MINELLO ET AL.: PREDATION ON BROWN SHRIMP 



was measured with an HF Instruments DRT-15 

 turbidimeter- (calibrated with a Formazin stan- 

 dard) and recorded as Formazin Turbidity Units 

 (FTUs). A typical turbidity curve for acclimation and 

 experimental periods is shown in Figure 1, and mean 

 values from turbid tanks for each experiment are 

 listed in Table 1. These turbidities were within the 

 range of values measured over a 2-yr period in the 

 Galveston Bay system (pers. obs.). Clear treatments 

 ranged between 0.1 and 2.4 FTUs. Light levels in 

 each tank were measured 13 cm below the surface 

 of the water with a LI-COR integrating quantum 

 meter (Model LI-188B) equipped with an under- 

 water sensor. This sensor measures radiation in the 

 400 to 700 nm waveband, and light energy is ex- 

 pressed in microeinsteins (^E s"^ m"^). Due to var- 

 iability in the thickness of the roof over the experi- 

 mental tanks, there were differences among the 

 tanks in incident light reaching the surface of the 

 water. During one experiment, light levels were 

 measured at the water's surface, and these values 

 were considered to be indicative of the differences 

 among tanks during all experiments. 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Experimental Design 



In all experiments, there were two replicate tanks 

 (each divided into two compartments) for each of 

 the four treatment combinations: clear water/no 

 sand, clear water/sand, turbid/no sand, and turbid/ 

 sand. Feeding by fish was restricted to daylight 

 hours. Twenty-four hours before the initiation of an 

 experiment, fish were placed in circular release 

 cages (0.75 m diameter) within experimental com- 

 partments, and clay was then added (200 mg/L) 

 through the settling system to four of the tanks over 

 a 3-h period. Twenty-five brown shrimp were placed 

 in each compartment (4.9 shrimp/m^) approximate- 

 ly 15 h before the start of an experiment. At 0600 

 h on the day of the experiment, turbidities were 

 measured and additional clay was added to elevate 

 the turbidity levels and reduce variability among the 

 four tanks. The release cages were lifted at 0700 

 h, and fish were allowed to feed for 12 h. The tanks 

 were drained at the end of the experimental period, 

 and missing shrimp were assumed to be eaten. For 

 each experiment, two control compartments (one 

 turbid and one clear) were stocked only with shrimp 

 to check survival and recovery of prey. 



The data were analyzed using the mean number 



PREDATORS 



PLACED IN 



RELEASE CAGES 



70r ^ 



PREY PLACED 

 IN TANKS 



PREDATORS 

 RELEASED 



REMAINING 

 PREY COUNTED 



EXPERIMENTAL PERIOD 



Ol L 



0800 1200 1600 2000 2400 0400 0800 1200 1600 2000 



TIME (HOURS) 



Figure 1.— Typical experimental sequence and turbidity (Formazin Turbidity Units) curve for predation 



experiments. 



61 



