Fish Hatchery Management - fisheries & aquaculture

82 FISH HATCHERY MANAGEMENTFIGURE 31. Pond fish being sampled with a lift net. The fish areattracted to the area with bait.as much to determine the condition and health of the fish, to adjust feedapplications, and to estimate harvest dates, as it is to estimate growth andsurvival. Usually, it is impractical to concentrate all fish in a pond together,so sampling is done on a small fraction of the population. Numericalcalculations based on such small samples may be biased and unreliableexcept as general guidelines.One way to sample pond fish is to attract them with bait and then capturethem, as with a prelaid lift net (Figure 3l). ttre problem with thistechnique is that fish form dominance hierarchies, and the baited areaquickly 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 extrapolatedto the whole pond if the seine sweeps a known area, if few fishescape the net, and if the population is distributed uniformly throughoutthe pond. The area swept by the net can be calculated with little difficulty;