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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

=> 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.