FRY: FISH AND SHRIMP MIGRATIONS 



89°50' 



GULF OF 



MEXICO 

 10 



KMS. 



29°20' 



29°10' 



FlGl'RE 2.— Sampling locations in the Barataria Bay complex of 

 Louisiana. This area is located landward of transect 8 (Fig. 1A). 1 = 

 Airplane Lake; 2 = Bay Rambo; 3 = Round Lake; 4 = Caminada 

 Bay; 5 = St. Mary's Point; 6 = Independence Island; 7 = Bayou 

 GarcL 



to tip of the telson) and weights (when possible) were 

 taken. Shrimp were often pooled by size categories in 

 which individuals typically did not differ by more 

 than 1 mm in length. Weights were obtained in three 

 ways: 1) Directly on a laboratory balance to the 

 nearest 0.1 g, 2) when seas were calm, estimated to 

 within ±15% with a pan balance, or 3) estimated from 

 length using the length-weight regressions of Fon- 

 taine and Neal (1971) for combined sexes. 



Fish were measured for total length to the nearest 

 millimeter, and, if not weighed directly in the 

 laboratory, their weights were estimated by using 

 Dawson's (1965) length-weight regressions. 



In the laboratory, tissue samples were rinsed in 

 freshwater, dried, and powdered. Gas samples for 

 mass spectrometry were prepared from powdered 

 tissues as follows: 1) For carbon, 3-8 mg subsamples 

 were combusted in sealed Pyrex 3 tubes at 590° C 

 using CuO as an oxidant (Sofer 1980); 2) for nitrogen, 

 10-20 mg subsamples were mixed with a CuO/Cu 

 mixture in quartz tubes and combusted at 900° C for 

 X A h (Macko 1981); 3) for sulfur, 0.5-1 g subsamples 

 were combusted in a Parr bomb, the resulting sulfate 

 precipitated with barium, and S0 2 subsequently 

 generated by thermally decomposing BaS0 4 in a 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



sealed quartz tube (Fry et al. 1982). Following com- 

 bustion, all sealed tubes were broken under vacuum, 

 and gases purified and transferred using liquid N 2 

 and dry ice/acetone mixtures. Gases were analyzed 

 for their stable isotope contents using a dual inlet, 

 isotope ratio mass spectrometer (VG-Micromass, 

 Model 620E). Results are reported in 8 notation 

 where 8X = [fl sample /i? standard ) -1] X 10', R = 13 C/ 12 C, 

 15 N/ I4 N, or 34 S/"S, and X = 13 C, 15 N, or 34 S. 



Values reported in this paper are given relative to 

 PDB carbonate, air, and Canyon Diable troilite stan- 

 dards for C, N, and S, respectively. Mass spectromet- 

 ry corrections were applied for oxygen contributions 

 in both carbon and sulfur measurements (Craig 

 1957; Nakai and Jensen 1964). Replicate deter- 

 minations showed that measurements were generally 

 precise to within ±0.3%o for # 3 C, ±0.2%o for 8 li N 

 and±0.5%ofor# 4 S. 



RESULTS 

 Regional Patterns 



Throughout the northern Gulf of Mexico, benthic 

 shrimp and stomatopod species that reside offshore 

 have # 3 C values that usually range between —14.5 

 and -17.5%o (Fry 1981a, b). Figure IB shows this 

 broad geographic similarity for one set of collections 

 obtained in October 1978; isotopic values among 21 

 composite samples showed only a 1.8%o range, from 

 —14.7 to — 16.5%o. In contrast to this relatively uni- 

 form distribution of 8 l3 C values, isotopic values for 

 migratory, estuarine dependent shrimp showed 

 regional patterns (Fig. 1C). The # 3 C values for 

 migratory shrimp averaged less negative than 

 offshore values along the south Texas and south 

 Florida coasts; along the Louisiana and north Texas 

 coasts, in contrast, isotopic values were more nega- 

 tive than the offshore values (Fig. 1C). The transition 

 between the south vs. north Texas regions occurred 

 at transect 4, approximately opposite Freeport, Tex. 

 (Fig. 1C). 



Further sampling showed that these less vs. more 

 negative regional divisions were consistently present 

 over the 3 yr of study (Figs. 3-5). The regional pat- 

 terns held true not only for shrimp but also for two es- . 

 tuarine dependent fish species, spot and croaker 

 (Figs. 3F, 5E, 5F). 



While striking regional patterns among estuarine 

 dependent animals were discernable in the carbon 

 isotope results, this was not true of the nitrogen and 

 sulfur results. Figure 6 shows that for both N and S, 

 isotopic values of estuarine dependent shrimp 



791 



