FRY: FISH AND SHRIMP MIGRATIONS 



Secondly, it is interesting to carefully contrast the 

 isotopic data for spot and Atlantic croaker collected 

 along the Louisiana and north Texas coasts (Fig. 5E, 

 F). Although the migratory biology of these two 

 species is generally held to be quite similar (e.g., 

 Parker 1971), the isotopic results show clear dif- 

 ferences. Of the two patterns, that of the Atlantic 

 croaker conforms closest to findings of trawl studies 

 which show that most animals leave estuaries as 

 juveniles < 25 g during late spring and early summer 

 migrations (Nelson 1969; Parker 1971; Kobylinski 

 and Sheridan 1979). The October collections of this 

 study should thus primarily reflect summer growth 

 offshore, and show the general isotopic convergence 

 that was observed in the Atlantic croaker data. The 



C, N, and S Food Web 

 Fractionations 



Table 3 summarizes C, N, and S isotopic values ob- 

 served in the Gulf of Mexico and other offshore 

 ecosystems. Relative to phytoplankton at the base of 

 the food web, some fractionations or changes in 

 isotopic compositions are evident at higher trophic 

 levels for C and N isotopes. Mean S values increase 

 for both C and N isotopes in these food webs (Table 

 3). Such cumulative fractionations have been at- 

 tributed to preferential respiration of 12 C0 2 in the 

 case of C isotopes (McConnaughey and McRoy 

 1979a) and to excretion of 14 N-enriched compounds 

 in the case of nitrogen ( Wada 1979). 



Table 3.— Stable isotope values for seawater and offshore marine biota. 1 



1 Whenever possible, values cited are those from the Gulf of Mexico or other temperate and tropical waters. 



2 References. 1 = Fry 1981 b; 2 = Fry and Parker 1979; 3= Gormly and Sackett 1977; 4= Sackett and Moore 1966; 5= Macko 

 1981; 6= Miyake and Wada 1976; 7= Hartmann and Nielsen 1969:8= Mekhtiyeva et a I. 1 976; 9= Kaplan etal. 1963; 10= Rees 

 et al. 1978; 1 1 = this study. 



3 Values for macroalgae. 



six individuals with deviant tf^C values that were 

 collected off Barataria Bay in October 1980 could 

 represent the much smaller pool of individuals that 

 continues to reside in bays until early winter weather 

 triggers their exodus (Gunter 1950; Suttkus 1955). It 

 is striking that this latter class of adult migrants 

 seems to represent a much larger fraction of the pop- 

 ulation for spot than Atlantic croaker (compare 

 Figure 5E andF). 



While further study in other seasons is required to 

 confirm these differences between Atlantic croaker 

 and spot, these data point out both strengths and 

 weaknesses of this isotopic approach to studying 

 migrations. This approach is weakest for tracing the 

 movements of small individuals because these 

 animals rapidly lose their estuarine isotopic tag dur- 

 ing offshore growth. The approach is much stronger 

 when applied to adult migrants that only gradually 

 lose their estuarine isotopic tag during metabolic turn- 

 over offshore. Trawling methods are probably su- 

 perior for studying movements of small juveniles that 

 cannot easily avoid nets, but for larger adults that 

 can, isotopic methods of following movements may 

 lead to a clearer understanding of seasonal and year- 

 to-year variations. 



For S isotopes, marine algae typically show a small 

 0-4%o fractionation relative to seawater sulfate and 

 thus closely reflect its +20%o isotopic composition 

 (Table 3). Previous studies show that marine animals 

 closely reflect the isotopic composition of their +17 

 to +20%o algal foods and seldom have values lower 

 than + 16%o (Kaplan et al. 1963; Mekhtiyeva et aL 

 1976). 



In this study, the majority of resident offshore 

 shrimp and stomatopod samples had somewhat 

 lower +13 to +17%o values (Fig. 6). This apparent 

 discrepancy may reflect an undersampling of soft- 

 bottom benthic fauna in previous studies. Recent 

 work in estuarine marshes has shown that sulfur with 

 low # 4 S values can enter rooted plants from 

 sediments, resulting in plant tissues with 5 34 S values 

 lower than +5%o (Carlson and Forrest 1982; Fry et 

 al. 1982). Consumption of low ^ 4 S benthic bacteria 

 or plants seems responsible for the low +6 to +8%o 

 values found in many estuarine and offshore samples 

 of pink and brown shrimp (Figs. 6, 7A, B). Among 

 offshore resident shrimp, the lowest 5 34 S values were 

 observed in Louisiana waters (Fig. 6), offshore of 

 Barataria Bay (Fig. 1, transect 8). The occurrence of 

 low 5 34 S values in this area may have as their cause 



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