82 FISH HATCHERY MANAGEMENT 



Figure 31. Pond fish being sampled with a lift net. The fish are 

 attracted to the area with bait. 



as much to determine the condition and health of the fish, to adjust feed 

 applications, and to estimate harvest dates, as it is to estimate growth and 

 survival. Usually, it is impractical to concentrate all fish in a pond to- 

 gether, so sampling is done on a small fraction of the population. Numeri- 

 cal calculations based on such small samples may be biased and unreliable 

 except as general guidelines. 



One way to sample pond fish is to attract them with bait and then cap- 

 ture them, as with a prelaid lift net (Figure 3l). The problem with this 

 technique is that fish form dominance hierarchies, and the baited area 

 quickly becomes dominated by the larger and more vigorous individuals. 



This will bias the sample. 



Most pond samples are taken with seine nets. Such samples can be ex- 

 trapolated to the whole pond if the seine sweeps a known area, if few fish 

 escape the net, and if the population is distributed uniformly throughout 

 the pond. The area swept by the net can be calculated with little difficulty; 



