Sea lice infection in wild Atlantic salmon has been happening for centuries, and salmon farmers have managed them in commercial production settings for many years. However, the need for increased production, changes in nutrition associated with poor physiological status and resistance to chemical treatments have made sea lice a major production challenge for the salmon industry. Sea lice are estimated to cost the global aquaculture industry about $1 billion USD per year.
Sea lice are very aggressive, damaging ectoparasites that feed on fish-skin mucus, blood and tissue, resulting in severe skin lesions which make fish more vulnerable to infection and possible mortality. Various control measures have been tested while others are being investigated, in combination or alone, including breeding programs that increase disease resistance, closed production systems, and biological treatments such as farming salmonids together with cleaner fish. A vaccine against sea lice has been developed, but its efficacy is still to be confirmed under commercial conditions.
Given the global trend to eliminate antibiotic use and other drugs that may result in pathogen resistance, development of functional feeds is gaining popularity. This strategy is a more natural, nutritious solution when combined with better production management to defeat sea lice challenges. Interestingly, trace minerals play key physiological roles in modulation of immune response and overall disease prevention. Among them, zinc is essential for growth and development of all animal species studied and may be limiting in practical diets of Atlantic salmon.
Zinc is known to exert beneﬁcial effects beyond growth, namely in modulating immune response and resistance to disease (Gammoh and Rink, 2017). Zinc deficiency has long been associated with delayed wound healing and erosion of ﬁns and skin (Ogino and Yang 1979, Hughes 1985). Impaired skin integrity is common in farmed Atlantic salmon, especially during stressful conditions, namely during the smoltification and transfer to seawater periods (Karlsen et al., 2018), which may increase susceptibility to infection, including sea lice.
To determine effective methods of reducing this susceptibility, Zinpro conducted a research study to evaluate the impact of supplemental zinc sources and levels on growth performance and resistance to sea lice in Atlantic salmon.
About the Study
The trace mineral research study was conducted at the Chile Quillaipe Aquaculture Center, Fundación Chile, Puerto Montt, Chile. Atlantic salmon of approximately 106 g (nearly 4 oz) were distributed into tanks (45 fish per tank) and fed for 60 days on a diet (See Table 1) produced from a basal diet containing 20% fish meal and supplemented with:
- Group 1: 120 ppm zinc from inorganic zinc as zinc sulfate
- Group 2: a combination of 60 ppm of zinc sulfate and 60 ppm of zinc from Availa® Zn
- Group 3: 60 ppm of Availa-Zn alone
At the end of the 60-day feeding period, a strain of Caligus rogercresseyi (Chile’s most problematic sea louse species) was used to topically infest tanks separately. Skin score was evaluated at the end of the 20-day challenge period, when Caligus reached adult stage.
What Results Did We See?
At the end of the 60-day feeding period, final weight, specific growth rate (SGR) and feed conversion ratio (FCR) differed between treatments.
- Availa-Zn supplemented at 60 ppm resulted in numerically higher final body weight and SGR when compared to full rate inorganic zinc (ZnSO4, 120 ppm).
Additionally, final body weight was significantly higher with 60 ppm of Availa-Zn than its 50:50 combination with zinc sulfate. FCR was significantly improved when zinc sulfate was replaced with a 50:50 combination of zinc sulfate and Availa-Zn (60 ppm zinc sulfate and 60 ppm Availa-Zn).
- 60 ppm of Availa-Zn resulted in a similar FCR as seen in diets supplemented with 120 ppm of zinc sulfate. Performance results indicate a better bioavailability of Availa-Zn compared to zinc sulfate.
- Availa-Zn supplemented at 60 ppm was significantly more efficient in reducing the number of adult stage sea lice when compared to zinc sulfate.
A skin damage score evaluation indicated fish fed 60 ppm of Availa-Zn had a more desirable level of skin integrity than fish fed either diet containing zinc sulfate, although the difference was not significant (P > 0.05).
Results support the role zinc plays in wound healing in fish (Jensen et al., 2015; Gerd et al., 2018; Ogino and Yang 1979; Hughes 1985).
Our trace mineral research study indicates that supplementation with zinc amino acid complex from Availa-Zn is more efficient than inorganic zinc forms in improving salmon growth performance and increasing its resistance to sea lice, which may help producers reduce the costs associated with sea lice treatment. Further studies are needed to investigate the exact mechanisms played by zinc amino acid complex alone or in combination with other potentially limiting trace minerals in modulating immune response and resistance to disease of salmonids in practical salmon diets. In addition, dietary trace mineral recommendations for salmon may need revision, as the total level and availability of essential trace minerals are becoming severely limited through progressive replacement of fish meal with plant proteins.
You can click here to read the full research study.
- Gammoh, N.Z. & L. Rink. 2017. Zinc in Infection and Inﬂammation. Nutrients 2017, 9, 624.
- Gerd, M.B., E., Sundh, H., Østbye, T.-K., Sveen, L., Sundell, K., A. & B. Ruyter 2018. Omega-3 fatty acids and zinc affect robustness and function of barrier tissues in Atlantic salmon Salmo salar. Book of Abstracts Aqua2018, August 25–29, 2018, Montpellier, France.
- Hughes, S.G. 1985. Nutritional eye diseases in salmonids: a review. Prog. Fish Cult., 47:81–85
- Jensen, L.B., Wahli, T., McGurk, C., Eriksen, T.B., A. Obach, Waagbø, R., Handler, A., & C. Tafalla. 2015. Effect of temperature and diet on wound healing in Atlantic salmon (Salmo salar L.). Fish Physiol. Biochem., 41:1527–1543.
- Karlsen, C., Ytteborg, E., Timmerhaus, G., Høst, V., Handeland, S., Jørgensen, S.M. & A. Krasnov. 2018. Atlantic salmon skin barrier functions gradually enhance after seawater Figure 3. Average Caligus abundance in salmon fed diets varying in zinc source and level for 60 days and challenged with Caligus for 20 days.