Szedlmayer and Lee: Diet shifts of Lutjanus campechanus 



371 



16 48 45 41 37 41 33 35 37 59 67 37 40 37 41 28 28 35 29 13 15 15 5 3 



20 40 60 80 100 120 140 160 180 200 220 240 



Size class (mm SL) 



Figure 4 



Stomach contents by specific-volume for ten higher taxonomic groups 

 over 10-mm size classes of red snapper {Lutjanus campechanus) from 

 both open and reef habitats in the northern Gulf of Mexico. Numbers 

 on the upper axis are the number of red snapper that contained prey 

 for each respective size class. 



Red snapper shifted diets with increasing size. For red 

 snapper <60.0 mm SL, diets were dominated by shrimp, 

 chaetognaths, squid, and copepods. Large red snapper 

 (60-280 mm SL) shifted to feeding on fish prey, greater 

 amounts of squid and crabs, and continued feeding on 

 shrimp (Fig. 4). 



The diets of juvenile red snapper changed as they 

 moved from open to reef habitats. Fish collected had 

 overlapping sizes of 70.0 to 160.0 mm SL from both open 

 and reef habitats, and the MDS analysis for this size 

 range showed a clear separation of diets between the two 

 habitats (Fig. 5). Two points that were outliers (R75, T155 1 

 were biased because they represented only one fish each, 

 and the third outlier (R85) was difficult to explain. 



The clear separation of red snapper diets shown by the 

 MDS analysis can be attributed to several prey shifts that 

 accompanied habitat shifts. For prey crabs, open-habitat 

 red snapper diets were dominated by Xanthidae, and 

 smaller amounts of Paguridae, Portunidae, Diogeninae, 

 and Pinnotheridae (Fig. 6), whereas diets of red snapper 

 from reef habitats shifted to a dominance by Portunidae 

 and Diogeninae ( Fig. 7). For prey shrimp, open habitat red 

 snapper diets were dominated by Penaeidae and Mysida- 

 cea (Fig. 8), whereas diets from reef habitats shifted to a 

 dominance of Sicyoninae, Hippolytidae, Alpheidae, and 

 Squillidae (Fig. 9). For prey fish, open-habitat red snap- 

 per diets were dominated by Engraulidae (although most 

 were unidentified; Fig. 10), whereas diets from reef habitat 

 clearly reflected prey fish from reef habitats and included 

 Blenniidae, Serranidae, and three prey fish identified to 

 genera, Centropristis spp, Halichoeres spp., and Sen-anus 

 spp. (Fig. 11). 



-3-2-1 1 2 3 



X(unitless) 



Figure 5 



Multidimensional scaling of diets for red snapper (Lutja- 

 nus campechanus) based on the Bray-Curtis dissimilarity 

 index computed for specific volume of prey taxa both within 

 and between habitats for overlapping size classes (70.0 to 

 159.9 mm SL). The letter and number accompanying each 

 point indicates the habitat and size class that each point 

 represents (e.g., T = trawl, R = reef, 75 = 75 mm SL size 

 class). Circles were drawn by hand. Axes are unitless. 



Discussion 



The present study provides a substantial sample size 

 (ft = 1639) for red snapper diet analysis and a relatively 



