68 FISH HATCHERY MANAGEMENT 



adding fish to a rearing unit with a uniform water flow until the oxygen 

 content is reduced to the minimum level acceptable for the species at the 

 outflow of the unit (.5 parts per million recommended minimum oxygen 

 level for trout). The information required for calculating the Flow Index 

 can also be determined with an existing weight of fish in a rearing unit by 

 adjusting the water inflow until the oxygen content is reduced to 5 parts 

 per million at the outflow of the unit. 



The Flow Index can then be used to determine the permissible weight of 

 any size fish ( W), by the formula: W = F x L x /. 



Example: In the previous example, a Flow Index of 1.5 was determined 

 for a raceway safely holding 900 pounds of 4-inch trout in 150 gallons per 

 minute water flow, (l) How many pounds of 8-inch trout can be safely 

 held? (2) How many pounds of 2-inch trout? 



(1) W = 1.5X8X 150 



W = 1,800 pounds of eight-inch trout 



(2) W = 1.5 X2X 150 



W = 450 pounds of two-inch trout 



Furthermore, when weight of fish is increased or decreased in a raceway, 

 the water inflow requirement can be calculated by the formula: 



1= W ^[F X L). 



For example, if 450 additional pounds of 8-inch trout are added to the 

 above raceway containing 1800 pounds of 8-inch trout, what is the re- 

 quired water inflow? 



/ = (1800 + 450) ^(1.5x8) 



/ = 188 gallons per minute water inflow 



The Flow Index shown in the example should not be considered a 

 recommended level for all hatcheries, however, because other environmen- 

 tal conditions such as water chemistry and oxygen saturation of the water 

 may influence the holding capacities at various hatcheries. 



Table 8, with an optimum Flow Index of 1.5 at 50°F, considers the ef- 

 fects of water temperature and elevation on the Flow Index. This table is 

 useful in estimating fish rearing requirements in trout and salmon 

 hatcheries. For example, a trout hatchery is being proposed at a site 4,000 

 feet above sea level, with a 55°F water temperature. Production of 4- inch 

 rainbow trout is planned. How many pounds of 4-inch trout can be safely 

 reared per gallon per minute water inflow (if the water supply is near 100% 

 oxygen saturation)? 



