A shift toward plant-heavy diets in aquaculture may be leaving rainbow trout looking a bit “washed out.” New research from Chile reveals that high levels of plant protein significantly hinder a fish’s ability to absorb astaxanthin, the key pigment responsible for that signature pink-orange hue.
Why the Color is Fading
The study, led by Dr. Alejandro Villasante, suggests the issue isn’t just the lack of fishmeal – it’s a metabolic bottleneck. Here is the breakdown:
- The Cholesterol Connection: High plant protein diets (at 80% fishmeal replacement) lower the concentration of cholesterol and bile acids in the fish.
- Absorption Failure: Without enough bile to emulsify fats, the fish cannot effectively absorb fat-soluble compounds like astaxanthin, especially in its “diesterified” form.
- The Result: Significant drops in pigmentation scores across both dorsal and abdominal areas, along with lower levels of the pigment in the plasma and fillets.
Beyond Just Aesthetics
The researchers found that the impact of high plant inclusion extends past skin deep, affecting the overall health and efficiency of the fish:
Metric |
Impact of 80% Plant Protein |
| Growth | Lower final weight and reduced weight gain. |
| Efficiency | Worse feed conversion ratios. |
| Digestion | Reduced lipid digestibility and altered gut microbiota. |
The Silver Lining: Moderate plant protein inclusion did not significantly compromise the color, suggesting there is a “sweet spot” for sustainable feeding without losing quality.
What’s Next?
The team at the Universidad de Chile and Universidad de las Américas isn’t stopping here. Their next steps include:
- Reversal Trials: Testing if adding cholesterol and bile salts to plant-based feed can restore the pink color.
- Mechanistic Validation: Investigating the specific “FXR transporter” pathway involved in bile metabolism.
- Broadening the Scope: Confirming if these findings hold true for other salmonids and identifying specific anti-nutritional factors in plant ingredients.