30 FISH HATCHERY MANAGEMENT 



Figure 13. Three linear clarifiers located at the Jones Hole National Fish 

 Hatchery, 114 f t x 41 ft and 6 ft deep. Each unit has a sludge scraper system for 

 sludge removal. These are long redwood boards attached by chain to move and 

 deposit sludge into the sumps at the upper ends of the clarifiers. This system is 

 designed to pump sludge to drying chambers. (FWS photo.) 



A warmwater fish rearing pond acts as its own settling basin. Except 

 when the water level is so low that any water movement scours the bottom, 

 draining a pond usually does not cause much waste escapement. However, 

 during seining operations when the bottom is disturbed, levels of suspend- 

 ed solids in the effluent can increase several hundred times. Special atten- 

 tion should be given to discharges at such times. If water flow through the 

 pond cannot be stopped until solids can resettle, the effluent may have to 

 be filtered or diverted away from receiving streams. Likewise, pollutant 

 loads from other hatchery operations can increase sharply at times. Periods 

 of raceway cleaning are examples, and there should be means available to 

 handle the added waste concentrations. Sometimes, raceways and tanks can 

 be vacuumed before they are disturbed, although this is labor-intensive 

 work (Figure 15). 



SOLID WASTE DISPOSAL 



Over half of the total nutrients produced by hatchery operations are in the 

 form of settleable solids. They must be removed frequently from lagoons 

 and clarifiers, because they rapidly decompose and would otherwise pollute 

 the receiving waters with dissolved nutrients. 



The "solid" wastes from settling basins and various filtration units 

 around a hatchery, being 90% water, can accumulate into large volumes 

 that must be disposed of. Hatchery sludge has considerable value as a fer- 

 tilizer. In warm climates and seasons, it can be spread directly on the 



