62 FISH HATCHERY MANAGEMENT 



daily, weekly, or monthly increment of length is nearly constant for some 

 species of fish during the first 1^ years or so of life. Carefully maintained 

 production records will reveal this growth rate for a particular species and 

 hatchery. 



Example: On November 1, a sample of 240 fish weighs 12.0 pounds. 

 The water temperature is a constant 50°F. From past hatchery records, it is 

 known that the fish have a condition factor C of 4,010 x lO'^ and that 

 their average monthly (30-day) growth is 0.66 inches. Will it be possible to 

 produce 8-inch fish by next April 1? 



(1) The average weight of the fish is 12 pounds/240 fish = 0.05 pounds 

 per fish. From the length-weight table for C — 4,000 x 10^ (Appendix l), 

 the average fish length on November 1 is 5.00 inches. 



(2) The daily growth rate of these fish is 0.66 inch/30 days — 0.022 

 inch/day. 



(3) From November 1 through March 31, there are 151 days. 



(4) The average increase in fish length from November 1 through March 

 31 is 151 days x 0.022 inch/day = 3.32 inches. 



(5) Average length on April 1 is 5.00 inches + 3.32 inches = 8.32 inches. 

 Yes, 8-inch fish can be produced by April 1. 



GROWTH AT VARIABLE WATER TEMPERATURES 



In the previous example a growth of 0.660 inch per month at 50°F was 

 used. If all factors remain constant at the hatchery, growth can be ex- 

 pected to remain at 0.660 inch per month and growth can readily be pro- 

 jected for any given period of time. Not all hatcheries have a water supply 

 that maintains a constant temperature from one month to the next. Unless 

 water temperature can be controlled, a different method for projecting 

 growth must be used. 



Growth can be projected if the average monthly water temperature and 

 increase in fish length are known for several months. The Monthly Tem- 

 perature Units (MTU) required per inch of growth must first be deter- 

 mined. Monthly Temperature Units are the average water temperature for 

 a one-month period, minus 32°F (the freezing point of water). Thus, a 

 hatchery with a monthly average water temperature of 50°F would have 18 

 MTU (50° — 32°F) available for growth. To determine the number of MTU 

 required for one inch of growth, the MTU for the month are divided by 

 the monthly gain in inches (available from past records). 



Consider a hatchery with a water temperature that fluctuates from a low 

 of 41°F in November to a high of 59°F during June. June would have 27 

 MTU (59°-32°F) but November would have only 9 MTU (41°-32°F). 



