Publication Abstract

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

B.P. North, J.F. Turnbull, T. Ellis, M.J. Porter, H. Migaud, J. Bron and N.R. Bromage

The welfare of farmed fish is a subject of increasing interest, and one of the principal areas of concern is the stocking density. The present study maintained juvenile rainbow trout in triplicate in 1.8 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), and individual morphometric (mass, fin condition) and physiological (haematocrit, plasma cortisol, glucose (lysozyme activity) indicators. Stocking density did not significantly affect growth or mortality, but the fish held at10 kg m^-3 had a significantly lower mean body condition factor and size variation was greater. Plasma cortisol levels were greater at 10 kg m^-3 treatment in 5 of the monthly sample points. Stocking density had a significant effect on fin condition, with the increasing density resulting in fish with significantly smaller fins, although the cause of the erosion remains unclear. Principal components analysis was used to produce objective welfare indices representing coherence that existed between the simultaneously measured individual welfare indicators. The resulting principal components supported the observations of high levels of cortisol and a trend for lower lysozyme activity in the 10 kg m^-3 and identified a group of fish common to all density treatments that had good dorsal and caudal fin scores, but low condition factor, which 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 negative effects on mean growth rate, or physiological indicators of welfare. 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 result in poor trout welfare.

Reference:

B.P. North, J.F. Turnbull, T. Ellis, M.J. Porter, H. Migaud, J. Bron, N.R. Bromage (2006) The impact of stocking density on the welfare of rainbow trout (Oncorhynchus mykiss). Aquaculture, 255(1-4): 466-479