SELECTIVE BREEDING ALTERS PHOSPHORUS REQUIREMENTS OF RAINBOW TROUT – A MODEL INTEGRATING BREEDING AND FISH NUTRITION

Introduction

Advances in selective breeding and fish nutrition have markedly improved performance of farmed fish. Nutrient requirements of fishes are typically estimated using costly dose–response trials, and the requirements are often treated as static despite known context-dependent variation. Selective breeding has improved feed efficiency, body composition and nutrient retention, potentially altering nutritional needs, yet feed formulation rarely accounts for these genetic changes. Factorial models that link fish traits to nutritional requirements exist (Lupatsch et al. 1998), but an integrated multitrait framework is lacking. Dietary phosphorus (P) is essential for growth and skeletal development but contributes to eutrophication, and hence P levels in aquafeeds have been actively reduced in some countries. Balancing adequate dietary P with environmental sustainability remains a major challenge. This study (Kause et al. 2026) aimed to: 1) Develop traits-to-feed equations linking genetic trait changes to dietary P requirements of rainbow trout; 2) Apply it to the genetic improvement of traits in the Finnish rainbow trout breeding programme, and 3) Forecast future dietary P needs.

Material and Methods

Traits-to-feed equations link trait changes to feed formulation using mass balance principles. Body P depends on fillet yield (YLD), viscerosomatic index (VSI), and P content of fillet (filletP), viscera (visceraP) and other parts of the body (otherP):

Optimal digestible dietary P can be calculated from feed conversion ratio (FCR), apparent digestibility coefficient of P (ADCP = digestible P / total P in feed), efficiency of P retention (REP = g P gained / g P intake), and bodyP as defined in equation 1:

Genetic trends of FCR, YLD, and VSI from 23 year-classes of the Finnish breeding programme (1992–2015; 547,246 fish) were used (Kause et al. 2026). To forecast future dietary P needed, genetic trends of the traits were projected to 2050, and the traits-to-feed equations were applied on the trends under three scenarios of P retention efficiency.

Results and Discussion

Changes in YLD and VSI had modest, linear effects on dietary P (Fig. 1a), whereas FCR had a strong, nonlinear effect - improving FCR substantially increased dietary P requirements (Fig. 1b). The average annual genetic improvement (in %) in the Finnish breeding programme was estimated to be -0.632% for FCR, +0.082% for fillet YLD%, -0.749% for VSI%, and 1.63% for body weight. Between birth years of 1992 and 2015, FCR has been genetically improved from 1.34 to 1.14, fillet YLD% from 64.7% to 65.3%, and VSI% from 11.1% to 10.7%. When equations 1 and 2 were applied over the genetic trends, the required dietary P were elevated across years (Fig. 2a), with future projections indicating a ~28% increase by 2050 if retention efficiency remains unchanged (Fig. 2b).

Figure 1. Effect of traits on optimal feed phosphorus levels, under different a) fillet yield (%) levels, and b) feed conversion ratios (FCR), when also phosphorus retention efficiency (REP) can differ.

Figure 2. Effect of genetic trends in FCR, fillet yield (YLD) and viscerosomatic index (VSI) on optimal feed phosphorus levels, under different phosphorus retention efficiency levels (REP), a) in 1993-2016 based on estimated and b) in 2016-2050 based on forecasted changes in FCR, YLD and VSI due to breeding.

The model links genetic improvement directly to dietary requirements. FCR is the main driver of nutrient needs, while body composition has smaller effects. Fish with low FCR require more nutrient-dense feeds. Selective breeding has increased optimal dietary phosphorus requirements. Current phoshorus levels in feed may already be near or below optimal, and future increases may create tension between environmental goals and fish nutrition. Improving phosphorus retention efficiency will be critical. The traits-to-feed framework integrates genetics and nutrition to predict dietary needs.

Acknowledgment

This work (No. 818367 – AquaIMPACT) has received funding from the EU's Horizon 2020 programme.

References

Kause A., Soares F., and Silva T.S. (2026) Aquaculture 612: 743171. https://doi.org/10.1016/j.aquaculture.2025.743171

Lupatsch I., Kissil G.W., Sklan D., and Pfeffer E. (1998) Aquaculture Nutrition 4(3):165-173. https://doi.org/10.1046/j.1365-2095.1998.00065.x