Publication Abstract

A review of the relationships between stocking density and welfare in farmed rainbow trout

T. Ellis, A.P. Scott, B. North, N.R. Bromage, M. Porter and D. Gadd

There is increasing public, governmental and commercial interest in the welfare of intensively farmed fish and stocking density practices have been highlighted as an area of particular concern. Here we review the evidence surrounding the issue of stocking density for rainbow trout to stimulate debate and focussed research. Although no explicit reference to welfare has previously been made, 43 separate studies have examined the effects of density on production and physiological parameters of rainbow trout. Increasing stocking density does not appear to cause crowding stress in rainbow trout. However, commonly observed effects of increasing density are an increase in fin damage and mortality, and a reduction in food intake, food conversion efficiency, nutritional condition and growth. By referring to the Five Freedoms, we illustrate that these indicators signify increasing density can be detrimental to trout welfare, although the magnitude of the effect is dependent upon study-specific conditions. There is dispute as to the cause of the observed effects of increasing density, with either water quality deterioration or increasing behavioural interactions being variably proposed. Both causes can theoretically generate the observed effects of increasing density. However, documentation of the relationship between density and the effects of aggressive behaviour at relevant commercial densities is lacking. Consequently only inferential evidence exists that increasing behavioural interactions generate the observed effects of increasing density, whereas there is direct experimental evidence that water quality degradation is responsible. Nevertheless, there is poor understanding of inappropriate levels of the key water quality parameters. The potential for non-aggressive behavioural interactions (abrasion, obstruction, collision) to affect trout welfare has, however, been largely overlooked.

Reference:

T. Ellis, A.P. Scott, B. North, N.R. Bromage, M. Porter and D. Gadd, 2002. A review of the relationships between stocking density and welfare in farmed rainbow trout. Journal of Fish Biology, 61(3): 493-531.