HATCHERY OPERATIONS 63 



Let US assume from past records that the fish grew 0.33 inch in November 

 and 1.00 inch in June. How many MTU are required to produce one inch 

 of growth? 



(1) In November, 9 MTU ^ 0.33-inch gain = 29 MTU per inch of 

 growth. 



(2) In June, 27 MTU ^ 1.0-inch gain = 27 MTU per inch of growth. 



Once the number of MTU required for one inch of growth is determined, 

 the expected growth for any month can be calculated using the equation: 

 MTU for the month ^ MTU required per inch growth = monthly 

 growth in inches. 



Example: From past hatchery records it is determined that 27 MTU are 

 required per inch of growth, and the average water temperature for the 

 month of October is expected to be 48°F. What length increase can be 

 expected for the month of October? 



(1) The MTU available during the month of October will be 16 

 (48°-32°F). 



(2) Since 27 MTU are required for one inch of growth, the projected in- 

 crease for October is 0.59 inch (16 -^ 27). 



If fish at this hatchery were 3.41 inches on October 1, the size can be pro- 

 jected for the end of October. The fish will be 4 inches long (3.41 + 0.59). 



Generally, monthly variation occurs in the number of MTU required per 

 inch of growth, and an average value can be determined from past records. 



Carrying Capacity 



Carrying capacity is the animal load a system can support. In a fish 

 hatchery the carrying capacity depends upon water flow, volume, exchange 

 rate, temperature, oxygen content, pH, size and species of fish being 

 reared, and the accumulation of metabolic products. The oxygen supply 

 must be sufficient to maintain normal growth. Oxygen consumption varies 

 with water temperature and with fish species, size, and activity. When 

 swimming speed and water temperature increase, oxygen consumption 

 increases. As fish consume oxygen they also excrete metabolic products 

 into the water. If the fish are to survive and grow, ammonia and other 

 metabolic products must be diluted and removed by a sufficient flow of 

 water. Because metabolic products increase with increased fish growth and 

 overcrowding, the water flow must be increased. 



Low oxygen in rearing units may be caused by insufficient water flow, 

 overloading with fish, high temperature which lowers the solubility of 



