34 



FISHERY BULLETIN OF THE FISH AISTD WILDLIFE SERVICE 



FACTORS AFFECTING FRY PRODUCTION 



In commercial bait-rearing operations, it is nec- 

 essary to obtain maximum production and sur- 

 vival of young from a minimum-sized brood stock. 

 Insofar as possible, optimum conditions of tem- 

 perature, salinity, food, sex ratio, and brood-stock 

 density are maintained. Some preliminary infor- 

 mation was obtained at the pilot plant and more 

 detailed information at the Kewalo plant on the 

 importance of these factors. 



TEMPERATURE AND SPAWNING 



Fry production started in December 1956 at the 

 pilot plant, and by mid- January 1957 it was appar- 

 ent that the brood stock in tank 3 was not as 

 productive as that in tank 1. 



Environmental conditions in the two tanks dif- 

 fered in several respects. The water tempera- 

 ture in tank 3 averaged 1° to 2° F. cooler than 

 in tank 1. This difference probably arose from 

 the fact that tank 3 was shaded by trees and a 

 storage shed most of the day, whereas tank 1 was 

 situated in more open surroundings and received 

 more hours of direct sunlight. In an attempt to 

 eliminate the temperature difference, the water 

 entering tank 3 was piped through a 100-foot 

 length of black, three-fourths-inch, garden hose 

 that was stretched across the roof of the storage 

 shed. On sunny days, the temperature of the 

 water flowing into tank 3 was raised as much as 

 5° to 10° F. and the temperature of the water in 

 the tank to about the same level as in tank 1 

 (table 5). 



A second major difference between the tanks 

 was the consistently lower oxygen concentration 

 in tank 3. This condition probably accounted 

 for the dead yolk-sac fry that were frequently 

 found in the detritus siphoned from the floor of 

 the tank. Aeration brought about a significant 

 increase in the concentration of oxygen in tank 3 

 (appendix table 1). 



The pattern of water circulation was also dif- 

 ferent in the two tanks. The flow of water from 

 inlet to outlet in tank 3 opposed the prevailing 

 wind while that in tank 1 was in the direction 

 of the prevailing wind. The resulting circulation 

 pattern kept tank 1 relatively clean and the water 

 usually green with algae, while tank 3 had much 

 detritus and genei'ally stagnant water. The 

 color of the water in tank 3 varied from clear to 



Table 5. — Average minimum and maximum temperatures 

 (° F.) in tanks 1 and 3 at the pilot plant 



1 No data; thermograph out of order. 



light green to dark brown. Improvement in 

 water circulation in tank 3, resulting from aera- 

 tion and slight heating of the water, was sufficient 

 to produce a significant increase in fiy produc- 

 tion during March 1957. Tables 6 and 7 give 

 the- production per female per month for these 

 brood tanks (tank 2, which was later converted 

 to a brood tank in August 1957, was similar in all 

 respects to tank 1). 



Table 6. — Fry produced in tank 1, per female and month, 

 at the pilot plant, December 1956- July 1958 



1 Because of mortalities and transfer of flsh, the number of females used to 

 calculate the production per female is based on the number of females that 

 were present in the tank for more than 2 weeks. 



