Urea and trimethylamine oxide in diets for seawater farmed rainbow trout: effect on fat belching, skin vesicle, winter ulcer and quality grading

Abstract

Three experiments were conducted in order to evaluate the effects of urea and trimethylamine oxide (TMAO) supplementation in diets for seawater-farmed rainbow trout Oncorhynchus mykiss. In all experiments, commercial extruded feed was used as basal control diet, whereas experimental diets were the basal diet supplemented with either 10 g kg–1 TMAO, 10 g kg–1 urea or 10 g kg–1 TMAO and 10 g kg–1 urea. The experiments were conducted both in research units and in a commercial fish farm. Quality was evaluated in three different commercial slaughterhouses, sorting the rainbow trout into superior, ordinary or processing grade. Significantly more TMAO was observed in muscle of rainbow trout given dietary supplementation of TMAO. However, the levels were low compared with levels in marine fish in general. Trimethylamine (TMA) was not found. No increase in muscle urea content was observed in trout fed urea in the diet. Significantly reduced fat belching, seen as reduced number of fat droplets surfacing in the net pen, was observed in groups given TMAO-supplemented feeds. At least twice as many rainbow trout with skin vesicles were observed in the control group as in the group fed urea-supplemented diet. In addition, skin vesicles developed into open sores in the winter only in rainbow trout fed control diet. In a large-scale trial, quality grading of almost 200 000 market-sized rainbow trout showed significantly more (88 vs. 83%) trout of superior grade in the group fed dietary urea compared with fish fed control diet. The main reason for downgrading from superior to ordinary and processing grade was skin lesions, supporting the observations made in the experimental units.