Weighing Trout by Water Displacement
Weighing fish by displacement has been widely used by trout
producers, processors, and live haulers in western North Carolina.
The displacement weight method, although widely used, is somewhat
misunderstood. This is an attempt to clarify some of the
uncertainties related to displacement loading.
Displacement loading uses the fact that 1.02 pounds of trout
displace 1.0 pound of water, regardless of fish size. Usually a
transportation tank is filled with water to a specific mark on the
inside of the tank. The fish are loaded until the water reaches a
second mark.
The procedure for displacement loading is as follows:
Step 1 – fill the tank to a specific "empty" point with
water and make a permanent mark
Step 2 – add water to the tank to a "full" level and make a
second permanent mark
Step 3 – remove the water from the tank, weighing the water
as it is removed and lowering the water level in the tank from the
"full" level to the "empty" mark
Step 4 – multiply the total weight of water removed by 1.02
to determine the pounds of trout
Example:
=> weight of the water
between the "full" mark and the "empty" mark = 448 pounds
=> 448 pounds of water x 1.02 = 457 pounds of trout
Tanks used to transport a standard weight are usually calibrated.
Water is filled to the same mark each time. Fish are added to a
second mark to load the specified pounds. Generally 2-2.5 pounds of
trout per gallon can be safely transported if sufficient
aeration/oxygenation is provided. The volume of the tank (gallons)
is determined from the "empty" mark.
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