1. Introduction
Olive flounder is a highvalue marine species cultivated extensively in East Asia. The industry seeks sustainable feed solutions that improve growth, health, and fillet quality while reducing reliance on fishmeal and synthetic additives. Marine macroalgae such as Sargassum fusiforme (a brown seaweed) and Ecklonia cava (a kelp species) are rich in polysaccharides, polyphenols, minerals, and essential fatty acids, making them promising candidates as functional feed ingredients.
This page reviews recent research on the inclusion of these seaweeds in olive flounder diets, focusing on growth performance, feed utilization, immune modulation, gut microbiota, and product quality.
2. Nutritional Profile of the Seaweeds
Both seaweeds provide a unique blend of macro and micronutrients:
- Protein: 1015% dry weight, containing all essential amino acids.
- Polysaccharides: Fucoidan (rich in sulfated fucose) in S. fusiforme; alginate and laminarin in E. cava.
- Polyphenols: Phlorotannins with antioxidant activity.
- Minerals: Iodine, calcium, magnesium, and trace elements.
- Fatty acids: Small amounts of EPA/DHA and a high 3/6 ratio.
These components have documented benefits for fish metabolism and disease resistance.
3. Experimental Design Overview
Typical trials last 812weeks and employ a completely randomized design with 35 dietary treatments:
| Diet | Seaweed inclusion | Form | Replicates |
|---|---|---|---|
| Control | 0% | None | 4 |
| SFusiforme 1 | 1% | Powder | 4 |
| SFusiforme 3 | 3% | Powder | 4 |
| ECava 1 | 1% | Powder | 4 |
| ECava 2 | 2% | Powder | 4 |
Key parameters measured include weight gain, specific growth rate (SGR), feed conversion ratio (FCR), survival, plasma biochemistry, antioxidant enzymes, and histological examination of the intestine and liver.
4. Growth Performance and Feed Utilization
Across multiple studies, low to moderate inclusion (13%) of either seaweed has produced the following trends:
- Weight gain: 512% higher than control when 12% of S. fusiforme is used.
- Specific growth rate: Increment of 0.150.30%day.
- Feed conversion ratio: Improved by 0.050.10 points, indicating better nutrient utilization.
- Higher inclusion (>4%) sometimes depresses performance, likely due to reduced palatability or excess fiber.
5. Immune and Antioxidant Responses
Both seaweeds enhance innate immunity:
- Increase in plasma lysozyme activity (up to 30%).
- Higher complement (C3) levels.
- Elevated respiratory burst activity of headkidney leukocytes.
- Antioxidant enzymessuperoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)show 1.21.5fold rises.
Challenge tests with Vibrio harveyi or Streptococcus iniae consistently report lower mortality (1525% reduction) in seaweedfed groups.
6. Effects on Gut Morphology and Microbiota
Histological examinations reveal:
- Longer villi and increased microvilli surface, improving nutrient absorption.
- Thicker mucosal layer, offering better barrier function.
Highthroughput sequencing shows a shift toward beneficial bacteria such as Lactobacillus and Bacillus, and a reduction in potential pathogens (Aeromonas spp.). The prebiotic role of fucoidan and alginate is implicated.
7. Influence on Flesh Quality
Seaweed enrichment influences fillet composition:
- Higher total omega3 fatty acids, especially EPA and DHA.
- Reduced lipid oxidation (lower TBARS values) during chilled storage.
- Slight increase in ash and mineral content, notably iodine.
Sensorial panels generally rate seaweedfed fillets as more fresh and milder in flavor, with no offnotes detected at 2% inclusion.
8. Economic and Environmental Considerations
Both seaweeds are cultivated locally in coastal China, Korea, and Japan, reducing transport costs. Their growth does not compete with arable land and sequesters carbon, supporting sustainability goals. A costbenefit analysis indicates that a 2% inclusion can increase net profit by 35% owing to better feed efficiency and lower diseaserelated losses.
9. Recommendations for Feed Formulation
- Incorporate 1.52% dried S. fusiforme or E. cava powder into a basal diet formulated for 45% protein and 12% lipid.
- Combine seaweed with a modest amount of probiotics to synergistically enhance gut health.
- Monitor pellet durability; excessive seaweed can increase water solubility.
- Maintain storage of seaweed powder at <10C and <65% RH to preserve bioactive compounds.
Further research should explore enzymatic hydrolysis of seaweed polysaccharides to improve digestibility and examine longterm effects on reproductive performance.
