Differences in energy utilisation between a lean and fat strain of rainbow trout (Oncorhynchus mykiss)

A good understanding of the utilisation of energy in fish diets is important for accurate feed formulation in aquaculture. One of the primary reasons for differences in the utilisation of digestible energy between fish species are differences that exist in the composition of growth (fat versus protein gain). However, it has also been observed that composition of growth can differ between genetic strains within a single fish species. The main focus of the current experiment was to investigate whether the genetic background of different strains of rainbow trout affects the relationship between digestible energy intake and retained energy. To test this, two different commercial trout strains were selected based on their differences in body fat content (a Lean-Strain and Fat -Strain) and therefore expected differences in composition of growth. Furthermore, this research investigated whether such a potential strain difference in the relationship between digestible energy and retained energy was dependent on the type of non-protein energy in the diet (a Carb-Diet versus a Fat-Diet). Three feeding levels were used in order to estimate the utilisation efficiency of digestible energy for retained energy, leading to a 2 by 2 by 3 factorial design. The results of this study showed that the relationship between digestible energy and retained energy was affected by both strain and diet, but not by an interaction effect between these two factors. Firstly, it was observed that the utilisation efficiency of digestible energy for growth (kgDE) was higher in the Fat-Strain (72% in the Lean-Strain versus 87% in the Fat-Strain) which may be related to a higher potential for fat deposition. This higher kgDE in the Fat-Strain was however balanced by a higher maintenance requirement (47 kJ/kg0.8 per day versus 28 kJ/kg0.8 per day) leading to a similar retained energy between strains in the current trial. Secondly, it was shown that the exchange of dietary carbohydrates for dietary fat on an isoenergetic basis also increased kgDE (74% for the Carb-Diet versus 85% for the Fat-Diet). The lack of an interaction effect between strain and diet showed that kgDE in both strains was affected by the exchange of carbohydrates for fat on an isoenergetic basis in a similar way. The results of the current trial demonstrated that both dietary macronutrient composition and the composition of growth of specific trout strains should be accounted for in calculating the true net available energy for fish in feed formulation.