MINELLO ET AL.: PREDATION ON BROWN SHRIMP 



shrimp eaten, however, showed no significant treat- 

 ment effects. Overall, Atlantic croaker ate 2.7% of 

 their weight in shrimp over the experimental period. 

 Differences among tanks in incident light apparently 

 increased the within treatment variability in the ex- 

 periment. There was a significant negative linear 

 correlation between incident light and the number 

 of shrimp eaten in the 16 experimental compart- 

 ments (r = -0.51, n= 16,P = 0.046). An ANCOVA 

 using incident light as a covariate lowered the er- 

 ror sum of squares, and the main effect of turbidity 

 became significant (Table 4B). This was the only ex- 

 periment in which variability in incident light among 

 tanks had a major effect on our results. Atlantic 

 croaker appeared to feed more actively at low light 

 levels, but predation rates were higher in clear 

 water than in turbid water. Turbidity therefore did 

 not appear to affect predation by simply reducing 

 the light in the water column. 



Turbidity and Burrowing of 

 Brown Shrimp 



Burrowing by juvenile brown shrimp was mea- 

 sured in both clear and turbid water to aid in the 

 interpretation of significant interactions in the 

 predation experiments. The percentage of shrimp 

 burrowed was reduced from a mean of 85.7% in 

 clear-water tanks to 46.9% in turbid tanks (Table 

 5). In the ANOVA the effect of turbidity was highly 

 significant (P < 0.001). The effect of day was also 

 significant (P = 0.041), and fewer shrimp burrowed 

 on the second day of the experiment. Overall, light 

 levels were lower on the second day, and a similar 

 ANOVA on light measured 13 cm below the surface 

 of the water also showed significant differences 

 related to turbidity (P < 0.001) and day (P = 0.011). 

 The turbidities used (30-47 FTUs) reduced the aver- 

 age light level in the water by 29% compared with 



values in clear tanks. Burrowing did not appear to 

 be related to shrimp size, and there was no signif- 

 icant difference between the mean length of bur- 

 rowed shrimp compared with nonburrowed shrimp 

 (paired t-test, P > 0.40, 14 df). 



DISCUSSION 



Effect of Turbidity on Predation 



Turbidity reduces predation on prey possessing 

 limited escape capabilities by reducing the visual 

 reactive distance of the predator (Moore and Moore 

 1976; Vinyard and O'Brien 1976; Gardner 1981). 

 Turbid water should have less of an effect on pre- 

 dation if the predator-prey size ratio is large (Moore 

 and Moore 1976; Vinyard and O'Brien 1976) or if 

 the predator has the ability to use sensory mecha- 

 nisms other than vision to detect prey. The signif- 

 icant decrease in predation rates by pinfish in our 

 experiments may be explained in part by the strict 

 reliance of this predator on vision for prey detec- 

 tion (Minello and Zimmerman 1983) and upon the 

 relatively small predator:prey size ratio (Table 6). 

 Turbidity appeared to have less of an effect on pre- 

 dation by Atlantic croaker. This predator does not 

 depend solely upon vision for prey detection, but also 

 uses olfaction and touch (Chao and Musick 1977). 

 The increased predation rates for southern flounder 

 in turbid water may be related to the ambush 

 feeding tactics of this predator and the effect of tur- 

 bidity on prey behavior. The activity level of brown 

 shrimp increased in turbid water as evidenced by 

 a decrease in burrowing and the frequent observa- 

 tion of actively swimming shrimp in turbid tanks. 

 According to the model of Gerritsen and Strickler 

 (1977), increased prey movement dramatically in- 

 creases encounter rates with slow moving or sta- 

 tionary predators. This effect of prey movement is 



Table 5.— The effect of turbidity on burrowing by juvenile brown shrimp. All measurements 

 were taken at approximately 11 00 h. Percentages of shrimp burrowed in each tank (generally 

 10 shrimp/tank) are listed with turbidity levels and light levels in the water column. 



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