The impact of stocking density on the welfare of rainbow trout (Oncorhynchus mykiss)

The welfare of farmed fish is a subject of increasing interest and one of the principal areas of concern is stocking density. The present study stocked juvenile rainbow trout in triplicate in 1.82 m3 flow-through tanks at densities of 10, 40, and 80 kg m− 3 over a 9-month period. Welfare was assessed by measuring a range of population (mortality, growth, size variation, FCR), individual morphometric (mass, fin condition, condition factor) and physiological (haematocrit, plasma cortisol, lysozyme activity) indicators. Stocking density did not significantly affect growth or mortality, but the fish held at 10 kg m− 3 had a significantly lower mean body condition factor and an increased size variation at the end of the study. Stocking density had a significant effect on fin condition, with the 40 and 80 kg m− 3 treatments resulting in fish with significantly smaller fins than the 10 kg m− 3 treatment. Plasma cortisol levels were greater at 10 kg m− 3 on 5 of the monthly sample points. Principal components analysis was used to produce objective welfare scores representing coherence that existed between the simultaneously measured individual welfare indicators. The resulting principal components identified a trend for high levels of cortisol and lower lysozyme activity in the 10 kg m− 3. The PCA also identified a group of fish common to all treatments that had good dorsal and caudal fin scores, but low condition factor. This may have represented individuals that had adopted a non-competitive feeding strategy. The experiment demonstrated that stocking densities of 80 kg m− 3 did not produce consistent effects on mean growth rate, or physiological indicators of welfare. Nevertheless, fin erosion increased with increasing density, although the cause of the erosion remains unclear. The evidence for stronger dominance hierarchies in the 10 kg m− 3 treatment, indicate that low as well as high stocking densities have the potential to adversely affect trout welfare.