MINELLO ET AL.: PREDATION ON BROWN SHRIMP 



flounder, burrowing also appeared to reduce the 

 number of shrimp eaten, but this reduction occurred 

 in both turbid and clear water as evidenced by the 

 nonsignificant tiirbidity/substratum interaction term 

 (P = 0.152). In fact, the reduction in mean preda- 

 tion rates associated with the presence of a sand 

 substratum was greatest in turbid water, and the 

 positive effect of turbidity on predation appeared 

 greatest in tanks without sand (Table 2A). Further 

 analysis of the effects of turbidity and substratum 

 on the activity of brown shrimp and the feeding 

 behavior of southern flounder would be needed to 

 explain interactions between these variables. Bur- 

 rowing does not appear to protect brown shrimp 

 from predation by Atlantic croaker, and there was 

 no experimental evidence for an interaction between 

 turbidity and substratum. 



Experiments With Red Drum, 

 Sciaenops ocellatus (Linnaeus) 



During the course of this study two experiments 

 were also conducted on the effects of turbidity and 

 substratum on predation rates of another fish pred- 

 ator, the red drum (420-592 mm TL). An in depth 

 analysis of these data was not included due to poor 

 survival of shrimp in control compartments (18% of 

 control shrimp died from unknown causes). Control 

 mortalities, however, did not appear to be related 

 to experimental variables, and data obtained in these 

 experiments suggested that predation rates of red 

 drum are not affected by turbid water or the pres- 

 ence of sand substrata. That substratum has no 

 significant effect on predation rates is supported by 

 additional unpublished but well-controlled experi- 

 ments in our laboratory, indicating that burrowing 

 does not protect juvenile brown shrimp from pre- 

 dation by red drum. Yokel (1966) described the feed- 

 ing behavior of these fish which consists of search- 

 ing along the bottom with the head down and lower 

 jaw rubbing along the surface of the substratum. He 

 concluded that this method of feeding would enable 

 the fish to locate animals in shallow burrows. 



CONCLUSIONS 



The artificial nature of these laboratory experi- 

 ments certainly must be considered when attempt- 

 ing to interpret the data in relation to natural 

 phenomena. One major advantage of the apparatus 

 used in our experiments was the relatively large size 

 of the experimental enclosures (5.07 m^ bottom 

 area) which allowed the use of prey densities com- 

 monly found in natural populations. The use of these 



large enclosures, however, made replicating treat- 

 ment combinations more difficult, hence reducing 

 the power of statistical tests. Despite this limitation, 

 general conclusions about relationships between 

 turbidity, substratum, and predation on brown 

 shrimp can be made on the basis of our experimen- 

 tal results. Under certain conditions, turbid estu- 

 arine water should provide juvenile brown shrimp 

 protection from fish predators such as pinfish and 

 Atlantic croaker. Turbidity does not appear, how- 

 ever, to reduce predation by southern flounder on 

 juvenile brown shrimp. The effect of turbidity on 

 predator-prey relationships apparently depends 

 upon the feeding behavior and morphology of pred- 

 ators and on the behavior of the prey. Burrowing 

 into the substratum also appears to protect brown 

 shrimp from some fish predators, and the ability of 

 brown shrimp to burrow is affected by substratum 

 characteristics (Williams 1958; Aldrich et al. 1968; 

 Rulifson 1981). A change from hard shell botton to 

 soft silty mud should enhance burrowing and reduce 

 predation by estuarine fish such as pinfish, south- 

 ern flounder, and perhaps spotted seatrout. Fishes 

 such as Atlantic croaker and red drum, however, 

 are apparently well adapted for feeding upon bur- 

 rowed organisms, and differences in estuarine sedi- 

 ments may not affect predation by these species. 

 Because turbidity and substratum do not appear to 

 alter predation of all fishes in a similar manner, the 

 effects of these habitat characteristics on the mor- 

 tality of juvenile brown shrimp should strongly de- 

 pend upon the dominant fish predators present in 

 an estuarine system. 



Comparisons of estuaries with regard to their pro- 

 tective capacity for juvenile brown shrimp are com- 

 plicated by interactions among habitat character- 

 istics and their effects. In addition to the type of 

 substratum, light levels (Wickham and Minkler 

 1975), temperature (Aldrich et al. 1968), and salin- 

 ity (Lakshmi et al. 1976) have been shown to affect 

 burrowing of brown shrimp. Starvation (Hughes 

 1968a), tidal movements (Hughes 1968b), shrimp 

 size (Eldred et al. 1961; Hughes 1968a; Moctezuma 

 and Blake 1981), and dissolved oxygen (Egusa and 

 Yamamoto 1961) affect burrowing of other penaeids 

 and may have a similar effect on brown shrimp. The 

 presence of rhizomes and roots of estuarine vege- 

 tation may also reduce burrowing by these animals. 

 All of these factors, therefore, can potentially inter- 

 act with predator-related mortality. In our experi- 

 ments, burrowing by brown shrimp was reduced in 

 turbid water, and this had a significant effect on 

 predation rates of pinfish. Interactions that control 

 the presence of protective habitat characteristics are 



67 



