FISHERY BULLETIN: VOL. 81, NO. 4 



regions of the northern Gulf of Mexico to find consis- 

 tent isotopic patterns among animals recruiting to 

 offshore areas. These patterns ultimately distinguish 

 estuarine habitats in which animals feed prior to 

 offshore migration. Secondly, isotopic data were 

 used to study the importance of estuarine foods to 

 the five species during their adult lives. By examining 

 the rapidity of isotopic convergence upon offshore 

 values it was possible to distinguish species that util- 

 ize estuarine foods well into adulthood. 



Methods 



Animals were collected with a 10.2 m otter trawl 

 during October 1978, 1979, and 1980, along 10 

 offshore transects in the northern Gulf of Mexico 

 (Fig. 1A). Additional May collections were made off 



the Texas coast along transects 1 and 2 (Fig. 1A) in 

 1980. Trawling stations were located at various 

 depths along these transects. Station depths ranged 

 from 5 m near the beach to 150 m on the continental 

 shelf; the majority of trawl tows were taken at depths 

 of 5-50 m. A bar seine was used to collect juvenile 

 shrimp from marshes and shallow bays of the 

 Barataria Bay region of Louisiana (Fig. 2). 



In all offshore collections, animals were frozen and 

 white muscle tissue dissected from abdomens 

 (shrimp) or areas above the lateral line (fish). For 

 some Barataria Bay collections, whole shrimp were 

 used as samples rather than muscle tissue. Whole 

 shrimp and stomach content samples were acid- 

 treated to remove carbonates prior to isotopic 

 analysis. 



For all penaeid shrimp, total length (tip of rostrum 



A. 



30 



- 25 N 



B. RESIDENT OFFSHORE SPECIES 



-17-1 



-'6-1 . . 



■15 



■U 





-i 1 1 1 



C. MIGRATORY SPECIES 



6 13 c 



-19 



-17- 



15- 



-13 



11 



•X 



I 1 1 1 



x 



x = individual 



• = composite 



— = the average of resident 

 nonmigratory shrimp 

 shown in I B 



2 3/. 

 Texas 



7 8 9 

 Louisiana 



Figure 1.— A, Collection areas in the Gulf of 

 Mexico. B, d ]i C values of offshore resident 

 species, October 1978; species included the 

 shrimps Sicyonia dorsalis, Trachypenaeus 

 similis, Solenocera vioscai, and Parapenaeus 

 longirostris, and the stomatopod Squilla etn- 

 pusa. C, 5"C values of migratory, estuarine- 

 dependent penaeid shrimp, October 1978; 

 data for brown, pink, and white shrimp 

 combined. 



10 

 Florida 



Transect 



790 



