The number and size of fish in cages or ponds has been an aspect of fish farming that has attracted a considerable amount of criticism. However, much of this criticism has been ill informed, says Professor Jimmy Turnbull from the University of Stirling.

This article attempts to explain the issues relating to the density of fish and their health and welfare, as well as explaining how research has generated an understanding of the issues and how that understanding has been turned into productive farming practices.

Many people’s opinions of animal welfare are related to images of chickens crammed into small cages and some people extrapolated from their feelings about conditions in terrestrial farming systems to fish farming. However, any extrapolation from terrestrial farming systems to fish farms has to be approached with a great deal of caution.

Farmed terrestrial animals, even birds, live in a two dimensional space. Therefore you can estimate the density as number of animals (or birds) per square meter or weight (biomass) per square meter. This is often referred to as the stocking density. Fish occupy three dimensional spaces and therefore any estimate of density, whether number of animals or biomass, has to be related to the volume not area and is therefore measured in cubic meters. Even this is not simple since the animals or fish may not be evenly distributed, especially in large volumes.

Generally farmed fish will shoal or school together, however, under some circumstances they may change their behaviour and start individually defending territories. These territories may be related to structures in the tank or cage or to the source of food. In either case this can result in increased aggression and reduced access to feed, for at least some fish.

There are many aspects of the fish’s environment that affect the change from territorial to shoaling behaviour including, the species and life stage, stocking density, the water velocity, water temperature and feeding system. However, in simple terms there may well be a lower limit to safe stocking density. Fish health, welfare and productivity will suffer below certain stocking densities.

Even when fish shoal the area they use may represent a greater or smaller proportion of the available space. In very cold weather some fish such as fresh water Atlantic salmon parr tend to gravitate to the bottom of tanks and remain there relatively inactive. Other factors also affect shoaling, such as light levels and the number of fish in cages. There is some evidence that high numbers of individuals and higher light levels may result in more dense shoals of fish. In addition tidal currents may reduce the volume of a cage available to fish and the flow through the cage will be reduced by the growth of organisms on the mesh, making estimated volumes inaccurate.

Because fish do not usually occupy all the available space overall stocking density (number of animals or biomass per unit volume) is not necessarily a good indication of what the fish experience. For this reason people have devised other measures of crowding or loading of the system. For tanks with water flowing through them these include carrying capacity (Kg of fish per litre of water per minute) or flow index (Kg of fish per litre per minute per cm) and others (Ellis et al. 2002).

All this information demonstrates that “stocking density” is neither a simple nor necessarily a meaningful term. If we then attempt to look at how stocking density affects the health, welfare and productivity of fish things get even more complex. Just putting more fish in a cage or tank does not cause direct harm or damage; most detrimental effects occur through social interactions and water quality (MacIntyre et al. 2008). For example, social interactions include aggression resulting in physical damage or reduced access to feed, and water quality could be related to lower levels of oxygen in some parts of the cage or increased levels of ammonia.

Therefore stocking density does not necessarily affect the experience of the fish and any affect is indirect. For this reason there is substantial evidence (Turnbull et al. 2008) that stocking density is not a good way of accurately predicting productivity or welfare.

Does stocking density mean anything? It does, but as a general indication of conditions, not a specific accurate measurement that can be used to control farming practices. Over a wide range of stocking densities you can get good or bad health, production and fish welfare depending on the quantity and quality of the water and many other factors such as the feeding system. Despite this complexity and uncertainty, if you keep putting more and more fish into the water eventually you will get to a stage where things will go wrong and problems will occur.

On most farms, experienced fish keepers will know how much their system can hold, if they exceed this level (number or weight of fish) then they have to be very careful or problems may occur. The acceptable level will be lower or higher depending on the local conditions and farming system. Even when things do start to go wrong they do not necessarily result in a collapse with catastrophic losses but may first appear as relatively mild damage to the fins. Although you cannot say exactly when problems will occur it is still possible to estimate when the probability of things going wrong increases.

Despite the complexity careful statistical analysis can identify stocking densities when the risk of things going wrong starts to increase. A project funded by the LINK Aquaculture scheme looked at stocking density in marine Atlantic salmon cages and found that the chances of poorer welfare started to increase at stocking densities above 20-25 kg/m3 (Turnbull et al. 2005).

This was the result of a great deal of data collection and very detailed and careful analysis. However, this does not mean that fish will inevitably suffer above this density and it also does not mean that fish will invariably be healthy and productive below this density. It means only that there is a greater risk of having problems above this density.

If experienced farmers can deal with stocking density why do we need scientific analysis? Many people who do not understand the complexity of the social and physical environment of fish have campaigned for lower stocking densities. A sustainable aquaculture industry has to be ethical but also environmentally and economically sustainable. Unnecessary reductions in the number of fish farmers can grow may render their business unprofitable and as explained above, low stocking densities can also be bad for fish.

What the aquaculture industry requires is a set of guidelines that allows the development of a sustainable industry, is good for their fish and is also acceptable to processors, retailers and consumers. For this reason both the salmon and trout industries in the UK approached us to try and develop guidelines for stocking density. This has resulted in scientific publications (Turnbull et al. 2005; North et al. 2006; Berrill et al. 2009;), but has also been incorporated in industry standards, welfare organisation fish farm standards and European guidelines.

This research has not been restricted to collaboration with fish farmers and we have had a variety of dialogues with other stakeholders, including retailers and welfare pressure groups to seek their opinion and inform them (Berrill et al. 2010). We are still working in this area and currently have funding from the RSPCA Freedom Food to work with salmon farming companies to examine stocking density in fresh water salmon. All of this has contributed the profitability and ethical nature of farmed fish production.


Berrill, I. K., Cooper, T., MacIntyre, C. M., Ellis, T., Knowles, T. G., Jones, E. K. M., Turnbull. J. F. (2010) Achieving consensus on current and future priorities for farmed fish welfare: A case study from the UK. Fish Physiology and Biochemistry.

Berrill, I. K., Kadri, S., Ruohonen, K., Kankainen, M., Damsgård, B., Toften, H., Noble, C., Schneider, O. and Turnbull, J. F. (2009) BENEFISH: A European project to put a cost on fish welfare actions. Fish Veterinary Journal. 11, 23–28.

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

MacIntyre, C., Ellis, T. North, B.P. & Turnbull, J.F. (2008) The influences of water quality on the welfare of farmed trout: a review. In: Fish Welfare. Ed. Branson, E. Blackwell Scientific Publications, London. 150-178.

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., North, B.P., Ellis, T., Adams, C., Bron, J., MacIntyre, C. & Huntingford, F.A. (2008) Socking density and the welfare of farmed salmonids. In: Fish Welfare. Ed. Branson, E. Blackwell Scientific Publications, London. 111-118.

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.

October 2010

the Fish Site Editor

Learn more