FISHERY BULLETIN: VOL. 81, NO. 4 



Regional results from Florida and south Texas more 

 clearly show the importance of feeding on 13 C- 

 enriched foods prior to offshore migration (Figs. 3, 4). 

 Such foods are found in shallow areas such as sea- 

 grass meadows and some shoreline algal mats (Fry 

 1981b) rather than in deeper open bays (Fry 1981a). 

 The isotopic data thus point to the importance of 

 these shallower habitats as feeding grounds for spot, 

 pink shrimp, and brown shrimp prior to their offshore 

 migrations. The N and S isotopic results shown in 

 Figure 7 A and B are consistent with this conclusion, 

 as the +6 to +8%o values found among small 

 migratory shrimp were also found in shrimp from 

 seagrass meadows (Fig. 6). Additionally, this conclu- 

 sion agrees well with previous studies that have 

 documented high abundances of these three species 

 in grassflats (e.g., Hutton et al. 1956; Hellier 1962; 

 Hoese and Jones 1963; Kobylinski and Sheridan 

 1979; Orth and Heck 1980). 



In summary, isotopic values found among small 

 migrants in all three offshore regions closely 

 mirrored isotopic values found in local estuaries. The 

 sharp division in 8 1} C values occurring opposite 

 Freeport, Tex. (Fig. 1A, transect 4), suggests little 

 migratory interchange between offshore regions, at 

 least for smaller sized shrimp. Future sampling near 

 transect 4 and in other areas where two isotopically 

 contrasting regions interface could be useful in trac- 

 ing the movements of shrimp and fish after their 

 offshore migrations. This should be easiest for 

 animals that migrate offshore as adults and show only 

 a slow convergence upon offshore values. 



Convergence 



To understand the four patterns of isotopic con- 

 vergence and nonconvergence that were observed, it 

 is useful to consider two extreme cases. In the first, 

 animals migrate offshore as small juveniles and re- 

 main permanently offshore during growth to adult 

 sizes. This leads to the pattern of early isotopic con- 

 vergence that was most often observed (e.g., Figure 

 7 A). Alternately, animals may mature in estuaries, 

 migrating offshore as large adults. When sampled 

 offshore, animals of this kind will show very little con- 

 vergence upon offshore values, for most of their 

 tissues have been formed from estuarine foods. 

 Results for spot most clearly conformed to this pat- 

 tern (Figs. 3F, 5E). 



Gradations exist between these extreme cases. For 

 example, if some species have mixtures of small 

 juvenile and large adult migrants, the result may be a 

 gradual convergence such as that observed for white 

 shrimp (Fig. 5G). Such mixed cases could also result 



1) if animals do not stay permanently offshore but 

 freely move between estuarine and offshore regions, 

 or 2) if animals stay offshore but consume small 

 migrants or detrital foods that come from estuaries. 



The isotopic results thus lead to some predictions 

 about the migratory life histories of these species — 

 predictions that can be checked against previous find- 

 ings of mark-recapture and trawl studies. The five 

 species studied can be divided into two groups on the 

 basis of migration patterns and offshore location as 

 adults. Trawling studies of both pink and brown 

 shrimp show that they move offshore as small 

 juveniles and continue to move into deeper waters as 

 they mature (Iverson et al. 1960; van Lopik et aL 

 1979 5 ). The other three species— white shrimp, 

 croaker, and spot— are more coastal, with the center 

 of their offshore ranges at < 10 m depths (Chittenden 

 and McEachran 197 6). For these latter three species, 

 several reports exist documenting 1) occasional or 

 frequent reentry into estuaries from the offshore Gulf 

 of Mexico (Simmons and Hoese 1958; White and 

 Chittenden 1977; van Lopik et al. footnote 4), and 2) 

 delayed migration until adult sizes are reached in es- 

 tuaries (e.g., Gunter 1950; Suttkus 1955; Hellier 

 1962; van Lopik et al. footnote 4). 



In general, the isotopic patterns of convergence 

 closely conformed to what was expected from these 

 previous life history studies. Pink shrimp and most 

 brown shrimp collections showed close convergence 

 to offshore values by the 20 g size in good accord with 

 observations that these shrimp leave estuaries at an 

 average size <6 g (Copeland 1965; Trent 1967; 

 Parker 1970; Ford and St. Amant 1971). The less 

 marked convergence observed among spot, white 

 shrimp, and, to some extent, Atlantic croaker, sug- 

 gests that these species continue to rely on estuarine 

 foods throughout their adult lives, and is consistent 

 with their closer association with estuarine areas. 



Two aspects of the isotopic data deserve special 

 mention. First, while most of the isotopic data 

 gathered are consistent with early migration of brown 

 shrimp as young juveniles, this was not always true. 

 Collections of brown shrimp made off the Louisiana 

 and north Texas coasts in October 1978, showed only 

 a gradual convergence (Fig. 6A) similar to that ob- 

 served for white shrimp (Fig. 6G). While the causes of 

 this one exceptional set of results are not clear, it ap- 

 pears that some variability can exist in migration 

 patterns. 



'Van Lopik, J. R, K. H. Drummond, and R. E. Con- 

 drey. 1979. Draft environmental impact statement and fishery 

 management plan for the shrimp fishery of the Gulf of Mexico, 

 United States waters. Gulf of Mexico Fishery Management Council. 



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