Fish Hatchery Management - fisheries & aquaculture

42 FISH HATCHERY MANAGEMENTFTGURE 17. A fish crowder for large-diameter circular pools. (1) Screens areinserted into the three-sided frame, after it is placed in the pool. (2) One end ofthe frame is anchored to the pool wall with a retaining rod, and the other end iscarefully guided around the circumference of the pool, herding the fish ahead ofthe crowder. (S) T'he fish can be readily netted from the rectangular enclosureformed by the three sides of the crowder and weighed. Note the hanging dialscale and dip net (see inventory methods in Chapter Z). (l) 1'ne crowder alsocan be used for grading fish when appropriately spaced racks are inserted in theframe. 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 additionaloxygen as the water circulates around the tank or pool. water introducedunder pressure at the head end of rectangular troughs or racewaysdoes not have the same opportunitythrough those units.to reaerate the water flowingAn example of the effect of water pressure on circular tank environmentsis presented in Table 5. At low pressures, the amount of dissolved oxygenlimits 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 bestsuited for feed distribution, self-cleaning action of the tank and the energyrequirement of continuously swimming fish. This environment may not besuitable for such fish as northern pike, which do not swim actively all ofthe time. When properly regulated, the flow Pattern in a circular tank willeffectively keep feed particles in motion and will eventually sweeP uneaten