Szedlmayer and Lee: Diet shifts of Lutjanus campechanus 



367 



ontogenetic shifts in red snapper diet with increasing size 

 and possible changes in diet with habitat shifts from open 

 substrate to structured habitat (artificial reefs). 



Materials and methods 



Red snapper were collected from open-flat substrate (sand 

 and mud) and reef habitats (artificial reefs; Fig. 1). The 

 open habitat was located approximately 6 km south of 

 Mobile Bay, Alabama (30'06'N, 88°03'W), and ranged 

 in depth from 12 to 20 m. Previous studies showed very 

 high concentrations of age- red snapper from these areas 

 (Szedlmayer and Shipp, 1994; Szedlmayer and Conti, 

 1999 ). The artificial reef habitats were located in the Hugh 

 Swingle artificial reef area, approximately 20 km south 

 of Mobile Bay, AL, and ranged in depth from 18 to 23 m 

 (Szedlmayer and Shipp, 1994; Szedlmayer, 1997). 



We collected fish from open substrate by trawl (7.62-m 

 head rope, 2.54-cm mesh, 2-mm codend mesh). Samples 

 were taken every two weeks from June to December 

 1994; however, time between samples was longer in the 

 winter because of poor weather. Each trawl was fished 

 for 10 min, and all age-0 and age-1 red snapper collected 

 were placed on ice, returned to the laboratory, and frozen 

 for later analysis. Bottom dissolved oxygen, salinity, and 

 temperature were sampled with a Hydrolab Surveyor II at 

 each location (Szedlmayer and Conti, 1999). 



Prior to diet analysis, red snapper were thawed, weighed 

 to the nearest 0.1 g, and measured to the nearest 0.1 mm 

 SL. The whole fish was preserved in 10% formalin if SL 

 was <50 mm, whereas for larger fish, stomachs were re- 

 moved and preserved. After 48 hours in formalin, stomach 

 samples were transferred to 75% isopropyl alcohol. 



Concrete block and PVC artificial reefs (1 m 3 ) were 

 placed in the Hugh Swingle reef area in August 1992 

 and July 1993 (Szedlmayer, 1997). "Reef is used here for 

 defining these artificial habitats. Reefs were not sampled 

 for a minimum of 3 months after placement. Red snapper 

 were collected from June 1993 through December 1994. 

 Fish were collected from these reefs by SCUBA divers 

 first placing a drop net (3.0 m radius, 1.3 cm square mesh) 

 over the reef and then releasing rotenone into the enclosed 

 area. Reef fish were placed on ice in the field and trans- 

 ported back to the laboratory. Approximately 12-18 h 

 after collection all reef fish were weighed to the nearest 

 0.1 g and measured to the nearest 1.0 mm. Stomachs were 

 fixed in 10% formalin, and after 24 h transferred to 75%' 

 isopropyl alcohol. Red snapper size classes were also esti- 

 mated by SCUBA visual surveys in July and August 1994. 

 On each visual survey, divers counted red snapper by 50- 

 mm size intervals. Bottom dissolved oxygen, salinity, and 

 temperature were sampled with a Hydrolab Surveyor II 

 during each survey. 



All stomachs were dissected and contents placed in petri 

 dishes. All prey were counted and identified to the lowest 

 possible taxon. Volume was calculated by using an adapta- 

 tion of the method described by Hellawell and Able (1971). 

 Each prey taxon from each stomach was placed into a glass 

 well of a known depth. A cover slide was placed on the well, 



Mobile 

 Bay 



X 



If 



FL 



Gulf of Mexico 



O 



Gulf of Mexico 



O 



o 



o 



30.00 N 



□ □ 



D na D nr 



5 10 Kilometers 



80.00 W 



Figure 1 



Collection sites for red snapper ( Lutjanus campechanus) in the 

 northern Gulf of Mexico. Open circles are open habitat trawl 

 sites, and gray squares are 1-m 3 concrete or PVC artificial reefs. 



depressing the prey taxon to a known depth (e.g., 1 mm). 

 The prey were video taped with a high-8 Sony camera 

 and images were digitized with Image Pro 2.0 software 

 (Media Cybernetics. Silver Spring, MD). Image size was 

 calibrated to 0.01 mm by a stage micrometer. The surface 

 area of each preparation was measured by using Image 

 Pro software. Volume was calculated by multiplying the 

 surface area by the known depth. Specific volumes for par- 

 ticular prey taxa were calculated by dividing prey volume 

 by individual fish weight (mnv'Vfish wt g). Comparisons of 

 diet shift by increasing fish size were made by grouping 

 prey taxa into ten prey groups and by calculating specific 

 volume for 10-mm-size intervals of red snapper. 



A dissimilarity index (Bray-Curtis) was calculated 

 from specific volumes of individual prey taxa, for overlap- 

 ping size classes of red snapper both within and between 

 habitats: Bray-Curtis = IW^-Yj/IiY^+Y^), where Y = 

 specific volume of j th species, and j and k are the samples 

 being compared (Field et al., 1982). The dissimilarities 

 were then used in a multidimensional scaling analysis 

 (MDS; Schiffman et al., 1981). The MDS provided a two- 

 dimensional "map" of the distances between samples (fish 



