HATCHERY OPERATIONS 



85 



Figure 32. A mechanical crowder used in concrete rearing ponds with an adjust- 

 able Wilco grader mounted on the crowder frame. (Courtesy California Department 

 of Fish and Game.) 



greater have adverse effects on most fishes. The use of l-3'/^i saline solution 

 for handling and moving fish has been recommended by some fishery 

 workers to reduce handling stress. Containers should be full of water. If the 

 water cannot slosh, fish will not be thrown against the sides of the 



container. 



A dip net and tub can be used to avoid physical damage when small 

 poundages of fish are moved. Large- meshed nets should be avoided, partic- 

 ularly when scaled fish are involved. Nets used for catfish commonly are 

 treated with asphaltum or similar substances to prevent damage due to 

 spine entanglement. 



Many warmwater fish hatcheries comprise a number of earthen ponds 

 that normally are harvested through a combination of draining and seining 

 (Figure 33). When large poundages of fish are present, a substantial portion 

 is removed by seining before the pond is lowered. The remainder are then 

 easily harvested from collection basins (Figure 34). Small fihgerlings are 

 harvested by lowering the pond water level as rapidly as possible without 

 stranding the fish or catching them on the outlet screen. 



If the contents of a pond cannot be removed in one day, the pond 

 should be partially refilled for overnight holding. Holding a partially har- 

 vested pond at a low level for long periods of time should be avoided 



