SICK, ANDREWS, and WHITE: REQUIREMENTS FOR SHRIMP CULTURE 



pellets 0.3 cm in diameter by approximately 1.5 

 cm in length and which sink in water, and hence, 

 pellets having these characteristics were used in 

 both environmental and nutritional experiments. 



RESULTS AND DISCUSSION 



SUBSTRATE STUDY 



A survival rate of 80% was obtained after 

 5 weeks in tanks without substrate, 80 to 90% 

 survival was maintained over much of the dur- 

 ation of the experiment among both treatments 

 having sand-shell substrates, and less than 60% 

 survival occurred among tanks having brick sub- 

 divisions (Figure 1). Although P. setiferus is 

 reported to burrow less than either P. duorarum 

 or P. aztecus (Anderson, 1966; Perez Farfante, 

 1969) , it apparently was able to avoid predation, 

 especially during the highly vulnerable moulting 

 period, quite successfully with or without a sand 

 substrate, since 5-week survival data among the 

 two sand-shell treatments and the bare tank bot- 

 tom treatment were not significantly different 

 (P < 0.05) (Duncan, 1955). If the type of 

 shelter is a factor in increased survival for 

 penaeids maintained under culture conditions, 

 the brick subdivisions should have enhanced sur- 

 vival. However, the markedly high mortality 



100 



80  



c 60 



40 



20 



SiJND-SHELL SUBSTRATE ON PLATFORM 

 SAND-SHELL ON TANK BOTTOM 

 NO SUBSTRATE 

 BRICK SUBSTRATE 



Figure 1. — Mean and standard error for percentage of 

 animals surviving after 5 weeks of growth on four dif- 

 ferent substrates. 



rate among this group, significantly different 

 from the other three treatments (P < 0.05), 

 may have resulted from either failure of the 

 shrimp to behaviorally segregate and thus fully 

 utilize this protection or from physical abrasion 

 against the sharp and coarse brick surface. Al- 

 though there may have been toxic substances in 

 the brick materials, the bricks were carefully 

 washed and assumed to be otherwise inert in any 

 chemical effect they may have had on the animals. 

 Although differences in volume of water caused 

 by placing various substrates in their respective 

 treatments was not controlled for, it was felt 

 that these differences in a running water system 

 were not critical to the survival of shrimp. Dif- 

 ferences in bottom area among the treatments 

 caused by placement of different types of sub- 

 strate were neither controlled for nor measured 

 but were also thought to be negligible compared 

 to differences found among treatment groups. 

 The high degree of cannibalism noted by Subrah- 

 manyam and Oppenheimer (1969) in tanks with- 

 out substrate was not observed in any groups. 



AERATION STUDY 



The group having oxygen supplied by injecting 

 air through airstones had significantly higher 

 survival rates (P < 0.05) when compared with 

 a treatment aerated by agitation of the water 

 column (Figure 2) . Although the average oxy- 

 gen levels were similar between the two treat- 

 ments (3.4-6.8 ppm), such levels in tanks aer- 

 ated by high-pressure nozzles often dropped for 

 short intervals due to clogging of the nozzles 

 with silt and biological debris. Electrical power 

 failures which affected water flow but not the 

 compressed air supply (equipped with stand-by 

 DC power) also caused intermittent drops in 

 oxygen levels. Such short-term irregularities 

 may have been more critical to shrimp toler- 

 ances than is indicated from reference to average 

 oxygen level values, per se. Also, at the rela- 

 tively high temperatures maintained throughout 

 the study, short drops in oxygen levels could have 

 been very critical. Decreased survival in tanks 

 with agitation of the 0.75-m water column may 

 also have resulted from physical agitation of the 

 animals. 



lor 



