HATCHERY OPERATIONS 79 



should be made whenever necessary to assure that records provide accurate 

 data. In any inventory, it is imperative that fish weights be as accurate as 

 possible. 



INTENSIVE CULTURE 



Fish can be weighed either by the wet or dry method. The wet method in- 

 volves weighing the fish in a container of water that has been preweighed 

 on the scale. Care must be exercised that water is not added to the 

 preweighed container, nor should water be splashed from it during weigh- 

 ing of the fish. This method is generally used with small fish. Dry weigh- 

 ing is a popular method of inventorying larger fish. The dip net is hung 

 from a hook at the bottom of a suspended dial scale. The scale should be 

 equipped with an adjusting screw on the bottom, so the weight of the net 

 can be compensated for. Dry weighing eliminates some fish handling and, 

 with a little practice, its accuracy is equal to that of wet weighing. 



The most common ways to determine inventory weights are the sample- 

 count, total-weight, and pilot-tank methods. 



In the sample-counting method, the total number of fish is obtained ini- 

 tially by counting and weighing the entire lot. In subsequent inventories, a 

 sample of fish is counted and weighed and either the number per pound or 

 weight per thousand is calculated (Figure 29). To calculate the number per 

 pound, divide the number of fish in the sample by the sample weight. To 

 calculate the weight per thousand, divide the sample weight by the number 

 of fish (expressed in thousands). The total weight of fish in the lot then is 

 estimated either by dividing the original total number of fish (adjusted for 

 recorded mortality) by the number per pound or by multiplying it (now 

 expressed in thousands) by the weight per thousand. This method can be 

 inaccurate, but often it is the only practical means of estimating the weight 

 of a group of fish. To assure the best possible accuracy the following steps 

 should be followed: 



(1) The fish should be crowded and sampled while in motion. 



(2) Once a sample of fish is taken in the dip net, the entire sample 

 should be weighed. This is particularly true if the fish vary in size. The 

 practice of weighing an entire net full of fish will obtain more representa- 

 tive data than that of weighing preset amounts (such as 5 or 15 pounds). 

 Light net loads should be taken to prevent injury to the fish or smothering 

 them. 



(3) When a fish is removed from water it retains a surface film of water. 

 For small fish, the weight of the water film makes up a larger part of the 

 observed weight than it does for larger fish. The netful of fish should be 

 carefully drained and the net bottom wiped several times before the fish 

 are weighed. 



