DEFINING NUTRITIONAL REQUIREMENTS OF RAINBOW TROUT FOR THE FUTURE BY REVISITING THE PAST

Introduction

Nutritional requirements of fish are classically determined through dose-response trials under controlled settings and generally defined as the nutrient amounts required to cover basal/maintenance costs plus whatever required to achieve a certain growth (or nutrient retention). Despite the cumulative knowledge acquired on the nutritional requirements of fish (see, e.g., NRC 2011), there is the possibility that, as fish traits change due to (e.g.) genetic improvement programs, fish nutritional requirements may be affected, eliciting the need to revise nutritional guidelines accordingly.

It is challenging to quantify the genetic improvement in feed utilisation and growth efficiency across years. As a complementary approach, it is possible to analyse annual trends of data on fish traits, with a “year effect” being used as a proxy of the “genetic effect” on fish traits. In particular, we assume that, if genetic improvement has affected fish traits in a meaningful way, one should be able to observe these changes along time, as genetic improvements accumulate.

We performed a meta-analysis of rainbow trout growth trials (from both published peer-reviewed sources and in-house trials) covering the 1979-2019 time period to determine: a) whether there is evidence of changes in important fish traits (e.g. feed intake rates, maximum growth capacity, growth efficiency) along time; b) what possible changes in nutritional requirements should we expect as a consequence of these changes in fish traits.

Materials and methods

For the analysis, information on a total of 24 rainbow trout growth trials (representing 172 tanks/cages/units) was collected from either scientific literature or in-house trials of SPAROS Lda, covering the time span between 1979 and 2019. This effort focused on sources which provided explicit information on growth, water temperature, feed intake and diet composition. When available, information on whole body composition was also collected from each source. For each period between measurements, aggregate estimates of relevant factors and responses (year, average temperature, average body weight, average growth, average feed intake, average digestible energy intake, average digestible protein intake, average energy gain, average protein gain) were calculated, resulting in a dataset with 534 points used for downstream analysis. Meta-analysis was performed by modelling relevant fish traits as a function of the factors (or transformed factors) using least-squares and quantile linear regression. Additional information about time-dependent trends of important traits (growth, FCR, fillet yield, viscerosomatic index) in rainbow trout, provided by Luke and obtained using an independent genetic trend analysis of the Finnish national breeding program, was also used to support this analysis (Kause et al. 2021).

Results

The results do not show strong evidence of year effects (and, thus, potential genetic improvement effects) on feed intake levels or maximum growth rates. Instead, there is a progressive improvement over time in traits related to growth efficiency (i.e. minimum FCR, maximum protein and energy retention efficiencies).

This improvement in growth efficiency is consistent with the independent estimates of the genetic improvement of rainbow trout traits in Luke’s breeding program (red points/lines in Figure 1, Kause et al. 2021), which have been estimated avoiding the confounding effects of nutritional factors.

Discussion and Conclusion

Overall, the results support the notion that, at least in rainbow trout, genetic improvements have led to cumulative changes in traits that can affect fish nutritional requirements. We discuss the possible impacts of these trait changes on nutritional requirements and optimal feed formulations.

Acknowledgements

This work was funded by the EU Horizon 2020 AquaIMPACT project (Genomic and nutritional innovations for genetically superior farmed fish to improve efficiency in European aquaculture; 818367).

References

National Research Council. (2011). Nutrient requirements of fish and shrimp. National academies press.

Kause A., Koskinen H., Nousiainen A. (2021). Role of selective breeding in the improvement of feed efficiency at rainbow trout farms. Aquaculture Europe 2020. Book of Abstracts.