HATCHERY OPERATIONS 



81 



The pilot- unit method utilizes a tank or raceway of fish maintained to 

 correspond to other tanks or raceways of the same type. The pilot unit is 

 supplied with the same water source and flow, and the fish are fed the 

 same type and amount of food per unit of body weight. All the fish reared 

 in the pilot unit are weighed and the gain in weight is used to estimate the 

 fish weight in the other rearing units. This method is more accurate than 

 sample counting for fish up to six inches long. 



EXTENSIVE CULTURE 



Fish grown in ponds are relatively inaccessible and difficult to inventory 

 accurately before they finally are harvested. Pond fish still are sampled fre- 

 quently, as they are in raceway culture, but the value of such sampling is 



Figure 30. A quarter- sampler can be used to accurately estimate the number of 

 fish per pound or weight per thousand fish, (l) A framed net with four remov- 

 able pockets in the bottom is designed to fit snugly in a large tub of water. (2) 

 Several netfuls of fish are put in the tub and when the frame is removed the fish 

 are divided into four uniform samples. (3) Only one-quarter of the" fish are actu- 

 ally used in the sampling. The fish are counted and then weighed. (4) A modi- 

 fied frame design has one of the net pockets closed (arrow) and the other three 

 open. As the frame is lifted out of the tub the fish in the closed pocket are 

 retained for counting. It is felt that a sample taken in this manner, from several 

 netfuls of fish, reduces bias in sampling. (FWS photos.) 



