FISHERY BULLETIN: VOL. 79, NO. 2 



Bay Migrations 



The 8^^C data also trace migrations between 

 different estuarine habitats. Brown shrimp are 

 smaller and grow more rapidly in estuaries than 

 offshore, further abbreviating the useful life of the 

 initial 8^^C tag a shrimp possesses when entering 

 a new feeding ground. Despite this, brown shrimp 

 S^^C values were highly variable at bay stations 

 60, 70, 86, and 92 (Figure 3). High variability may 

 indicate that these stations lie on migration routes 

 where shrimp converge from several isotopically 

 distinct feeding grounds. In support of this idea, 

 one may note that a high 8^^C range, 6.71., was 

 observed among migrating individuals at Aransas 

 Pass Inlet, and that offshore, small migrating 

 brown shrimp caused a larger 8^^C range than 

 that found in offshore resident species (Figure 4). 



To identify which stations lie on migratory 

 routes, one must distinguish the normal variabil- 

 ity expected from local feeding conditions from a 

 higher variability due to migration. Local varia- 

 tion is probably 2.2%.i or less and 3L or less for 

 composite and individual samples, respectively. 

 These estimates are based on the 8^^C ranges ob- 

 served at frequently sampled bay stations, i.e., 

 stations 90, 100, and 102 for composite samples 

 and station 50 for individual samples (Figure 3). 

 Further study will undoubtedly refine these esti- 

 mates, or, alternatively, future studies may use 

 large differences between shrimp tissue 8^^C and 

 local food 8^^C as a criterion to identify locations 

 where migratory activity occurs. These 

 techniques should allow the delineation of under- 

 water migratory routes whose existence has been 

 suggested by Parker (1970). 



CONCLUSION 



This paper proposes to trace shrimp movements 

 by matching the 8^^C values of migrating shrimp 

 with the 8^^C values of shrimp living in habitats 

 such as sea grass meadows and planktonic bays 

 where available shrimp foods, and hence the 

 shrimp themselves, differ substantially in their 

 8*^C values. There are several limitations to this 

 method, as well as some advantages. The utility of 

 the method is limited by the number of habitats 

 that 8*^C analysis can differentiate. Future 

 analyses of other stable isotopes such as hydro- 

 gen/deuterium may allow additional distinctions 

 between habitats such as marshes and sea grass 

 meadows. The rapid growth rate of shrimp im- 



344 



poses a second limitation. The maximum amount 

 of information about migratory movement is 

 gained by sampling slower growing animals soon 

 after they begin their migrations. For shrimp, 

 sampling males offshore or in migratory passes 

 during the times of peak migrations will give 

 clearest results. Seasonal changes in shrimp 8^^C 

 occur, but are gradual and can be avoided by sam- 

 pling during the 1-3 mo peak migration period. 



This method should be equally applicable to 

 other migratory animals, both terrestrial and 

 aquatic. Fish movements may be easier to analyze 

 than those of shrimp because of the age-specific 

 8^^C historical records that fish lay down in their 

 scales. Such analyses could yield information not 

 only about which areas an animal had fed in, but 

 also at what stage in the life cycle and with what 

 frequency these feeding episodes occurred. The 

 initial data presented for the south Texas coast 

 show that shallow water habitats such as sea grass 

 meadows are important suppliers of shrimp to 

 commercial fisheries. Direct extensions of these 

 investigations should yield quantitative informa- 

 tion on which estuarine habitats supply the bulk 

 of shrimp and fish caught in commercial fisheries. 



ACKNOWLEDGMENTS 



Gill Gilmore and the staff of the Coastal 

 Fisheries Division, Texas Parks and Wildlife De- 

 partment, kindly made their spring shrimp collec- 

 tions available for analysis. P. L. Parker and R. S. 

 Scalan provided support and technical assistance. 

 Chris L. Kitting, Joan Holt, and two anonymous 

 reviewers made useful criticisms of early drafts of 

 this manuscript. This work was supported by NSF 

 grant OCE 77-27009. 



LITERATURE CITED 



Cook, h. L., and m. j. Lindner. 



1970. Synopsis of biological data on the brown shrimp 

 Penaeus aztecus aztecus Ives, 1891. FAO Fish. Rep. 

 57:1471-1497. 

 COPELAND, B. J. 



1965. Fauna of the Aransas Pass Inlet, Texas. I. Emigra- 

 tion as shown by tide trap collections. Publ. Inst. Mar. 

 Sci. Univ. Tex. 10:9-21. 



Craig, H. 



1953. The geochemistry of the stable carbon isotopes. 

 Geochim. Cosmochim. Acta 3:53-92. 

 DEGENS, E. T, R. R. L. GUILLARD, W. M. SACKETT, AND J. A. 

 HELLEBUST. 

 1968. Metabolic fractionation of carbon isotopes in marine 

 plankton — I. Temperature and respiration experi- 

 ments. Deep-Sea Res. 15:1-9. 



