Background
Aquaculture production in Norway has grown immensely in terms of quantity and value compared to other developed nations in the last decade [1]. This tremendous growth depends directly on fish feed and fish health along with other best management practices contributing to fish welfare. Also, chemical compounds used in aquaculture have increased concomitantly and have been used widely for feed and health management. However, a further increase in aquaculture should occur in a sustainable manner without harming the environment or posing a threat to biodiversity and consumer safety. This can be achieved by replacing synthetic compounds, such as the antioxidant (eg. ethoxyquin) and antibiotics with natural, efficient, non-toxic alternatives identified among bioactive compounds from plant-based sources.
In the present study, we have screened 339 extracts from various plant parts of different varieties of 9 herbs (Artemisia annua, Achillea millefolium, Rhodiola rosea, Mentha piperita, Oreganum vulgare, Humulus lupulus, Leuzea carthamoides, Rosmarinus officinalis, Portulaca oleracea) grown in Norway for antioxidant (AO) property and antimicrobial (AM) activity against the Atlantic salmon pathogens, Aliivibrio spp. Moritella spp. and Tenacibaculum spp. The selected pathogens mainly cause ulcerative disease in salmonids, which is among the main reasons for reduced welfare and increased mortality in on-growing salmon in Norway [2]. Furthermore, intensive farming conditions can lead to increased bacterial and parasitic diseases, which in turn lead to huge losses in farmed fish [3]. The objective of this study was first to assess the bioactivity of herbs produced in Norway based on evaluation and comparison of the antioxidant and antimicrobial properties of the plant extracts, and second to select the potential phytogenic compound or whole extract that revealed the best properties to be included as an additive in fish feed.
Methods
The herbs were grown in open field or in green house in Norway. The extracts were prepared by successive extraction with three different extraction solvents: dichloromethane (DCM), ethanol and water. The commonly used antioxidant methods, ABTS and Folin Ciocalteu, were used to measure the antioxidant capacity of the extracts. Agar disc diffusion assay was used to test the susceptibility of bacterial strains to extracts. Tested strains were grown in two different growth media mimicking external sea water environment and internal tissue of salmon. Extracts that revealed the highest AO and AM activities were tested in salmon skin cells in vitro (keratocytes) and cytotoxic activity assessed.
Results
Among 9 herbs studied, Origanum vulgare, Artemisia annua, Humulus lupulus and Rosmarinus officinalis had an interesting effect on AO and AM properties. The DCM extracts of the selected plants revealed particular inhibition of the salmon pathogens. However, the extracts were found to have a lethal effect on salmon keratocytes in a dose-dependent manner. Therefore, further studies are required to fully understand its suitability as a natural additive in fish feed and its impact on fish. Our study also indicated that the bioactivity may be caused by a combination of compounds present in the whole extracts, which cannot be obtained using purified individual compounds from the same plant.
Conclusion
The results from screening of Nordic herb extracts using salmon pathogens and salmon keratocytes show a great potential in selecting a suitable herbal extract/compound that can be used in the salmon farming. Moreover, plant extracts or individual bioactive compounds may have additional benefits other than AO and AM activities, but might also show adverse effects.
Reference
[1] Garlock, T., Asche, F., Anderson, J., Bjørndal, T., Kumar, G., Lorenzen, K., Ropicki, A., Smith, M. D., Tveterås, R. (2020). A global blue revolution: aquaculture growth across regions, species, and countries. Reviews in Fisheries Science & Aquaculture. 28(1), 107-16.
[2] www.vetinst.no:fiskehelserapporten , pg 86.
[3] Torrissen, O., Jones, S., Asche, F., Guttormsen, A., Skilbrei, O. T., Nilsen, F., Horsberg, T.E., Jackson, D. (2013). Salmon lice–impact on wild salmonids and salmon aquaculture. Journal of fish diseases. 36(3), 171-94.