TMAO: A Crucial Osmolyte for Fish Health in Winter

TMAO is emerging as a critical osmolyte in protecting fish health during challenging winter months, especially when traditional marine ingredients are substituted in aquafeeds.

By Marianne Nergård, a veterinarian specializing in fish health at Aker QRILL Company

Winter poses significant challenges for farmed salmon, as cold temperatures slow down their metabolism, impacting various physiological processes such as digestion, immune function, and recovery from skin and mucosal injuries. Compounding these issues, salmon reduce their drinking, making them more vulnerable during this period.

The Vital Role of Fluid Balance in Fish Health

Salmon inhabit seawater that is much saltier than their internal body fluids. Their skin acts as a barrier to prevent dehydration through osmosis. While this barrier helps, some water is still lost. To maintain fluid balance, salmon drink seawater and expel excess salts through kidneys, gills, and gastrointestinal secretions.

During winter, reduced drinking necessitates alternative strategies for fluid regulation. One such strategy involves increasing TMAO (Trimethylamine-N-oxide) levels. TMAO helps maintain fluid balance and protect proteins from denaturation during pressure changes, crucial for marine species like salmon.

Osmolytes: Key Components in Marine Raw Materials

“While not classified as essential nutrients, osmolytes play a crucial role in fish feed and marine ingredients,” stated Antony J Prabhu Philip at the IFFO Conference 2024. With 62.8 million salmon deaths in 2023, mostly due to diseases and osmoregulatory dysfunction, the importance of osmolytes cannot be overlooked.

Research indicates potential issues when marine ingredients, rich in valuable osmolytes including TMAO, are replaced by novel, often plant-based alternatives. Known benefits like omega-3 fatty acids (EPA and DHA) decline, suggesting other essential nutrients within marine ingredients may also be critical. The ongoing “OsmoProtect” project funded by the Research Council of Norway aims to uncover these secrets and enhance fish health through challenging periods.

Krill: A Rich Source of TMAO

Krill, a significant component of the salmon diet, rely on TMAO due to their daily migrations between deep and shallow waters. Protecting them from pressure-induced damage, TMAO is crucial for survival.

Krill meal is a valuable source of TMAO, preserving its natural high concentration. This process ensures the nutritional integrity of krill is maintained and remains effective when incorporated into fish feeds.

Krill supports fish health with its natural omega-3 fatty acids, antioxidants, vitamins, and minerals. As more is understood about osmolytes, TMAO’s role in enhancing fish health, especially in challenging conditions, becomes increasingly apparent.

Knowledge Is Power: Developing Diverse Raw Materials

Expanding the use of diverse raw materials in aquafeeds is essential for a growing aquaculture industry. Krill, rich in vital nutrients and TMAO, enhances fish health and could make introducing other novel ingredients more successful.

References:

• Bockus, A. B., Powell, M. S., Sealey, W. M., Zito, P., Kurerov, Y., Sbisa, T., Carrillo-Longoria, J. A., & Gaylord, T. G. Dietary trimethylamine oxide alters digestibility, intestinal histopathology, and gene expression in soy fed rainbow trout (Oncorhynchus mykiss). Aquaculture, 596, 741810 (2025). https://doi.org/10.1016/j.aquaculture.2024.741810
• Rørvik, Steien, Nordrum, Lein, and Thomassen. Urea in feeds for sea water farmed Atlantic salmon: effect on growth, carcass quality and outbreaks of winter ulcer. Aquaculture Nutrition, 7: 133-139 (2001). https://doi.org/10.1046/j.1365-2095.2001.00173.x
• Carroll, S., Kelsall, C., Hazon, N. and Eddy, F.B. Effect of temperature on the drinking rates of two species of flatfish, flounder and turbot. Journal of Fish Biology, 44: 1097-1099 (1994). https://doi.org/10.1111/j.1095-8649.1994.tb01280.x
• Gaten, E., Tarling, G., Dowse, H. et al. Is vertical migration in Antarctic krill (Euphausia superba) influenced by an underlying circadian rhythm? J Genet 87, 473–483 (2008). https://doi.org/10.1007/s12041-008-0070-y

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