Influence of improved feed quality and food conversion ratios on phosphorus loadings from cage culture of rainbow trout, Oncorhynchus rnykiss (Walbaum), in freshwater lakes

Abstract

The expansion of freshwater cage culture in Scotland during the 1980s led to concern with respect to the impact of nutrient, particularly phosphorus (P), discharges on water quality in lakes. The primary route by which P enters the aquatic environment from cage farms is through the feed administered to the fish. In recent years, there have been considerable technological advances in feed manufacturing and improved feeding practices at farm level. This paper investigates how P inputs to freshwater lakes from cage farms may be reduced through changes in diets and improved food conversion ratios. The results demonstrate that there has been a significant improvement in the P content of freshwater trout diets in recent years and that it is possible to reduce waste P loadings from cage sites through the use of better-quality diets and improved feed management. In Scotland, production levels in freshwater cages are commonly set by the regulatory authority assuming a soluble waste P loading of 10 kg P tonne−1 of fish produced. This study suggests that actual soluble waste P loadings are likely to be far lower than this and that, as a result, increases in water column total P levels may be overestimated.