During planning of experiments scientists frequently face the choice of experimental unit size. On the one hand, the experiments should be designed to mimic realistic large-scale farming situations as far as possible. On the other hand, smaller unitsare often preferred because running costs are lower, the conditions are easier to standardise, and it is more feasible to run many replicates. Furthermore, with reference to the three R's in animal research, small units enable reduction in number of animals. Despite advantages with small experimental units, professional staff at many research facilities have observed that unit size matters regarding fish performance in experiments, and that transfer of fish, especially from larger to smaller units, mayresult in weeks without normal feed intake. However, there are a few studies showing effects of scale on growth (Boeuf and Gaignon, 1989). In addition, the study by Boeuf and Gaignon (1989) had too few replicates (N=2) to enable statistical analyses andhence conclusions. The aim of this experiment was to investigate the effects of rearing tank scale, and different scaling histories on Atlantic salmon performance during five months.
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