Stocking density practices of commercial UK rainbow trout farms

Despite the perceived increase in the likelihood for poor welfare at higher densities, there is a lack of information on actual practices and the factors affecting stocking density on commercial farms. This article collates such information from the UK trout production industry from a voluntary self-reporting postal questionnaire. The response rate achieved was 30%, allowing representation of 88 individual farms, which were conservatively estimated to represent 48% of the total UK production of rainbow trout in 2000. The responses indicate that fish size is an important factor in the selection of density, smaller fish almost universally being reared at lower densities. Stocking density practices differed markedly between individual farms with maximum densities varying from < 20 to > 80 kg m− 3. Stocking density was linked to type of production, with farms producing fish for the table market operated at higher densities than farms rearing fish for the restocking/fisheries market. Most farms surveyed (81%) used additional means to oxygenate the water, with oxygenation being more prevalent on table farms, whilst aeration was prevalent on restocking/fisheries farms. There is recognition within the industry that excessive densities can cause welfare problems, although individual responses indicated that water quality (oxygen level and water replacement) was perceived to be more significant than density per se.