Selective breeding alters nutritional needs of farmed fish – a new model supports welfare-oriented feeding 

A biometric model developed by the Natural Resources Institute Finland (Luke) can predict how fish feed composition should be adjusted as selective breeding changes the biological traits of fish. The study revealed that selectively bred rainbow trout require more phosphorus in their feed than before. 

Thanks to selective breeding, today’s farmed rainbow trout grow faster, utilize feed more efficiently, and produce more edible fillet than ever before. Breeding also changes the fish’s nutritional requirements. If feed composition is not adapted to this development, fish welfare may be compromised. 

The research team developed a new biometric model that links measurable fish traits—such as feed and nutrient utilization, fillet yield, and body composition—directly to the optimal feed composition for the fish. The model can predict how feed composition should change as selective breeding alters the biological characteristics of fish. 

“Traditionally, fish feed composition has been determined through laborious and expensive feeding trials. The new model helps researchers and feed producers design feeds and feeding strategies that support fish health and welfare and meet the biological needs of modern farmed fish strains,” says Antti Kause, Principal Scientist at Luke. 

The new model provides a predictive tool for safeguarding fish welfare by mathematically integrating fish genetics and nutrition into the same framework. It can also be applied to protein, energy, and other essential nutrients, paving the way for precision feeding that supports both fish welfare and sustainable production.  

Selective breeding increases the phosphorus requirement of rainbow trout 

The study focused on phosphorus (P), an essential nutrient for bone and cell development in fish. The developed model showed that selectively bred rainbow trout require more phosphorus in their feed than before—mainly because their feed efficiency has improved significantly. 

In recent decades, efforts have been made to reduce phosphorus in fish feeds for environmental reasons. Too little phosphorus causes skeletal deformities, poor growth, and reduced welfare, while excessive phosphorus in feeds can lead to eutrophication of water bodies. “Finding the right balance is therefore both an environmental and an animal welfare issue,” Kause explains. 

Feed efficiency in rainbow trout has improved by about 50% since the early 1980s, and the amount of nitrogen and phosphorus (kg) entering water bodies per 1,000 kg of fish produced has decreased by over 70%. At least one-third of this improvement in feed conversion ratio is estimated to result from selective breeding. 

If selective breeding continues at the current rate, model predictions suggest that the optimal feed phosphorus level for rainbow trout will increase by about 28% by 2050, unless phosphorus utilization efficiency improves simultaneously. This information helps feed producers prevent welfare risks in advance. “The results send a clear message for fish welfare: as selective breeding produces fish that grow faster and utilize feed more efficiently, feed development must keep pace,” Kause says. 

Selection programs already aim to improve production, product quality, and welfare traits in a balanced way. Improving nutrient digestibility—either by developing feeds or through breeding—supports the health and welfare of farmed fish populations and is also important for environmental sustainability. 

Reference: 
Kause A, Soares F, Silva TS. 2026. A biometrical traits-to-feed model integrating fish nutrition and genetic improvement of feed utilisation traits: Predicting changes in optimal feed phosphorus levels for selectively bred rainbow trout. Aquaculture 612: 743171. 
https://doi.org/10.1016/j.aquaculture.2025.743171