Effect of density on growth and survival of rainbow trout

Groups of rainbow trout (Salmo gairdneri) were kept at five different densities during an initial feeding period of 42 days and a subsequent growth period of 180 days. They were then kept at the same density for 55 days. Differences between groups in mean weights and mortality were found as early as the end of the initial feeding period. At the end of the 180-days growth period the differences in mean weight between density groups were large. Differences in condition (K) factor were also found at the end of this period. Groups at high densities had the lowest growth rate and K factor. When groups were kept at the same density, those originally at high densities showed the highest relative growth. It is concluded that high densities depressed the growth rate, and that compensatory growth occurred when the densities were standardized. It is important to take density into account when ranking families or progeny groups for growth performance.     

Selection experiments with salmon: III. Genetic and environmental sources of variation in length and weight of Atlantic salmon in the freshwater phase

Atlantic salmon (Salmo salar) brood stock were taken from 32 strains in Norway for 3 consecutive years. Each year, after fertilization, the offspring in each family from each strain were reared in separate egg trays and fingerling tanks. Individual weights and lengths were recorded on a sample of fish from each tank. Environmental and genetic factors influencing the growth rate were studied. The length and weight at a standardized age were significantly affected by hatching date and number of fish held in a tank. There was also a significant ‘tank effect’ which influenced the heritabilities estimated. Significant differences between strains were found for all three year-classes (P<0.01). The estimates of heritabilities found for different year-classes are very consistent. When ‘tank effect’ was removed the heritabilities estimated from the total material were 0.08 for weight and 0.12 for length calculated from the sire components of variance. Calculated from the dam components, the heritabilities were 0.15 for weight and 0.17 for length. The genetic correlation between length and weight estimated from the total material was 1.00. For the ‘trait’ condition factor there were no significant sire and dam components of variance when tank effect was removed.