Feed conversion ratio explained

FCR measures how efficiently fish turn feed into body mass. It varies from 1.0 for tilapia to 2.0+ for trout, and it determines how much waste your plants receive.

Feed conversion ratio (FCR) is the amount of feed required to produce one unit of fish body weight gain. An FCR of 1.5 means 1.5 kg of feed produces 1 kg of fish growth. The rest leaves the fish as waste: ammonia from gills, solid waste from digestion, and uneaten feed that decomposes. In aquaponics, that waste is the plant fertilizer. FCR connects the fish side of the system to the plant side.

How FCR works

FCR = total feed given / total weight gained

If you feed 10 kg of pellets over a grow-out period and the fish gained 7 kg of body weight, FCR is 1.43. The remaining 3 kg became dissolved waste (ammonia, urea), solid waste (feces, uneaten feed), and metabolic heat.

Lower FCR means more efficient conversion. Fish are the most feed-efficient livestock on the planet. For comparison: broiler chickens have an FCR around 1.6-1.8, pigs around 2.8-3.5, and beef cattle 6-8. Fish beat all of them because they're cold-blooded (no energy wasted on body heat), live in a buoyant medium (no energy supporting their own weight), and excrete nitrogen as ammonia through their gills rather than converting it to urea or uric acid, which is metabolically cheaper.

FCR by species

These numbers assume commercial-quality pelleted feed at appropriate protein levels, fed at recommended daily rates:

Tilapia: 1.0-1.5. Tilapia are the most feed-efficient warm-water aquaculture species. Young tilapia (under 100g) can achieve FCR below 1.0 because they're converting feed to growth extremely efficiently. Market-size tilapia (500-800g) are closer to 1.4-1.5 as growth rate slows with size.

Channel catfish: 1.5-2.0. Slightly less efficient than tilapia, but tolerant of a wider range of feed quality and water conditions.

Trout (rainbow): 1.2-1.8. Trout require higher-protein feed (40-45% vs 28-32% for tilapia), which costs more per kilogram. Their FCR is good, but the feed cost per kg of fish gained is higher than tilapia because of the protein requirement.

Barramundi: 1.3-1.6. Efficient converter, but requires warm water (26-30°C) and high-protein feed.

Largemouth bass: 1.5-2.0. Predatory species have higher FCR partly because they're adapted to intermittent feeding on high-protein prey.

Bluegill: 1.5-2.0. Often used in temperate aquaponics systems. Slower growth rate than tilapia but tolerates lower temperatures.

Carp (common): 1.5-2.2. Hardy, cold-tolerant, efficient on lower-protein feed. Legal restrictions in some areas because they're invasive.

Goldfish: 2.0-3.0. Not bred for feed efficiency. They grow slowly and convert poorly compared to food-grade species. Useful in aquaponics for their cold tolerance and legality, but they produce less waste per kg of feed than tilapia.

Why FCR matters for aquaponics

The waste produced by fish feeding is the nutrient source for plants. Higher FCR means more waste per unit of fish growth, which sounds like a good thing for plants, but it also means more feed cost per unit of plant nutrients generated.

In practice, the relationship that matters is feed input to plant nutrients output. A tilapia system fed 100g of feed per day produces roughly 3-4g of nitrogen as ammonia (bacteria convert it to nitrate), plus phosphorus, potassium, and micronutrients from the solid waste. The fish-to-plant ratio calculator uses these conversion rates to estimate how many plants a given fish stocking level can support.

Fish with poor FCR waste more feed, which means more solid waste accumulating in the system, more frequent solids removal, and more strain on the biofilter. Efficient converters (tilapia, trout) keep the system cleaner.

What affects FCR

Feed quality. Pellets formulated for the species with appropriate protein levels (28-32% for tilapia, 40-45% for trout) produce better FCR than generic pond feed. Cheap feed with fillers results in higher FCR and more solid waste.

Water temperature. Fish are ectotherms. Their metabolism tracks water temperature. Below the optimal range, digestion slows, appetite drops, and feed efficiency worsens. Tilapia below 22°C eat less and convert poorly. Trout above 18°C are stressed and lose efficiency. Matching species to your water temperature is the single biggest factor in FCR.

Feeding rate. Overfeeding doesn't make fish grow faster. It reduces FCR because the excess feed isn't consumed or is consumed but poorly digested. Feed 1-3% of body weight per day (higher percentage for small fish, lower for large fish). Adjust based on appetite: if pellets are still floating or sinking uneaten after 5 minutes, you're feeding too much.

Fish size. FCR worsens as fish grow. A 50g tilapia fingerling has an FCR around 1.0. A 500g market-size tilapia is closer to 1.5. Growth rate slows as the fish approaches its maximum size, and more of the energy from feed goes to maintenance metabolism rather than new tissue.

Stocking density. Moderate density (15-25 kg/m^3 for tilapia) produces the best FCR. Too low and the fish aren't stimulated to feed competitively. Too high and stress, poor water quality, and disease reduce feed efficiency.

Understanding FCR helps you predict feed costs, waste production, and the plant-carrying capacity of your system. It's the link between the fish tank and the grow beds, and getting it right means both sides of the aquaponics loop perform better.

Improving FCR in practice

Several factors push FCR higher (worse) than the theoretical minimum for a species. Addressing them improves conversion efficiency:

Feed quality. Cheap feeds with lower protein content, more filler (grain starch, binders), and lower digestibility produce more waste per gram of growth. Premium feeds with 35-40% protein from fishmeal or insect meal and high digestibility cost more per kg but require less feed per kg of fish growth. The total feed cost per kg of fish produced is often similar or lower with premium feed.

Water temperature. Fish digest and metabolize feed most efficiently in their optimal temperature range. Tilapia at 28 C convert feed to growth more efficiently than tilapia at 22 C because enzymatic activity is higher. Outside the optimal range, more feed energy goes to maintaining body temperature and less goes to growth.

Feeding frequency and portion size. Multiple small feedings (2-3 per day) are more efficiently converted than one large feeding. The gut can only process a certain volume at once; excess food passes through partially digested, wasting nutrients. Feed what the fish consume in 3-5 minutes. If food is still floating after 5 minutes, you fed too much.

Stocking density. Overcrowded fish spend more energy on social stress and competition, leaving less energy for growth. Optimal stocking density for growth efficiency is lower than the maximum density the fish can survive.

The fish-to-plant ratio calculator uses FCR to estimate nutrient output from your fish stock.