A new project will study innovative feed technology methods to improve zinc availability in salmon feeds.
A new Norwegian project: Technical optimisation of zinc supplementation strategies for enhancing zinc availability, improving fish health and minimising environmental impact (TOP-Zinc), will study innovative methods in feed technology to improve zinc availability in salmon feeds.
Zinc use in salmon feeds
Zinc is essential for maintaining good barrier function in the skin, intestine and gills, osmoregulatory functions, wound healing, bone health, and the functioning of several proteins at the molecular level. The increasing use of plant-based ingredients in Atlantic salmon feeds has reduced zinc levels and availability in the feed, thereby increasing the dietary levels needed to meet the requirement.
The zinc requirement for Atlantic salmon parr was estimated to range between 37-60 mg/kg. However, in commercial salmon feeds with high inclusion of plant ingredients, total dietary zinc levels of 140-160 mg/kg feed are required for freshwater parr and 200-240 mg/kg for seawater post-smolts. Further, improved health and welfare effects on cataracts, wound healing and barrier functions have been demonstrated at increased total zinc levels up to 300 mg/kg.
Increasing total zinc levels for health benefits in feed reduces availability and retention, ultimately increasing zinc emissions. Estimates from recent Atlantic salmon production data indicate that only 15% of the zinc fed in the diet is retained in the fish, with the rest lost to the environment. Increased zinc emissions can pose a risk of toxicity to marine organisms and promote the development of multi-resistant bacteria in the marine environment.
Legislation
Accordingly, to prevent negative environmental impact, the European Commission (EC) legislation on animal feeds has set a maximum limit of 180 mg Zn/kg in salmon feeds, implemented in 2016 following the EFSA feed additive panel recommendation, reducing it from the former limit of 200 mg/kg feed. Further reduction to 150 mg/kg has been suggested by EFSA, and an evaluation of the environmental impact is currently underway; the outcome could result in a further reduction of the zinc limit in salmon feeds.
The challenge is to safeguard the health of fish and the environment
Changes in feed composition, increased dietary requirements, declining fish health and welfare, feed legislative limitations, and potential negative environmental impacts have increased the need for improved utilization of dietary zinc. Therefore, strategies to increase salmon's zinc status without increasing total zinc levels in feeds should be prioritised, e.g., by improving dietary zinc availability.
TekSink - Nofima technology
Research efforts thus far to improve zinc availability in salmon feeds have focused on alternative zinc sources, while overlooking the role and impact of feed processing technology on zinc availability. Post-extrusion addition of heat-sensitive feed additives is known; however, it has not been tested for minerals. The concept of top-coating zinc after extrusion to salmon feeds was developed and tested in TekSink (a Nofima innovation project) with highly encouraging results. Top-coated feeds had 30% (10-15 percentage points) higher apparent availability of zinc than conventional premix supplemented feeds and consequently less zinc in faecal emissions.
The TOP-Zinc project
TOP-Zinc is built on the findings from TekSink. We aim to further optimise the coating technology, evaluate long-term effects on zinc deposition in fish/tissues, health effects, and model the potential environmental benefits from lower zinc emissions through novel top-coating technologies.
"In TOP-Zinc, we take a holistic one-health approach to the zinc challenge facing salmon farming and society at large. We propose to optimise novel top-coating technologies for zinc to improve dietary zinc availability and status in salmon post-smolts. Further, the impact on fish health and potential environmental risks and dispersion modelling will be studied, culminating in a long-term validation trial in sea cages," Antony J Prabhu Philip, Project Leader and Senior Scientist at Nofima explained.
The project is funded by FHF - Norwegian Seafood Research Fund and led by Nofima in cooperation with MOWI Feed, Akvaplan-NIVA, IMR, NIVA and Huvepharma.
