42 FISH HATCHERY MANAGEMENT 



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Figure 17. A fish crowder for large- diameter circular pools, (l) Screens are 

 inserted into the thret -sided frame, after it is placed in the pool. (2) One end of 

 the frame is anchored to the pool wall with a retaining rod, and the other end is 

 carefully guided around the circumference of the pool, herding the fish ahead of 

 the crowder. (3) The fish can be readily netted from the rectangular enclosure 

 formed by the three sides of the crowder and weighed. Note the hanging dial 

 scale and dip net (see inventory methods in Chapter 2). (4) The crowder also 

 can be used for grading fish when appropriately spaced racks are inserted in the 

 frame. Small fish will swim through the racks, leaving the larger ones entrapped. 

 Aluminum materials should be used to construct the crowder to reduce weight. 

 (FWS photo.) 



Air, driven into the water by the force of the inflowing water, provides ad- 

 ditional oxygen as the water circulates around the tank or pool. Water in- 

 troduced under pressure at the head end of rectangular troughs or race- 

 ways does not have the same opportunity to reaerate the water flowing 

 through those units. 



An example of the effect of water pressure on circular tank environments 

 is presented in Table 5. At low pressures, the amount of dissolved oxygen 

 limits the carrying capacity; at high pressures the buildup of metabolites 

 (ammonia) limits production before oxygen does. 



There must be a compromise between velocity and the flow pattern best 

 suited for feed distribution, self-cleaning action of the tank and the energy 

 requirement of continuously swimming fish. This environment may not be 

 suitable for such fish as northern pike, which do not swim actively all of 

 the time. When properly regulated, the flow pattern in a circular tank will 

 effectively keep feed particles in motion and will eventually sweep uneaten 



