Fish Hatchery Management - fisheries & aquaculture

62 FISH HATCHERY MANAGEMENTdaily, weekly, or monthly increment of length is nearly constant for somespecies of fish during the first I j years or so of life. Carefully maintainedproduction records will reveal this growth rate for a particular species andhatchery.Example:On November I, a sample of 240 fish weighs 12.0 pounds.The water temperature is a constant 50'F. From past hatchery records, it isknown that the fish have a condition factor C of 4,010 x l0 ; and thattheir average monthly (fO-auy) growth is 0.66 inches. Will it be possible toproduce 8-inch fish by next April l?(l) The average weight of the fish is 12 pounds/24O fish : 0.05 poundsper fish. From the length-weight table for C : 4,000 x l0i (Appendix I),the average fish length on November I is 5.00 inches.(2) The daily growth rate of these fish is 0.66 inch/3O days : 0.022inch/day.(3) From November I through March 31, there are 151 days.(4) The average increase in fish length from November 1 through March31 is l5l days x 0.022 inch/day : 3.32 inches.(5) Average length on April I is 5.00 inches + 3.32 inches : 8.32 inches.Yes, 8-inch fish can be produced by April l.GROWTH AT VARIABI-E WAT'ER TEMPERATURESIn the previous example a growth of 0.660 inch per month at 50'F wasused. If all factors remain constant at the hatchery, growth can be expectedto remain at 0.660 inch per month and growth can readily be projectedfor any given period of time. Not all hatcheries have a water supplythat maintains a constant temperature from one month to the next. Unlesswater temperature can be controlled, a different method for projectinggrowth must be used.Growth can be projected if the average monthly water temperature andincrease in fish length are known for several months. The Monthly TemperatureUnits (MTU) required per inch of growth must first be determined.Monthly Temperature Units are the average water temperature fora one-month period, minus 32'F (the freezing point of water). Thus, ahatchery with a monthly average water temperature of 50'F would have l8MTU (50" - 32'F) available for growth. To determine the number of MTUrequired for one inch of growth, the MTUthe monthly gain in inches (available from past records).for the month are divided byConsider a hatchery with a water temperature that fluctuates from a lowof 4l'F in November to a high of 59"F during June. June would have 27MTU (59'-32'F) but November would have only I MTU (+t'-sz"F).