NATURAL STABLE CARBON ISOTOPE TAG TRACES 

 TEXAS SHRIMP MIGRATIONS* 



Brian Fry^ 



ABSTRACT 



A 1978 spring and early summer survey of Texas brown shrimp, Penaeus aztecus, showed that stable 

 carbon isotope ( '^C/'^C or S'^C) analysis is useful for tracing shrimp movements. At least four isotopi- 

 cally distinct shrimp feeding grounds (three estuarine and one offshore) exist along the Texas coast. 

 Mean 6'^C values for brown shrimp in these feeding grounds during the spring and early summer were 

 -12.8 to -15.4 (sea grass meadows), -16.2 to -16.8 (offshore), -17.9 to -19.6 (open bays, group 1), and 

 - 20.1 to - 21.7 (open bays, group 2). Longer term seasonal studies offshore and at two sea grass stations 

 showed that shrimp 6'^C values become less negative by 1.2 to 2.4'Z. in the fall versus spring/early 

 summer Many small subadult brown shrimp collected offshore and during outgoing tides in a channel 

 leading to the offshore Gulf of Mexico had 5'^C values typical of individuals in sea grass meadows. 

 These and possibly other shallow- water habitats appear to supply more shrimp to south Texas offshore 

 fisheries than do deeper estuarine bays. 



Migratory movements of many commercial 

 marine species are difficult to follow. Traditional 

 methods include mark-recapture techniques to 

 follow individual growth and movement while se- 

 quential trawling studies follow mass migrations. 

 Newer techniques based on underwater acoustics 

 or genetic differences among stocks have ex- 

 panded our ability to follow marine migrations, 

 but many movement patterns remain unresolved. 

 Recently, the study of stable carbon isotope 

 ratios, ^^C/^^C or 6^^C, has shown that animals 

 acquire a natural isotopic label or tag from their 

 diet. The carbon in animals is generally isotopi- 

 cally similar within a range of ±2% to the carbon 

 in the diet (DeNiro and Epstein 1978; Fry et al. 

 1978; Teeri and Scholler 1979). Photosynthesis in- 

 troduces 8^^C variations among different plant 

 species (Park and Epstein 1960; Smith and Ep- 

 stein 1971), and the 8^^C values of animals may be 

 interpreted in terms of the relative carbon contri- 

 butions from plants at the base of food chains. In 

 the Gulf of Mexico, marine sea grass species have 

 the least negative S^^C values ( - 3 to - 151) while 

 phytoplankton and particulate organic carbon are 

 usually more negative (-18 to —25) and other 

 species of marine algae are intermediate ( - 12 to 

 - 20) (Craig 1953; Parker 1964; Smith and Epstein 



'Contribution No. 441 from the University of Texas, Port Aran- 

 sas Marine Laboratory. 



^Harbor Branch Institution, RR 1, Box 196-A, Fort Pierce, FL 

 33450. , / 



Manuscript accepted September 1980. 

 FISHERY BULLETIN: VOL. 79, NO. 2, 1981. 



1971; Eadie and Jeffrey 1973; Fry and Parker 

 1979). Animals in sea grass meadows are usually 

 less negative than animals found offshore, reflect- 

 ing the general 8^^C difference between sea grass 

 plants and phytoplankton (Parker and Calder 

 1970; Thayer et al. 1978; Fry and Parker 1979; 

 McConnaughey and McRoy 1979). 



This study assesses the potential of using 8^^C 

 variations to trace shrimp movements. Since food 

 availability differs between habitats such as sea 

 grass meadows and phytoplankton-dominated 

 open bays, the S^^C label acquired during feeding 

 will differ between such habitats. Measuring the 

 S^^C values of migrating shrimp should provide an 

 indication of which habitats they came from and a 

 basis for evaluating which habitats are relatively 

 important numerical contributors to commercial 

 shrimp fisheries. 



During 1978, I analyzed the 8^^C values of the 

 migratory brown shrimp, Penaeus aztecus, and to 

 a lesser extent, of the pink shrimp, P. duorarum. 

 These shrimp share a similar life history pattern 

 of offshore spawning, juvenile growth in estuaries, 

 subsequent offshore migration of subadults, and 

 final maturation to adults that spawn offshore 

 (Cook and Lindner 1970). Several nonmigratory 

 shrimp and one offshore stomatopod species were 

 also collected to facilitate interpretation of 6^^C 

 patterns as due to local conditions vs. migration. 

 Seasonal studies assessed the rate at which local 

 food variations cause a change in shrimp 8^^C. <^ 



337 



