The age at which fish are transferred to seawater and the feeding strategies employed during the breeding season affect the economics underlying production of rain trout, as well as the environmental loading of phosphorus and nitrogen. An experiment was undertaken to evaluate the effects of i) age at seawater transfer, ii) feeding level and iii) use of a finishing feed, upon production economics. Monosex female rainbow trout (500-900g grown to 3500g) transferred to seawater at either 1 or 2 years of age, were examined. Feeding level was either that normally used by the farm or reduced by 20%. A commercial feed type with a protein-fat content of 48-28% was used throughout the experiment. Seven weeks prior to slaughter, fish were placed on a finishing diet with a protein-fat content of 60-9%. A set of economic-environmental models were established to evaluate the effects of the different production protocols upon farm profitability. Calculations were based upon fish performance evaluated as daily growth rate (% body wt/d), feed conversion rate, mortality, and weight data by means of visceral loss and gonadal development. The results of the present study illustrate that it is not possible to optimize both environmental impact and economic benefit during the production of trout.
Present address: Bureau Veritas Quality International, Denmark, Oldenborggade I B, DK-7000 Fredericia, Denmark