the precise quantities of feed at prescribed inter- 

 vals. The more advanced of these devices monitors 

 water temperature, fish size, and adjusts the daily 

 ration accordingly. 



Where the pondfish culturist is basically an 

 ecologist. the trout and salmon culturist is a 

 physiologist. He must know the requirements for 

 space, for the physical and chemical components of 

 the water, and for the nutritional requirements of 

 the fish. Through careful control of these factors, 

 each kilogram of trout or salmon produced may 

 require as little as 1.2 kg of feed. 



Growth rates can be calculated with utmost accu- 

 racy. Haskell (1959) concluded that the increase in 

 length of trout up to 25 cm in size is at a constant 

 rate. He developed a ""temperature unit" theory in 

 which he states that the growth rate can be pre- 

 dicted for any temperature between 3.7° and 

 !5.6°C. We have found it practical to restate 

 Haskell's hypothesis to include 0°C as the zero 

 point for growth and consider it to be a straight line 

 relationship when plotted against time up to 15°C. 

 This has proven a useful tool in projecting growth 

 rates and forecasting the time when the fish will 

 reach a given length. 



Fish densities in trout and salmon rearing ponds 

 are not as critical as the quality and quantity of the 

 water flowing through the unit. Ample exchanges of 

 water between 0° and 2rC.. free of to.xic metals 

 such as zinc, copper, and manganese,- and from ex- 

 cessive levels of such gases as nitrogen, are essen- 

 tial. Oxygen levels must be maintained above 5 ppm 

 throughout the tank, and ammonia should not ex- 

 ceed 1 ppm for long periods of time. If these water 

 quality criteria are met, most salmonids can be 

 reared at densities exceeding 50 kg/M^. 



The raceway, a linear pond whose length is ap- 

 proximately 10 times its width, has been the most 

 popular type of rearing unit for salmonids in the 

 United States up to this time. Second in popularity 

 has been the circular pond. Many other types of 

 ponds have been tried, but have not been widely 

 adopted. In 1958 Burrows and Combs at Longview, 

 Wash., developed a circulating pond that is being 

 widely copied in the northwestern United States. 

 This pond is rectangular in shape, but employs turn- 

 ing vanes to cause the water to flow in a circular 

 pattern. The merits of this pond are its higher water 

 \elocities and thorough circulation w hich give good 

 distribution of feed and render the pond virtually 

 self cleaning. The increased velocities improve 

 stamina and result in better survival of the fish fol- 



lowing stocking. Water supplies, both entering 

 and leaving a fish pond, must be monitored for their 

 chemical content. Sufficient oxygen must be sup- 

 plied by the incoming water to permit the fish to use 

 the food. The relationship between food eaten and 

 the oxygen required is so constant that many fish 

 culturists calculate the oxygen content of the water 

 entering a pond as a means of determining the carry- 

 ing capacity. Investigations indicate that 100 g of 

 oxygen are required to metabolize 450 g of trout 

 pellets (1,200 calories) (Willoughby, 1968). This 

 1.4:5 ratio should hold constant over the tempera- 

 ture range of 4°-16°C. 



While oxygen is usually the first limiting factor in 

 the hatchery environment, it is not the only one. As 

 oxygen is used to break down foods for energy and 

 growth, by-products are formed. Prominent among 

 these are carbon dioxide and ammonia. Carbon di- 

 oxide poses few problems. Ammonia, however, is 

 another matter, as it is very difficult to remove by 

 mechanical means. Thus, when water is reused 

 from one fishpond to another, it can be aerated to 

 renew its oxygen content but ammonia continues to 

 accumulate and soon reaches toxic levels. 



A recent development which has revolutionized 

 fish culture in the United States is the use of bacte- 

 rial filters, which convert free ammonia to more tol- 

 erable nitrates. This development has opened up 

 possibilities for a tenfold increase in the quantity of 

 fish that can be reared in a given water supply. This 

 reconditioning system makes it possible to reduce 

 the quantity of freshwater by as much as 95%. 



The Dworshak National Fish Hatchery, 

 Ahsahka, Idaho, utilizes a water reuse system for a 

 part of its ponds. This hatchery began operation in 

 1968. There are 84 circulating ponds, 25 of which 

 operate on the water reuse system. In this system 

 approximately 10% of the water used in the ponds is 

 added as fresh water after being filtered, disin- 

 fected, and either cooled or heated. The water goes 

 through aerators for oxygenation and is supplied at 

 the rate of 27 liters/sec to each pond. When water 

 returns from the ponds, it goes through biological 

 filters where the pH is buffered and ammonia ox- 

 idizes to harmless nitrates. 



The hatchery was designed and built to replace 

 the spawning and nursery areas for the steelhead 

 trout which will be lost by the construction of the 

 Dworshak Dam. In addition to the rearing of 

 steelhead trout for release into the north fork of 

 the Clearwater River, this station is also participat- 

 ing in the management of the reservoir above the 



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