Projects - Stocking Density

LINK Aquaculture Sal 18 & Defra AW1203

Assessing the welfare of any animal under commercial conditions is very challenging. The difficulties associated with observing fish behaviour and the large population sizes in commercial aquaculture systems complicate things further. In the face of a poorly understood and difficult to observe system, one approach has been to look for simple production parameters, over which there is some capacity to exert control. Stocking density has been seized upon as one such parameter.

The UK FAWC report on the welfare of farmed fish highlighted stocking density as a concern and recommended the introduction of a maximum stocking density limit for trout and salmon. The report also recommended the urgent need for research into stocking density resulting in the funding of two projects:

Outputs from both of these projects are now available in peer-reviewed publications. Perhaps the most important finding from both projects was that there was a poor association between increasing stocking density and welfare status i.e. other production factors have a greater potential to impact on welfare than stocking density. Both projects also highlighted the complexities that need to be taken into account when considering stocking density in aquaculture systems. For example, the most commonly used unit of stocking density is biomass / rearing unit volume (kg m3). However, this assumes an even distribution of fish throughout the system, which is not a true refection how fish behave in commercial systems. Other facts such as net deformation due to water currents will also have significant effects on the actual volume of space available to the fish. In flow-through aquaculture systems commonly used to farm rainbow trout, water exchange rates and oxygen availability had a stronger influence on most aspects of fish welfare than stocking density.

Although there is undoubtedly a limit to the number of fish that an aquaculture system can support, there are many other factors that need to be considered. If the rest of the farming system is good then stocking densities can be relatively high with no adverse effect. If the rest of the system is bad then even acceptable lower stocking densities will not ensure the welfare of the fish. However, there are some circumstances such as high water temperature in cages or low rates of water exchange in flow-through systems where stocking density would need to be controlled.

Although these projects have focused on salmonids in systems typical of the UK, stocking density is also an important issue for other aquaculture species that are traditionally farmed elsewhere in the world.

For some species such as Arctic charr (Salvelinus alpinus) and tilapia (e.g. Oreochromis sp. and Tilapia sp.), high stocking densities have actually been shown to have benefits such as improved growth and reduced aggression.  At low stocking densities these species are highly territorial, but as density increases they will to shoal.

High stocking densities are commonly used for catfish species such as African catfish (Clarius sp.),Channel catfish (Ictalurus sp.) and the Pangasius species commonly farmed in Asia.  The ability of most catfish species to breath air and the high turbidity of the water from their natural habitats means that catfish are particularly tolerant to high density culture.

catfishtank tilapia

Above: High density culture of catfish (left) and tilpia (right) on a Vietnamese fish farm

Project outputs:

Ellis, T., North, B., Scott, A.P., Bromage, N.R. & Porter, M. (2001). What is stocking density? Trout News. 32, 35-37.

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

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

Turnbull, J.F., Bell , A., Adams , C., Bron, J. & Huntingford, F.A. (2005). Stocking density and welfare of cage farmed Atlantic salmon: application of a multivariate analysis. Aquaculture. 243, 121-132.

Turnbull, J.F., North, B.P., Ellis, T., Adams, C., Bron, J., MacIntyre, C. & Huntingford, F.A. (Accepted). Stocking density and the welfare of farmed salmonids . Proceedings of the 1 st Symposium on Fish Welfare, Edinburgh , 2004