FRY: FISH AND SHRIMP MIGRATIONS 



some estuarine animals prior to their offshore 

 migrations, a limited number of estuarine samples 

 was analyzed. The causes of the more negative S li C 

 values observed among estuarine dependent animals 

 off the Louisiana coast were investigated in estuarine 

 collections from the Barataria Bay region (Table 1). 

 With the exception of two brown shrimp collections 

 from Caminada Bay (S iy C = -13.9 to -15.1%o), 

 shrimp values fell between —16.6 and — 22.0%o and 

 did not differ significantly between the Spartina and 

 open bay habitats sampled (Table 1). The —16.6 to 

 — 22.0%o estuarine range is in good accord with the 

 — 17 to — 21.8%o range observed off Louisiana 

 among small recruiting shrimp (Fig. 5A-D, G). 



For # 15 N and tf^S, a few samples were analyzed 

 from seagrass meadows in Texas and Florida and 

 from both Spartina marshes and open bays in 

 Louisiana (Fig. 6). Low +5.6 to +8.4%o # 5 N and S 34 S 

 values were frequently observed among shrimp from 

 seagrass meadows (Fig. 6); animals recruiting off 

 south Texas and Florida initially possessed values in 

 this range (Fig. 7A, B). Sampling in Louisiana 

 marshes and open bays yielded scattered results for 



both 8 i5 N and # 4 S (Fig. 6) but showed that values of 

 + 10%o or lower are not confined to seagrass 

 meadows. 



Isotopic Convergence 



During offshore feeding and growth, the isotopic 

 values of estuarine dependent animals should 

 gradually converge upon average offshore values. In 

 fact, four patterns of convergence and noncon- 

 vergence were evident, when isotopic values were 

 plotted against animal weight. Most commonly, a 

 close and rapid convergence toward the mean value 

 of resident, offshore species was observed for C, N, 

 and S (e.g., Figure 7B). Less frequently, convergence 

 occurred, but the end-value reached by larger 

 migratory animals was significantly different than 

 the mean value for offshore residents (Fig. 5C; Fig. 

 7A, (S^S data). Nonconvergence, a third pattern, was 

 most evident in the <S n C data for spot (Figs. 3F, 5E). 

 Finally, a mixed pattern of very gradual convergence 

 with a few deviant large individuals was indicated in 

 two cases (Fig 5A, G). 



TABLE 1.— «J 13 C Values of estuarine shrimp from the Barataria Bay region of Louisiana, 1980. BS = 

 brown shrimp; WS = white shrimp; GS = grass shrimp. 



1 Locations are shown in Figure 2. 



2 Xiphopenaeus kroyen. 



3 Rephcate samples. 20 brown shrimp each. 



4 Hindgut, rather than stomach {proventrtculus} sample. 



795 



