RECENT EXPERIMENTATION 



Beginning in 1969, the major objective of the re- 

 search at Galveston was to develop methods 

 whereby larval shrimp could be cultured more effi- 

 ciently and economically. It was realized that the 

 economic success of shrimp culture was largely de- 

 pendent upon the costs of producing larval shrimp in 

 quantity. Two key problems contributing to the 

 costs were: I ) costs of food production and 2) costs 

 of labor. Research was initiated that was designed to 

 reduce the investment required for the construction 

 of a shrimp hatchery, to increase the efficiency of 

 algal and larval culture procedures, and to reduce the 

 amount of labor required in the hatchery. 



The approach used has been to grow unialgal cul- 

 tures separately from the larval shrimp and to add 

 only that number of algal cells needed to maintain the 

 shrimp population. However, when algal densities 

 were low, large volumes of the culture had to be 

 transferred to the shrimp rearing tanks. This resulted 

 in changes in the temperature of the larval culture 

 media which frequently caused mortalities. In addi- 

 tion, the medium in which the diatoms were grown 

 was slightly toxic to the larval shrimp. For these 

 reasons, it was decided to separate the cells from 

 their culture medium. 



Separation with a centrifuge has been successful 

 with several types such as a table model, a continu- 

 ous centrifuge, or a large cream separator. When a 

 continuous centrifuge or cream separator is used, 

 the algal concentrate accumulates within the cen- 

 trifuge and is removed by disassembling the 

 machine. If the speed of the centrifuge is adjusted so 

 that the cells are not damaged, the resulting concen- 

 trate is a satisfactory food. The cells are then sus- 

 pended in a known volume of water and a series of 

 counts made to determine cell density. The concen- 

 trate is then measured into a number of suitable 

 containers in volumes predetermined to provide 

 appropriate feeding levels in the larval rearing tanks. 



Experimentation with methods of preserving algal 

 concentrates was initiated in an effort to increase the 

 reliability of the larval culture procedure. 



In the past it had been necessary to begin algal 

 cultures several days before the gravid female 

 shrimp were captured to insure adequate volumes of 

 the culture for feeding. Often cultures were ready, 

 but gravid shrimp could not be captured, or if gravid 

 shrimp were captured, the algal cultures failed. 



Refrigeration has been used successfully to hold 

 the concentrates for periods of 96 hr. For storage 



under refrigeration the concentrate is placed in a 

 plastic container and diluted to a volume of 6 to 8 

 liters, then held at 5C and aerated gently. 



Freezing in a deep freeze at -19° to -22°C has 

 also been a suitable method of holding algae. Frozen 

 algal concentrates have been held 7 mo without 

 apparent damage to the cells. Research has also been 

 done on algal foods which are freeze-dried alone or 

 in the presence of protectants. Brown (1972) re- 

 ported that freeze-dried diatoms are suitable foods 

 for larval shrimp, although they are inferior to live 

 diatoms. 



The final modification in procedures made possi- 

 ble by the concentration of algae is the use of a 

 continuous feeding device consisting of a small 

 peristaltic pump. Either freeze-dried, frozen, or 

 fresh concentrated algae is suspended and diluted 

 slightly so that it can be pumped into a larval culture 

 at rates as slow as a few milliliters per hour. Larval 

 densities of 100-500 per liter have been maintained in 

 tanks up to l,80()-liter capacity using this technique 

 (Mock and Murphy, 1971). 



The entire procedure of centrifuging, freezing, 

 and feeding automatically has been performed with 

 cultu-res of Skeletonemu, Tetruselmis. Tha- 

 lassiosira, -and Cyclotella. Single species and mixed 

 species of algal concentrations have been tested. In 

 every case the algae used were reared in unialgal 

 cultures. Each step in this procedure contributes to a 

 more efficient hatchery operation, and the freezing 

 and automatic feeding reduce the labor requirements 

 of the operation significantly. 



TYPICAL EXPERIMENTAL RESULTS 



For purposes of demonstrating the value of re- 

 search conducted in small tanks, the results of two 

 experiments conducted in 1971 are presented below. 

 The results of these experiments were not particu- 

 larly outstanding, but they can be used to illustrate 

 the type of information which can be obtained using 

 this procedure. 



Experiment I 



Data are presented for a single tank from Experi- 

 ment I conducted March 31, 1971 (Table I). This 

 was the first use of frozen algae as food during the 

 protozoeal stages at the Galveston Laboratory. The 

 spawn from two brown shrimp were placed in a 

 1 , 520-liter fiber glass tank ( 1 .8 m in diameter, 0.9 m 

 high. v\ilh a flat bottom) in a greenhouse. Twelve 



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