Atlantic salmon growth is affected by environmental factors (e.g. temperature, photoperiod and water quality), social parameters (e.g. stocking density and hierarchical structures), genetic factors and nutrition. Atlantic salmon parr are known to develop a bimodel size distribution due to individual differences in growth rate (Thorpe et al., 1992). Two groups of fish, slow and fast-growing parr, differ in their age of smolting and migration to sea; thus growth of salmon under natural and farm conditions is highly variable. Farmed salmon cultured in freshwater hatcheries under controlled temperature and photoperiod on well-balanced diet reach the smolt stage (50–80 g) within one year (Figure 4). Smolts transferred to sea cages reach harvest size (~ 4 kg) in 10–15 months (Figure 5). Growth rates of farmed Atlantic salmon are much higher than those of wild fish. Sea-run sexually mature salmon returning to their rivers range from 2.3 to 9.1 kg in weight (Scott and Crossman, 1973). However, the weight of farmed broodstock salmon may range from 6 to 20 kg, depending upon their genetic background and whether fish are single or repeat spawners. Some small salmon, often referred as “grilse” (<1 kg), attain early sexual maturity, and they are culled from stocks to be used for reproduction.
