The Atlantic salmon (Salmo salar) faces a unique challenge: transitioning from freshwater to saltwater environments. This change requires significant osmoregulation, the ability to maintain a stable internal balance of water and minerals, and a surprising secret weapon plays a key role: dietary minerals!
Salmon raised on a mineral-rich diet show greater tolerance and survival when entering saltwater. This is because their bodies undergo a crucial transformation, remodeling their gills to cope with the drastic change in salt concentration.
New research conducted by scientists from Wageningen University and Research and the Institute of Marine Research sheds light on how the mineral composition of salmon diet (dietary electrolyte balance, dEB) influences their ability to regulate water and salt levels in their digestive system. This plays a crucial role in their successful adaptation to saltwater environments.
The importance of minerals
Studies demonstrate that salmon benefit from a mineral-rich diet when entering seawater. This helps them tolerate the change and survive better. The key lies in the gill remodeling induced by salt. As salmon encounter higher salinity, their gills undergo structural and molecular changes. This allows them to regulate ions more effectively, a crucial part of adapting to saltwater.
The role of the intestine in osmoregulation
While gill function is well known, recent research highlights the importance of the intestine in osmoregulation (maintaining water balance). When salmon transition from freshwater to saltwater, their intestines need to adapt to different osmotic conditions. Inadequate preparation in the intestine can negatively affect their performance in seawater.
The challenge of modern aquaculture feeds
Modern fish feed formulations often rely on plant-based ingredients rather than traditional fish meal and oil. While this offers benefits, it can also alter the electrolyte balance of the diet (dEB). dEB refers to the balance between essential minerals such as potassium and sodium compared to chloride in the feed.
Scientists are now focusing on dietary electrolyte balance (dEB). This refers to the balance between different minerals (sodium, potassium, chloride) in the feed. It turns out that dEB can significantly influence how the salmon’s digestive system regulates water and salt levels.
Knowledge gap
Despite the importance of dEB, our understanding of its role in regulating osmoregulation within the salmon’s digestive system is limited. Most research on dEB in fish has focused on freshwater environments.
A new hypothesis
This research proposes that the effect of dEB on osmoregulation mechanisms in salmon differs between freshwater and saltwater. This suggests that feed formulations should be adapted to support optimal adaptation during the transition.
To investigate this hypothesis, researchers are studying the impact of contrasting dEB levels and water salinity on water, ion, and nutrient flows within the digestive tract of Atlantic salmon smolts.
Scientists investigated how contrasting dEB levels (-100 vs. 500 mEq kg-1 MS) and water salinity (freshwater versus seawater) affect water, ion, and nutrient movement within the digestive tract of Atlantic salmon smolts.
Understanding this relationship is critical to optimizing the health and survival of salmon during this crucial life stage. By adapting feed formulations to support intestinal function alongside gill adaptation, researchers hope to improve the success rate of Atlantic salmon transitioning to seawater in aquaculture environments.
Key findings
- Diet affects stomach acidity: Low dEB diets resulted in a more acidic stomach environment.
- Primary impact of salinity: Seawater significantly increased the pH of the overall digestive system and the concentration of dissolved particles.
- Interaction between dEB and salinity: The interaction between dEB and salinity produced the most fascinating results:
- Freshwater: Fish with high dEB diets absorbed more water in the stomach.
- Seawater: The picture flipped! Salmon with low dEB diets showed much higher water uptake in the stomach and upper intestine. They also drank much more seawater (50% more!).
Conclusion
This research suggests that dEB plays a crucial role in how salmon manage water balance during the critical transition to seawater. A low dEB diet appears to trigger a compensatory response in seawater, encouraging salmon to drink more and absorb water more efficiently in the digestive tract.
Understanding how dEB influences water balance in different environments could revolutionize salmon aquaculture. By adjusting dEB in feed according to the salmon’s life stage (freshwater or seawater), we can potentially:
- Improve adaptation success: Minimize stress and health risks associated with transitioning to seawater.
- Optimize growth and survival: Ensure proper water balance for optimal health and growth.
- Promote sustainable aquaculture: Develop more efficient and responsible aquaculture practices.
The study was funded by the Aquaculture and Fisheries group (AFI) at Wageningen University and Research (Netherlands) and the Institute of Marine Research (IMR) in Bergen (Norway).
Contact
Antony J. Prabhu Philip
Nutrition and Feed technology group, Nofima
5141 Bergen, Norway
Email: antony.philip@nofima.no
Reference (open access)
Ciavoni, E., Schrama, J. W., Sæle, Ø., & Philip, A. J. P. (2024). Dietary electrolyte imbalance alters drinking rate and gastrointestinal tract water fluxes of Atlantic salmon (Salmo salar) smolt in seawater. Aquaculture, 585, 740685. https://doi.org/10.1016/j.aquaculture.2024.740685
