Introduction
The increasing inclusion of plantbased raw materials in aquafeeds reduces phosphorus (P) availability due to phytate, increasing dependency on inorganic P supplementation and contributing to environmental P discharge. Exogenous phytase is widely used to improve phytatebound phosphorus utilisation in fish. Beyond phosphorus release, phytase supplementation has been associated in terrestrial species with socalled extraphosphoric effects, potentially mediated by myoinositol liberation from phytate. Evidence for such mechanisms in salmonids, however, remains limited. This study evaluated the effects of a nextgeneration phytase on growth performance, nutrient digestibility, mineral status and myoinositol metabolism in rainbow trout.
Materials and Methods
An 84day feeding trial was conducted using rainbow trout (Oncorhynchus mykiss; initial body weight 69.5���g) housed in a recirculating aquaculture system. Fish were fed a plantproteinrich, moderately phosphorusdeficient diet (negative control; total phosphorus 0.65%) or a positive control diet supplemented with inorganic phosphorus (total phosphorus 0.87%). The negative control diet was supplemented with a nextgeneration phytase at 1,000 or 3,000���phytase units���kg-1. Each treatment consisted of three replicate tanks with 20 fish per tank. Growth performance, feed conversion ratio, apparent digestibility coefficients of phosphorus and zinc, plasma phosphorus and myoinositol concentrations, and mineral content of vertebrae and scales were determined at the end of the trial.
Results and Discussion
Phytase supplementation significantly improved feed conversion ratio compared with the negative control, reaching values comparable to the positive control. Apparent phosphorus digestibility increased significantly at both phytase inclusion levels, while zinc digestibility improved at the higher dose. Plasma phosphorus concentrations were restored to levels similar to those observed with inorganic phosphorus supplementation. Notably, phytase inclusion resulted in a marked and dosedependent increase in plasma myoinositol concentrations. Enhanced mineral deposition was observed in both vertebrae and scales, with scale mineral content closely reflecting vertebral mineralisation, supporting its use as a practical indicator of skeletal mineral status.
Conclusions
Supplementation of a moderately phosphorusdeficient trout diet with a nextgeneration phytase improved phosphorus utilisation, mineral status and feed efficiency, enabling a reduction in inorganic phosphorus supplementation. The pronounced increase in plasma myoinositol indicates that extraphosphoric effects of phytase are relevant in rainbow trout, extending observations previously described in terrestrial species. These findings support phytase supplementation as an effective nutritional strategy to improve performance and sustainability of plantbased salmonid feeds.
References
Lemos, D. & Tacon, A.G.J. (2017). Use of phytases in fish and shrimp feeds: a review. Reviews in Aquaculture, 9, 266–282.VerlhacTrichet, V. et���al. (2014). Phosphorus utilisation in rainbow trout fed phytasesupplemented diets. Journal of the World Aquaculture Society, 45, 367–379.