What feed conversion ratio measures and the levers that move it

Feed conversion ratio is the figure every farm reports and few farms calculate the same way. One site quotes 1.1, another 1.3, and the difference can come from the feed, the fish, the water or the arithmetic behind the number.

That matters because feed is the largest cost line in most fed aquaculture. FAO puts feed at 40 to 60% of production costs in semi-intensive and intensive systems, and Mississippi State researchers put it at about 50 to 60% of variable operating costs in US commercial catfish culture. A drift of 0.1 or 0.2 in FCR is a real cost change, provided the number is measured consistently.

Biological and economic FCR

FCR is feed fed divided by live-weight gain over a defined interval. It uses as-fed feed weight and live fish weight, which suits farm management because feed is bought as fed and fish are sold live. Some trials report feed efficiency, the inverse, so check which one a table shows.

The two common versions differ in what counts as gain and as feed. FAO describes biological FCR as the net feed used to produce 1 kg of fish, while economic FCR takes in all the feed used, so feed losses and mortality are included. In practice:

• Biological FCR divides feed consumed by all weight gained, including gain carried by fish that later died.
• Economic FCR divides all feed fed or purchased by net weight gain in fish that were harvested or are still in inventory. Losses to disease, predation, handling, grading, theft and escapes all load it.

Economic FCR is never lower than biological FCR, and FAO stresses that it reflects the whole farm management system, not the diet alone. A widening gap points to mortality or feed waste; a rising biological FCR points to the fish, the water or the feed.

Why each 0.1 of FCR matters

Feed cost per kilogram of gain is FCR multiplied by feed price per kilogram. At an example price of CAD 1,900 per tonne, 0.1 of FCR costs CAD 0.19 per kg of gain. On an example farm adding 1,000 t of biomass a year, a 0.1 rise in FCR means 100 t more feed, about CAD 190,000 at that price.

The chart below plots feed cost per kilogram of gain as FCR rises from 1.0 to 2.0 at three example feed prices: CAD 1,250, 1,900 and 2,400 per tonne. The lines are straight and fan out as price rises, so the same slip costs more on an expensive diet: each 0.1 of FCR adds between CAD 0.125 and 0.24 per kg of gain.

This argues against buying feed on price per tonne alone. FAO's trout farming manual notes that a cheaper feed with a higher FCR can prove more expensive than a costly one with an outstandingly low FCR. FAO also cites a bio-economic model of a 600 t per year recirculating farm for Japanese meagre, in which FCR rising from 1.0 to 1.6 lifted feed from 36.2 to 46.1% of production costs and raised annual operating costs by 22.9%.

Diet, ration and feeding frequency

Diet sets the ceiling. Mississippi State researchers attribute the typical 1.0 to 1.2 FCR of Atlantic salmon and rainbow trout largely to feeds with more protein, much more fat and less fibre than catfish feeds. FeedOptima's reference ranges put typical grower diets at about 19 to 23 MJ/kg digestible energy for salmonids and 12 to 15 MJ/kg for tilapia and carp, one reason their FCRs are not directly comparable.

Ration is the lever the farm controls every day. Overfeeding wastes feed and raises FCR sharply; mild underfeeding improves conversion but usually lowers yield; severe restriction pushes FCR up again because most of the feed goes to maintenance rather than growth. The useful target is the ration that maximizes margin, not growth or FCR alone.

FAO's trout manual recommends splitting the daily ration into 2 to 24 portions, feeding younger fish more often and increasing frequency as water warms. Extra meals help only if each one stops when the fish stop eating.

Temperature, oxygen, fish and pellets

In a controlled study on Atlantic salmon post-smolts, the optimum temperature for feed conversion efficiency fell as fish grew, from 13.4 C at 70 to 150 g to 11.0 C at 150 to 300 g, while the optimum for growth rose from 12.8 to 14.0 C. For larger fish, the best temperature for growth is not the best one for conversion.

Oxygen interacts with temperature. In post-smolts of about 0.3 to 0.5 kg, the dissolved oxygen needed to sustain maximum feed intake rose from 42% to 76% of air saturation between 7 and 19 C. FeedOptima's reference ranges treat 7.5 mg/L as optimal for rainbow trout and 5.5 mg/L as the level below which intake and welfare are limited.

Smaller fish convert better because their gain is mostly lean tissue, which holds less than half the energy of body fat, so FCR climbs through a cycle. Sick fish eat poorly, and in US catfish mortality may be the major cause of FCRs above 2:1.

Pellet quality decides how much of what is fed can be eaten. FAO notes that better milling and binding reduce fines, improve pellet hardness and water stability, and improve economic FCR. A Global Seafood Alliance article puts uneaten feed at about 2 to 5% of dry matter for fish feeds and 10 to 15% for shrimp feeds, and in cages the whole nutrient load from feed passes through the net.

Measuring FCR and comparing it fairly

The feed side is the easier half: opening stock plus deliveries minus closing stock, per unit and period. FAO's trout manual recommends a register of opening and closing fish numbers and weights, mortalities and feed used for each period, with dead fish counted and weighed daily.

Biomass is the weak half. It is usually average sampled weight multiplied by the number stocked minus counted mortalities, and a Global Seafood Alliance review notes that manual sampling carries an inherent inaccuracy of 15 to 25%. Short periods magnify that error: as an illustration, a tank that goes from 10.0 t to 11.0 t on 1.2 t of feed shows an FCR of 1.2, but if the closing biomass is really 10.8 t, the true figure is 1.5. Biomass estimated from feed fed and an expected FCR, which the review also flags as imprecise, just returns the assumed FCR.

Comparisons also fail across species and sizes. The table gives FeedOptima's typical whole-cycle FCR values beside typical grower-diet energy, as planning baselines rather than stage targets.

Species	Typical FCR	Typical grower diet DE, MJ/kg
Rainbow trout	1.1	19 to 22
Atlantic salmon	1.2	20.5 to 23
Barramundi	1.4	16 to 18.5
Whiteleg shrimp	1.5	13 to 15
Nile tilapia	1.6	12 to 14
Common carp	1.6	13 to 15
European sea bass	1.8	18 to 20
Gilthead sea bream	1.9	17 to 19.5

Periods with mortality need care: FAO notes that losing larger fish raises both feed loss and economic FCR. In US catfish, research ponds typically reach 1.8 or less while the industry farm-level figure has averaged about 2.5, a gap attributed mainly to unaccounted mortality, wasted feed and fish held past efficient size. Season distorts too: winter catfish ponds have reported FCRs of 6:1 or higher, because most of the feed was likely never eaten.

In practice

1. Write one FCR definition for the site and report biological and economic FCR side by side.
2. Count and weigh mortalities every day and record the cause, so lost gain can be separated from poor conversion.
3. Set accounting periods long enough that sampling error cannot reverse the result, and reconcile biomass estimates against harvest weights.
4. Log water temperature and dissolved oxygen with each day's feed, and read FCR against the species' optimum ranges.
5. Sieve a sample from each feed delivery for fines, and check for uneaten pellets with waste collectors or cameras.
6. Benchmark only within one species, size band and temperature range, by stage rather than as one site average.
7. Compare feeds on cost per kilogram of gain, FCR multiplied by delivered price, not on price per tonne.

FeedOptima's free analysis checks water conditions against species ranges and projects feed conversion from the species baseline, so you can run an optimization before changing a feed.