FISH AND SHRIMP MIGRATIONS IN THE NORTHERN GULF OF 

 MEXICO ANALYZED USING STABLE C,N, AND S ISOTOPE RATIOS 1 



Brian Fry 2 



ABSTRACT 



Natural stable isotope tags were used in the northern Gulf of Mexico to interpret migrations of five commer- 

 cial fish and shrimp species: Leiostomus xanthurus, Micropogonias undulatus, Penaeus aztecus, P. duorarum, 

 and P. setiferus. Along the south Texas and Florida coasts, isotopic analyses showed that seagrass meadows 

 and possibly other shallow estuarine habitats are important feeding grounds for shrimp that are later caught 

 in offshore fisheries. Thus stable carbon, nitrogen, and sulfur values of juvenile shrimp in grassflats coin- 

 cided with isotopic values of small shrimp collected offshore. These values were —11 to — 14%ofor# 13 C, and 

 +6 to +8%o for both S" 5 N and # 4 S. In contrast to these south Texas and Florida results, <S"C, # 5 N, and (? 4 S 

 values showed a second pattern off the Louisiana and north Texas coasts. This difference was most pro- 

 nounced in the S"C values which ranged from —17 to — 24%o instead of — 11 to — 14%o. Because isotopic 

 values were similar in Spartina marshes and open bays along this northern coast, no conclusions could be 

 reached about the relative importance of Spartina marshes as inshore feeding grounds. 

 During feeding and growth offshore, eventual convergence about offshore isotopic values should result for 

 the migratory species studied. However, striking differences in convergence patterns were evident for the 

 five species, ranging from close convergence at small, subadult sizes (P. aztecus and P. duorarum) to noncon- 

 vergence among adults (L. xanthurus). These differences point to contrasts in the basic life history patterns 

 of migration (especially the juvenile vs. adult size at which offshore migration occurs), and, for one species, 

 showed that isotopic methods can trace yearly variations in these patterns. 



Migrating animals constitute one form of export from 

 estuaries to offshore waters. Most of the commercial 

 species in the offshore Gulf of Mexico are estuarine 

 dependent, migrating offshore after a juvenile 

 growth phase in coastal bays (Lindall and Saloman 

 1977). Through their sheer numbers, estuarine 

 dependent animals constitute an important part of 

 benthic communities in the Gulf of Mexico (Hilde- 

 brand 1954; Moore et al. 1970). They also constitute 

 an energy subsidy to the many offshore animals that 

 consume them. 



In this paper, I examine stable C, N, and S isotope 

 distributions in five estuarine dependent species 

 from the northern Gulf of Mexico. These species are 

 brown shrimp, Penaeus aztecus; pink shrimp, P. 

 duorarum; white shrimp, P. setiferus; spot, Leio- 

 stomus xanthurus; and Atlantic croaker, Mi- 

 cropogonias undulatus. Previous work on fish (Fry 

 and Parker 1979) and shrimp (Fry 1981a) has shown 

 that offshore animals have very constant isotopic 

 values within an approximate 0.6-2.0%o range. 

 Against this rather uniform isotopic background, re- 

 cent migrants from estuaries are often identifiable 



'Contribution No. 570, Port Aransas Marine Laboratory, Univer- 

 sity of Texas, Port Aransas, TX 78373. 



2 Port Aransas Marine Laboratory, University of Texas, Port Aran- 

 sas, Texas; present address: Department of Biology, Jordan Hall 

 142, University of Indiana, Bloomington, IN 47405. 



Manuscript accepted March 1983. 



FISHERY BULLETIN: VOL. 81, NO. 4, 1983. 



via their deviant isotopic values. These deviant 

 values arise from consumption of foods that are 

 isotopically more diverse in estuaries than offshore 

 (e.g., McConnaughey and McRoy 1979a, b). 



For animals migrating from estuaries, offshore 

 feeding should lead to eventual convergence upon 

 offshore isotopic values. Laboratory experiments 

 and model calculations show that this convergence 

 should be essentially complete to within ±l%o 

 following a fourfold increase in weight for rapidly 

 growing animals (Fry and Arnold 1982). While this 

 rapid convergence is primarily due to simple growth, 

 metabolic turnover should also lead to eventual con- 

 vergence upon offshore values for adult migrants that 

 are not actively gaining weight. These considerations 

 have two consequences for an offshore sampling pro- 

 gram directed at studying patterns of recruitment 

 from estuaries: 1) Animals recruiting as adults will re- 

 tain isotopic traces of their estuarine past for 

 relatively long times; and 2) recruiting juveniles, in 

 contrast, rapidly lose their estuarine isotopic values 

 and hence must be sampled soon after offshore 

 migration. 



This study of estuarine dependent species had two 

 objectives. The first was to use offshore catches to in- 

 fer important estuarine feeding grounds utilized 

 prior to offshore migration. Collections were made 

 during several seasons and years in three different 



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