ROSS: TROPHIC ONTOGENY OF LEOPARD SEAROBIN 



The total amount of food ingested, as shown by 

 the mean volume of stomach contents, increased 

 rapidly with increasing fish size; log transformed 

 values of total prey volume varied linearly with 

 fish size over most size classes (Figure 4). The total 

 number of prey per fish also increased rapidly with 

 increasing fish size up to the 60- to 80-mm size 

 class, but then declined markedly for larger size 

 groups (Figure 4). The decline in number of prey 

 ingested occurred somewhat prior to a detectable 

 increase in mean prey size (cf. Figure 5). Searobins 

 smaller than the 90- to 100-mm size group showed 

 an asymptotic relationship of fish length and 

 linear prey size, while prey sizes increased rapidly 

 over the larger size groups. Since linear prey mea- 

 surements may be misleading, I also examined the 

 average volume (cubic centimeters) of prey items 

 eaten by size classes of P. scitulus. Prey volume 

 was calculated from the total sorted food volume 

 from each 10- or 20-mm size class, divided by the 

 total prey number for each size class. Mean prey 

 volume did not increase over small size classes of 

 searobins, but at 90-100 mm it initiated a rapid 

 increase (Figure 6). Consequently, the rapid rise 

 in total stomach volume of the leopard searobin 

 occurred initally through the capture of increas- 

 ing numbers of small prey, followed (after 90-100 

 mm) by the capture of fewer, but progressively 

 larger, prey. 



Relative prey biomas's (mean prey volume/mean 

 wet weight) was initially very high but then de- 



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Meon Stomach Volume „, nn'O' 



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 FISH LENGTH (mm SL) 



FIGURE 4. — The relationship of mean volume of stomach con- 

 tents (cubic centimeters) and mean prey number (logarithmic 

 scales) to fish length for Prionotus scitulus, Tampa Bay, Fla., 

 1972-73. The vertical lines indicate 1 SE on either side of the 

 mean, sample sizes are shown above the upper graph. 



creased with fish size to the 61- to 70-mm size 

 class, followed by an increase for fish larger than 

 the 90- to 100-mm size class (Figure 6). 



Increases in prey size with increasing predator 

 size might occur through shifts in the utilization of 

 progressively larger prey kinds, or through the 



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 40 - 

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 65 



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 85 



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125 135 



Figure 5. — Mean prey length versus 

 standard length groups ofPrionotus sci- 

 tulus from Tampa Bay, Fla. Vertical 

 lines are ranges; cross-bars and open 

 rectangles arex ± 2 SE. 



FISH LENGTH (mm SL) 



229 



