Feed efficiency of Rainbow trout (Onchorynchus mykiss) kept at high and low stocking density

Feed efficiency is a trait of high economic importance in fish production, and is highly related to feeding regimes employed and stocking density. However, feed efficiency is difficult to estimate because measurements of individual feed intake are generally not available in fish that are usually reared in groups in tanks. An alternative is to estimate feed efficiency using tank as the unit of measurement. The objective of this study was to investigate tank residual feed intake in rainbow trout kept at high (HD) and low (LD) stocking density during 42 days (Day 0-14 and Day 14-42) and the consequences of subsequently reducing density in the HD treatment between 42 and 78 days (Day 42-61 and Day 61-78). HD fish weighed less at all times than LD fish (P < 0.05). LD fish grew faster than HD fish (P < 0.05) but not between Day 42-61. The coefficient of variation of body weight was larger in HD fish than in LD fish (P < 0.05) at Day 14 and Day 42. LD fish ate more than HD fish between Day 14-78 (P < 0.01). HD fish were less food efficient than LD fish between Day 0-14 but more food efficient between Day 42-61. A higher coefficient of variation of body weight in the HD tanks suggests that growth and feed intake were inhibited because of dominance relationships at a high stocking density and possibly competition for food. After relocating HD fish to a low density treatment, HD fish showed compensatory growth and compensatory feed efficiency. Although it is not practical to estimate residual feed intake individually in fish, this research shows that calculation of tank residual feed intake can be used as an alternative, especially when used to compare families for family trait selection.