In aquaculture, few figures attract as much attention as the FCR. For years it has been the flagship indicator used to assess productive performance and, in many cases, it is still the first number people look at. Yet, as with any simplified metric, the problem lies not in the figure itself but in what we assume it explains. A low FCR — even below 1 — can be real, correct and physiologically sound. On its own, however, it is not enough to claim that a production system is performing well.
When an FCR below 1 appears, the usual reaction is disbelief. How can a fish gain more weight than it receives in feed? The answer lies in fish biology and in the way the indicator is defined. FCR compares feed supplied as dry matter with fish biomass gain measured as wet weight. This asymmetry is crucial. Commercial feed contains around 90% dry matter, while fish are composed of 70–80% water. As fish grow, a substantial part of the weight gain is water incorporated into body tissues rather than newly formed dry matter.
This is reinforced by a key physiological advantage. Fish do not expend energy maintaining a constant body temperature. As ectotherms, their maintenance costs are far lower than those of birds or mammals, allowing a greater proportion of ingested energy and nutrients to be channelled into growth. Under conditions of rapid growth, highly digestible diets and favourable environments, this efficiency can legitimately result in FCR values below 1.
Seen in this context, an FCR below 1 does not mean that fish “create matter” or that all gained biomass is high-value muscle. It simply reflects the combination of dry and wet weight in the calculation and the high growth efficiency of fish. Problems arise only when the figure is interpreted in isolation, without considering the nature of the growth or its wider consequences.
Productive performance is not a static snapshot but a complete story. It begins with growth, yet it also depends on survival, consistency and animal health. Indicators such as specific growth rate and average daily gain help determine whether fish are responding properly to diet, temperature and management. Survival, although less visible, is equally decisive: a system cannot be considered efficient if it continuously loses animals, regardless of how fast the remaining fish grow.
Feed efficiency remains central to this story. FCR is a valuable operational tool, but good performance is defined not just by a low value, but by a stable one. Unexplained fluctuations often signal management issues, stress or feed quality problems. To understand what lies behind FCR, other indicators become essential. PER reveals how efficiently dietary protein — an expensive and strategic resource — is being used, while nitrogen retention links growth directly to nutrient utilisation and environmental impact.
Productive performance also depends on how growth is distributed within the stock. Well-managed systems tend to produce homogeneous fish, whereas increasing size variation usually undermines overall efficiency, even when average figures look acceptable.
Sustainability adds another unavoidable dimension. Indicators such as FIFO highlight the extent to which productive performance depends on limited marine resources. A system may be biologically efficient yet fragile if it relies heavily on fishmeal and fish oil.
Ultimately, all these indicators converge on economics. Feed cost per kilogram produced is often the clearest test of whether efficiency is real or only apparent. Measuring productive performance, therefore, is not about finding a single perfect number, but about reading several together. FCR remains an important piece of the puzzle, but it only makes sense when interpreted alongside growth, survival, nutrient use and environmental impact.
Related
