FRY: NATURAL STABLE CARBON ISOTOPE TAG 



direct temperature effect on the carbon me- 

 tabolism of shrimp. 



Offshore Migrations 



As brown shrimp migrate offshore from the es- 

 tuaries, they encounter a new feeding ground. 

 During subsequent offshore growth, shrimp 

 metabolize away the estuarine carbon present in 

 their tissues, and dilute the remainder of this old 

 carbon with new offshore carbon. Summer d^^C 

 values of nonmigratory offshore shrimp were 

 quite constant around -16.5Z, and the offshore 

 feeding grounds appeared to be isotopically uni- 

 form over the three stations sampled (Table 2). A 

 model that incorporates these features and pre- 

 dicts the change of shrimp 8^^C during offshore 

 growth is shown in Figure 6. This simple model is 

 a first approximation in that only dilution of the 

 old estuarine carbon during offshore growth is 

 considered, while metabolic loss of this estuarine 

 carbon is ignored. The 8^^C value of shrimp at any 

 time after emigration can be calculated from the 

 equation: 



-20 



S'^C = 



^''C^MiWe) + iW,-W,)i8''C^^J 



w„ 



where We = initial weight at emigration, 



Wc ~ weight at time of collection offshore, 

 S^^Coid = S^^C at emigration, 

 S^^Cnew = mean offshore shrimp S^^C. 



The weights and S^^C values at the time of offshore 

 emigration shown in Figure 6 represent two indi- 

 vidual brown shrimp collected in May at Aransas 

 Pass Inlet (see Figure 4A). The 5 g wet weight is 

 typical of most brown shrimp during the annual 

 peak spring and early summer migrations (Cope- 

 land 1965; Trent 1966; Parker 1970; King 1971; 

 pers. obs. 1978). 



The model predicts that these migrating 5 g 

 shrimp will rapidly become isotopically indistin- 

 guishable from resident shrimp during offshore 

 growth (Figure 6). This prediction agrees well 

 with the results shown in Figure 4 in which 

 shrimp weighing >15 g are generally not signifi- 

 cantly different than the -16.5 average of non- 

 migratory resident shrimp. Growth from 5 to 15 g 

 should occur in about 1.1 and 2.9 mo for female and 

 male brown shrimp, respectively (Parrack 1979), 

 so that the effective offshore life of the estuarine 



-16 



-12 



.SHRIMP A 



i5 20 3^ 4^ 50 60 

 WET WEIGHT, GRAMS 



Figure 6. — Isotopic diet-change model for two shrimp migrat- 

 ing offshore from Texas estuaries. During offshore growth, these 

 two shrimp gradually approach the characteristic 5'^C value of 

 the offshore area, - 16.5 /.., according to the equation presented in 

 the text. Dashed lines as in Figure 4 indicate S'^C values at 

 which the estuarine S'^C tag is no longer distinguishable from 

 normal offshore values. 



tag is rather brief. Larger weights at emigration 

 and increased differences between Sold and 8new 

 result in a more gradual approach to the mean 

 offshore value, while smaller weights, decreased 



^Id 



>new differences, or metabolic loss of es- 

 tuarine carbon during offshore growth would re- 

 sult in a shorter detectable life for this estuarine 

 8^=^C tag. 



The data presented in Figure 4 allow a prelimi- 

 nary assessment of the probable estuarine origins 

 of most brown shrimp recruits to the offshore 

 fishery. A trend among smaller shrimp towards 

 S^^C values more typical of sea grass meadows, 

 rather than those typical of open bays, was evident 

 at both Aransas Pass Inlet in May and June and 

 offshore during late July (Figure 4). Shrimp from 

 two other shallow-water habitats, salt marshes 

 (Haines and Montague 1979), and blue-green algal 

 mats (Fry, unpubl. data) also have isotopic compo- 

 sitions similar to shrimp from sea grass meadows. 

 The S^^C label thus distinguishes deeper water, 

 open bays from shallow marsh, algal mat, or sea 

 grass habitats. These initial results suggest that 

 the shallow-water habitats are the more impor- 

 tant feeding grounds for shrimp that enter the 

 south Texas offshore commercial fishery. This con- 

 clusion is in good agreement with other studies 

 that show that shrimp are exceptionally abundant 

 in sea grass meadows (Loesch 1965; Young 1978) 

 and that sea grass meadows are quite extensive in 

 south Texas estuaries (Diener 1975). 



343 



