Energetic savings when switching from a whole-fish type diet to a commercial pelleted diet in California yellowtail (Seriola dorsalis)

Feed is one of the largest costs in most aquaculture operations, and as such, aquaculturists are continually striving to improve on ingredient sourcing and formulations to increase growth and performance while reducing costs. Analysis of the energy expended in digestion of different feeds can be a valuable early indicator of the suitability of a new ingredient or formulation. We use respirometry, an activity level analysis, and proximate analysis of fecal samples to compare the metabolic cost of digestion in California yellowtail (Seriola dorsalis) fed two different diets – a meal of whole sardine fillets representing a whole fish diet and a commercial pelleted feed designed specifically for Seriola aquaculture. Respiration trials reveal that the specific dynamic action coefficient (SDACoefficient), SDA coefficient (SDACoefficient), or percent of energy expended on digestion in relation to the total energy available in a meal, was more than double for a whole-fish type meal (20.8 ± 0.1%) than for the pelleted meal (10.2 ± 0.8%). Analysis of fish activities indicated 12.3% greater activity during the whole-fish type meal, as compared with the pelleted meal. After 24 h, the total amount of protein and lipid remaining in the feces from the whole-fish type meal (4.2 ± 0.7 kJ, 2.2 ± 0.4 kJ respectively) was significantly higher than the pelleted meal (1.8 ± 0.2 kJ, 0.9 ± 0.2 kJ respectively). Digestibility of the whole-fish type meal was slightly lower than the pelleted meal at 95.3 ± 0.5% and 97.7 ± 0.2%, respectively. The reduction in the energetic cost of digestion observed in fish fed the commercial pellet diet is likely attributed to the additional processing during manufacture, which liberates a significant portion of the energy normally allotted towards digestion. The results of this work reveal and quantify the benefits of switching from whole-fish feed to a commercial compound feed, as the reduced energetic costs of digestion allow for greater amounts of energy to be available for somatic growth. •Energetic cost of digestion in California yellowtail fed two different diets - a whole-fish type feed and commercial pelleted feed.•The comparative advantages of a commercial feed are revealed in terms of lowered energetic costs of digestion.•Evaluation of metabolic response during digestion can be a preliminary indicator of the comparative suitability for aquaculture diets.