I. Effects of High Temperature on Koi Physiology

1. Decreased Digestive Enzyme Activity
   When water temperature exceeds 28°C, the activity of digestive enzymes such as proteases decreases by 30%–50%. This reduces protein breakdown efficiency, and incompletely digested proteins increase intestinal burden.
2. Exacerbated Metabolic Heat Production
   The digestion and absorption of protein generate "diet-induced thermogenesis." Since koi have limited heat dissipation capacity in high temperatures, excessive protein intake can easily lead to internal heat accumulation, triggering metabolic stress.
3. Ammonia Metabolic Stress
   Digesting approximately 1 gram of protein produces about 0.16 grams of ammonia. In high temperatures, koi kidney function in excreting ammonia weakens, while reduced dissolved oxygen in water inhibits nitrification, doubling the toxicity of ammonia.
   

II. Specific Protein Adjustment Plan

1. Phased Control of Protein Content
   • Water temperature 28–30°C: Reduce protein content to 28–32% (a decrease of 5–8% from the original feed).
   • Water temperature 30–33°C: Reduce protein content to 25–28%.
   • Water temperature >33°C: Reduce protein content to 22–25%, paired with easily digestible carbohydrates.
2. Selection of High-Quality Protein Sources
   Prioritize animal-based proteins such as fish meal and shrimp meal (digestibility >85%), and reduce the proportion of plant-based proteins like soybean meal (<15%) to minimize metabolic waste.
3. Optimization of Amino Acid Balance
   Add limiting amino acids (e.g., lysine, methionine) so that essential amino acid requirements are still met even with reduced total protein.
   

III. Supporting Nutritional Adjustment Strategies

1. Shift in Energy Sources
   Increase the proportion of fat for energy to 8–12% (using easily digestible fats like fish oil and phospholipids), and adjust the carbohydrate proportion to 35–40% (using pre-gelatinized starch).
2. Functional Additives
   • Add vitamin C (500 mg/kg of feed) to enhance heat stress resistance.
   • Supplement with probiotics (Bacillus subtilis) and digestive enzymes (protease preparations).
   • Use bile acid preparations (0.1%) to promote fat emulsification and absorption.
3. Feeding Method Adjustments
   • Switch to 2–3 daily feedings of low-protein feed.
   • Reduce single feeding amounts by 30%, adopting a "small, frequent meals" approach.
   • Feed during cooler periods, such as at night.

IV. Coordinated Water Quality Management

1. Enhanced Ammonia Monitoring
   Test ammonia concentration daily. If it exceeds 0.2 mg/L, immediately:
   • Increase aeration by 30%.
   • Add zeolite powder (100 g per ton of water).
   • Supplement with nitrifying bacteria preparations.
2. Physical Cooling Measures
   • Install shade nets (lowering water temperature by 2–3°C).
   • Inject cool groundwater in the afternoon (temperature difference ≤2°C).
   • Install surface wave makers to promote evaporative cooling.
     

V. Special Considerations

1. Transition Period Management
   Protein adjustments should be implemented gradually over 3–5 days. Sudden formula changes may cause feed refusal or digestive disorders.
2. Differentiated Treatment Based on Physique
   • Young-of-the-year koi can maintain 30% protein to ensure growth foundation.
   • Older koi (>8 years) should have protein reduced to below 25%.
   • Broodstock post-spawning require additional vitamin E (200 mg/kg) to promote recovery.
3. Observation and Warning Indicators
   Further reduce protein content if the following signs appear:
   • Feces are white and mucus-like.
   • Increased mucus secretion on the body surface.
   • Frequent surface gasping in the afternoon.