O'NEIL and WEINSTEIN: FEEDING HABITATS OF SPOT 



BLEVINS CK 



 CREEK 

 □ SHOAL 



Figure 2. — Relative densities of spot at tidal creeks (values shown are monthly means of 

 both creek stations) and shoal sampling localities. Asterisk indicates that May values for 

 Blevins Creek are drawn to half scale. Values above histograms are ratios of creek to shoal 

 densities. 



Trophic Analysis 



During this study, over 1,750 spot stomachs 

 were removed and analyzed. In both creeks, spot 

 underwent size-related, as well as temporal and 

 spatial, changes in food utilization. Food utiliza- 

 tion differences owing to size-related (ontoge- 

 netic) changes were examined by cluster analysis 

 (Fig. 3). Calanoid copepods were the dominant 

 prey of the smallest spot size classes (Fig. 4). The 

 26-30 mm size class had begun to consume more 

 substrate-oriented prey (polychaetes and ne- 

 matodes). All the spot examined between 40 and 

 100 mm SL had considerable overlap in a wide 

 variety of food items. The great majority, how- 

 ever, were benthic organisms, e.g., maldanid and 

 nereid polychaetes, Leptocheirus amphipods, free- 

 living nematodes, and oligochaetes. Spot over 101 

 mm were clustered separately because of Leucon 

 americanus in the diet. It thus appears that onto- 

 genetic changes in spot diet shifted from a spe- 

 cialist mode when small to a more opportunistic 

 strategy in larger size classes. 



Size-class data were also subjected to reciprocal 

 averaging ordination (Fig. 4). Results closely par- 

 allel those in the numerical classification. Axis 1, 

 accounting for 49% of the variance, defined the 

 small, planktonic size classes, which consumed 



mostly calanoid copepods. The spot over 101 mm 

 were separated along Axis 2, with Leucon ameri- 

 canus and Monoculodes edwardsi the dominant 

 food items. The remaining size classes lay in the 

 plane of Axes 2 and 3 in association with a large 

 variety of benthic prey. 



The dendrogram representing the differences 

 between stations for all size classes of spot pooled 

 (Fig. 5) indicated that there are two main clusters 

 that correspond to the food distinctions between 

 the two creeks. In addition, both shoal stations 

 clustered as distinct outliers. 



Dominant prey items at the Goalders Creek 

 sites included nereid polychaetes, clam siphons, 

 a gammarid amphipod (Leptocheirus plumulo- 

 sus), and harpacticoid copepods. At Blevins 

 Creek, spot utilized proportionately more ne- 

 matodes, maldanid polychaetes, and oligochaetes. 

 At both locations spot made significant use only 

 of specific parts of some prey items, i.e., clam 

 siphons and tails of maldanid polychaetes (Currin 

 et al. 1984). 



Prey utilization differences were also noted be- 

 tween the creek stations and the adjacent shoal. 

 At Goalders Creek the amphipod Monoculodes 

 edwardsi, which dominated feeding on the shoal, 

 was partially responsible for the separation noted 

 in the dendrogram (Fig. 5). In the polyhaline sys- 



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