). With this exception, convergence 

 was complete by subadult, 20 g sizes. In contrast to 

 pink and brown shrimp, white shrimp exhibited only 

 gradual convergence that was incomplete well 

 beyond 20 g sizes (Fig 5G). The two fish species also 

 showed contrasts in their convergence patterns. 

 With the exception of six 50-100 g individuals col- 

 lected off Barataria Bay in October 1980, Atlantic 

 croaker in all size categories exhibited near-offshore 

 values (Fig. 5F). Spot of all sizes, however, showed 

 widely variable S li C values both off south Texas (Fig. 

 3F) and north Texas plus Louisiana (Fig. 

 5E). 



# ? C, £ 15 N, and # 4 S Correlations 



If two diets differ simultaneously in their C, N, and S 

 isotopic compositions, and animals switch from one 

 diet to the other, isotopic shifts occurring on the new 

 diet should be parallel and therefore correlated for all 

 three isotopes. In general, correlations between C, N, 

 and S data followed this prediction and were good for 

 the three examples examined (Fig. 7; Table 2). While 

 C and N appeared consistently correlated (Table 2), 

 correlations for S vs. N or C in the smaller set of north 

 Texas samples (N = 6 vs. 10-12 in the other regions) 

 were much weaker (Table 2). 



*P< 0.01. 



"P< 0.05. 



N.S. = not significant. 



DISCUSSION 



The two most striking results of this study were 1) a 

 strong regional division in the carbon isotopic com- 

 position of estuarine dependent species (Fig. 1) and 

 2) marked contrasts in the rate at which carbon 

 isotopic values of these species converged on 

 offshore values (Figs. 3, 4, and 5). The comparative C, 

 N, and S data also allow preliminary observations on 

 isotopic fractionations occurring in food webs of the 

 Gulf of Mexico. 



Regional Patterns in ^ 3 C 



The striking regional patterns found in £ 13 C values 

 of estuarine dependent shrimp closely parallel 

 regional # 3 C patterns previously documented in 

 nearshore sediments. These sediments have #"0 

 values averaging less negative than deeper offshore 

 sediments along the Florida and south Texas coasts 

 (Plucker 1970; Calder 1971 4 ; Fry et aL 1977; 

 BehrensetaL 1980), but more negative than offshore 

 sediments along the north- central Gulf coast (Sack- 

 ett and Thompson 1963; Shultz and Calder 1976; 

 Hedges and Parker 1976; Gearing et al. 1977). 

 Similar less vs. more negative regional patterns were 

 also evident in the S li C data for estuarine dependent 

 animal species (Figs. 1, 3, 4, 5). For sediments, the 

 regional variations have been ascribed to average 

 6^'C differences of the carbon that nearshore 

 sediments receive from rivers and estuaries. In the 

 former two regions, abundant U C- enriched sea- 

 grasses and macroalgae(5 13 C— — 10 to — 15%o) grow 

 or are exported offshore, while near the large rivers of 

 the north- central Gulf coast, — 27%o terrestrial mat- 

 ter is carried offshore (Plucker 1970; Calder footnote 

 3; Shultz and Calder 1976). Interestingly, the 

 isotopic transition between the north-central Gulf vs. 

 the south Texas region occurs at about the same 

 place for both sediments and shrimp— approxi- 

 mately offshore of Freeport, Tex. (Fig. 1, transect 4; 

 Gearing et al. 1977). 



While regional patterns are thus very consistent, 

 their origin along the north- central Gulf coast is 

 somewhat puzzling. Extensive stands of — 13%o 

 Spartina marshes replace seagrasses as sources of 

 13 C-enriched carbon along this coast (Chabrech 

 1972; Diener 1975), yet this replacement is not suffi- 

 cient to maintain the 13 C- enrichments (less negative 

 6 V]3 C values) observed along the Florida and south 

 Texas coasts (Fig. 1C). Sampling small juvenile 

 shrimp within Spartina marshes showed that even in 

 areas where Spartina influences should be strongest, 

 shrimp o^C values were substantially more negative 

 than Spartina values and ranged from — 17 to — 22%o 

 (Table 1). Similar discrepancies between — 13%o Spar- 

 tina values and more negative values for C in animals, 

 sediments, and water-borne particulates in marshes 

 have also been found in previous studies (Haines 

 1976a, b, 1977; Spiker and Schemel 1979; Hackney 

 and Haines 1980), and Peterson et al. (1980) have 

 discussed one possible explanation for these 

 discrepancies. 



4 Calder, J. A. 1971. Carbon isotope ratios of shelf sediments. 

 Paper presented at the 1971 Annual Fall Meeting of the American 

 Geophysical Union, San Francisco, Calif. 



797 



