The relationships between stocking density and welfare in farmed rainbow trout

By T. Ellis, B. North, A. Scott, N. Bromage, M. Porter, D. Gadd. 2002

729 citations recorded by the source corpus.

There is increasing public, governmental and commercial interest in the welfare of intensively farmed fish and stocking density has been highlighted as an area of particular concern. Here we draw scientific attention and debate to this emerging research field by reviewing the evidence for effects of density on rainbow trout. Although no explicit reference to ‘welfare’ has been made, there are 43 studies which have examined the effects of density on production and physiological parameters of rainbow trout. Increasing stocking density does not appear to cause prolonged crowding stress in rainbow trout. However, commonly reported effects of increasing density are reductions in food conversion efficiency, nutritional condition and growth, and an increase in fin erosion. Such changes are indicative of a reduced welfare status—although the magnitude of the effects has tended to be dependent upon study-specific conditions. Systematic observations on large scale commercial farms are therefore required, rather than extrapolation of these mainly small-scale experimental findings. There is dispute as to the cause of the observed effects of increasing density, with water quality deterioration and/or an increase in aggressive behaviour being variously proposed. Both causes can theoretically generate the observed effects of increasing density, and the relative contribution of the two causes may depend upon the specific conditions. 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 aggressive behaviour generates the observed effects of increasing density, whereas there is direct experimental evidence that water quality degradation is responsible. Nevertheless, there are contradictory recommendations in the literature for key water quality parameters to ensure adequate welfare status. The potential for welfare to be detrimentally affected by non-aggressive behavioural interactions (abrasion, collision, obstruction) and low densities (due to excessive aggressive behaviour and a poor feeding response) have been largely overlooked. Legislation directly limiting stocking density is likely to be unworkable, and a more practical option might be to prescribe acceptable levels of water quality, health, nutritional condition and behavioural indicators.

Subfields: Aquatic Science, Immunology, Nature and Landscape Conservation

View Paper: https://www.semanticscholar.org/paper/bc1aac7cee57203092b6255996cff1162c7e2e7c

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Citations in the corpus

The effect of density on sea bass (Dicentrarchus labrax) performance in a tank-based recirculating system (2009, background): “Large range of densities are commonly applied in European farms, from 10 to more than 100 kg m-3, according to the rearing system, the fish species and size (Ellis et al., 2002).”
Stocking density of Nile tilapia in cages placed in a hydroelectric reservoir (2013, background): “Fish welfare can be affected by density, and the evidence for this can be examined in terms of productivity, health, condition and stress level (Ellis et al., 2002).”
Effects of Rearing Density and Structural Complexity on the Pre‐ and Postrelease Performance of Atlantic Salmon (2007, background): “Density-dependent growth in hatcheries has been demonstrated in several previous studies (Ellis et al. 2002).”
Farmed fish welfare-suffering assessment and impact on product quality (2009, background): “Poor welfare due to a given stocking density can be shown in one flow regime but not under other conditions (Ellis et al., 2002).”
Stocking density and welfare of cage farmed Atlantic salmon: application of a multivariate analysis (2005, background): “Therefore impaired welfare at high densities in this study did not appear to be dependent on reduced water quality in any simple way, as suggested by Ellis et al. (2002) for rainbow trout (Oncorhynchus mykiss).”
The biological requirements for post-smolt Atlantic salmon in closed-containment systems (2011, background): “…responses, including increased mortality, increased metabolism, reduced growth rate, increased size variation, reduced feed intake, increased FCR, poor health condition and agonistic behaviour (Wedemeyer, 1997; Ellis et al., 2002; Boujard et al., 2002; Turnbull et al., 2005; Portz et al., 2006).”