FISHERY BULLETIN: VOL. 85, NO. 1 



Table 6. — Summary data on possible factors affecting predatlon rates for the species 



of predators examined. 



reduced as predator speed increases, and changes 

 in prey activity should only have a negligible effect 

 on encounter rates with more active predators such 

 as pinfish and Atlantic croaker. Increased predation 

 rates by fish in turbid water may also be related to 

 the effect of turbidity on the reactive distance and 

 escape behavior of prey. The ability of the predator 

 to detect the prey before the prey detects the preda- 

 tor is dependent upon differences in visual acuity, 

 apparent size, and motion (Cerri 1983; Howick and 

 O'Brien 1983). Although brown shrimp have the 

 ability to visually detect predators and avoid attack, 

 the acuity of the crustacean compound eye is much 

 lower than that of the vertebrate eye (Waterman 

 1961; Goldsmith 1973), and shrimp do not respond 

 to stationary predators. This last prey characteristic 

 may explain why the southern flounder is a very ef- 

 fective predator on brown shrimp. 



Effect of Substratum on 

 Predation Rates 



Juvenile brown shrimp readily burrowed in experi- 

 mental tanks with fine sand substrata, but they 

 could not burrow in tanks without sand. Burrow- 

 ing should reduce the apparent density and avail- 

 ability of brown shrimp to visually feeding predators 

 (Minello and Zimmerman 1984). Predators using 

 olfactory or tactile mechanisms of prey detection, 

 however, may have less difficulty detecting and 

 feeding upon burrowed shrimp. Predation rates for 

 pinfish and southern flounder, both visual feeders, 

 were significantly reduced in tanks with sand sub- 

 strata. Predation rates of Atlantic croaker were not 

 affected by the presence of sand which suggests that 

 burrowing does not protect brown shrimp from this 

 predator. In other clear-water experiments con- 

 ducted in our laboratory with Atlantic croaker 

 (Albrecht et al. 1983^), we have been unable to 

 detect any reduction in predation on brown shrimp 

 related to the presence of sand substrata. This pred- 



ator does not depend solely on vision to detect prey 

 (Minello and Zimmerman 1983), and Chao and 

 Musick (1977) hypothesized that Atlantic croaker fed 

 mostly by olfaction and touch. These fish also search 

 through the upper layers of the substratum while 

 foraging for food (Roelofs 1954; Chao and Musick 

 1977), and this behavior may reduce the number of 

 burrowed shrimp. 



The presence of sand may also affect predation 

 by altering the activity levels of both prey and pred- 

 ator. Increased activity of brown shrimp in tanks 

 without sand may have increased encounter rates 

 with southern flounder in accordance with the model 

 of Gerritsen and Strickler (1977). In addition, south- 

 ern flounder periodically burrow in sand, and 011a 

 et al. (1972) found that burrowed summer flounder 

 did not respond to the presence of prey. 



Interactions Between Turbidity 

 and Substratum 



Burrowing by juvenile brown shrimp is reduced 

 in turbid water (Table 5), and in situations where 

 burrowing protects shrimp from predators, an inter- 

 action might be expected between the effects of 

 turbidity and substratum on predation rates. This 

 type of interaction was present in the pinfish ex- 

 periments. Predation rates of pinfish were reduced 

 in the presence of a sand substratum only in clear 

 water; in turbid water predation was not significant- 

 ly affected by substratum. Turbidity reduced pre- 

 dation in tanks without sand, but in tanks with sand 

 substrata the effect of turbid water on feeding by 

 pinfish was apparently attenuated by a reduction in 

 shrimp burrowing and an increase in the number of 

 available prey. In experiments with southern 



^Albrecht, C, T. J. Minello, and R. J. Zimmerman. 1983. The 

 role of substrates in predation on brown shrimp (Penaeus aztecv^) 

 by Atlantic croaker {Micropogonias undulatus). NOAA/NMFS 

 Unpublished Report to Laboratory Director, SEFC, Galveston 

 Laboratory, 18 p. 



66 



