Quantification of differences in digestibility of phosphorus among cyprinids, cichlids, and salmonids through a mathematical modelling approach

Accurate estimates of phosphorus (P) digestibility are essential to enable the formulation of cost-effective diets meeting P requirement of fish but resulting in minimal P waste outputs. Evidence suggests that differences in P digestibility exist among fish species but they have not been quantified. A modelling approach was employed in this study to quantify these differences. As a first step, an existing mathematical model of P digestibility developed for salmonids was statistically evaluated to determine if this model could be extrapolated to predict digestible P content in feeds of two widely cultured groups of freshwater fish species, tilapias (cichlids species) and carps (cyprinid fish species) or if modification was required. Subsequently, two novel mathematical models specific for tilapias and carps were developed. Results suggest that significant differences in P apparent digestibility exist among cyprinids, cichlids, and salmonids. Cyprinid species appear to have low ability to digest P compounds of low solubility. They do not appear to be able to effectively digest bone P (digestibility of bone P was estimated to be nil), and their ability to digest dibasic calcium phosphates is low (30%) in comparison to that of cichlids and salmonids (which is in the ranges of 62-64%). These differences could be attributable to the absence of an acid stomach in cyprinids. Cyprinids and salmonids appear to be unable to digest phytate-P from plant ingredients, while tilapias seem to be able to digest phytate-P to a certain degree (approximately 27%). These mathematical models could be useful tools to estimate the digestible phosphorus content of feeds formulated with a wide variety of ingredients fed to fish species with different digestive anatomy and physiology.