Coordinating Working Party on Fishery Statistics (CWP)

Conversion factors

In fishery and aquaculture statistics the term conversion factor is used principally when converting the volume or weight of a product at one stage in the production chain to its volume or weight at another stage in the chain. Perhaps the most common use of conversion factors is for the conversion of the landed weight of a product to its live weight equivalent when it was removed from the water.

Conversion factors depend primarily on the species and its morphology. In addition there are a number of elements to consider:

  1. Extent and method of processing including on board fishing vessels
  2. Size of the fish, as the proportion of skeletal material to flesh change during growth, such a change could affect the conversion factor
  3. Area and season of capture; e.g. ratio of the length to the depth of the body may vary with area, and the state of maturity and fat content may vary with season. Such differences may affect the conversion factor.

Conversion factors are often presented for a species caught or grown in a particular area and processed by a method. However, there could be considerable differences in the method of presentation of a nominally identical product. For example, machine gutting, heading and filleting could result in very different products (in terms of the proportion of the product removed) compared to hand processing. Therefore, a conversion factor is usually the result of a lengthy, thorough, and therefore expensive investigation and once a factor has been established, in many situations it is not revised on a regular basis. However, CWP advises that national authorities should regularly review the factors that are in use. The accuracy of such factors is important, for example, in analyses of landings (e.g. for stock assessment and management) and the resultant stock management measures, where quantities are often expressed in live weight equivalent.

Conversion factors and data collection systems

FAO maintains records of the conversion factors used by the national authorities through a questionnaire, FISHSTAT CF1, on which the national authorities are requested to indicate the appropriate conversion factors. This is not an annual questionnaire but is distributed on rotation after a certain number of years. FAO requests national authorities that introduce significantly different conversion factors in the intervening period, to submit them to FAO.

Conversion factors introduce uncertainty in the estimation of the catch and this uncertainty can be considerable when converting highly processed products, e.g. fillets to live weight equivalent. Caution is advised when studying the conversion factors used by the different national authorities. While most countries use conversion factors for an identified product that are in reasonable agreement (±10%) there are instances where the differences are significantly greater. However, it would be dangerous to suggest automatically that a factor was unrealistic. Experience has shown that these factors can be justified frequently, because the brief descriptor of the product hides significant differences in the presentation of the product.

Conversion factors from product to live weight

The above notes have referred largely to factors to convert the landed weight to the live weight equivalent of the catches. However conversion factors may be used for other purposes. Trade statistics are normally by products and the comparison of the use of raw material for the various products requires that these are converted to a common denominator very often live weight equivalent. This requires the use of conversion factors. Another example is found in the section Supply Balance Sheets. An end product of balance sheets is the apparent per capita consumption of the product and by the judicial use of conversion factors it is possible to express this in various ways. It is often useful to compare the input of fish to the diet and compare it with the input from other protein foods such as meat and eggs. In this case a conversion factor would be applied to the product weight (or other unit in which the balance sheet had been calculated) to produce a figure for the consumption, for example, in live weight per year. Another use is for comparisons among countries and also for a gross assessment of national self-sufficiency in fish and fishery products.

Annex I.1 presents indicative factors for converting product weight to live weight for a selection of major fishery commodities.

Bibliography

FAO. 1980. Quantity conversion factors: Atlantic fish species - landed or product weight to live weight. FAO Fisheries Circular No.725. Rome, FAO. 45 pp. (also available at https://www.nafo.int/portals/0/pdfs/sc/1986/scs-86-025.pdf).

FAO. 2000. Conversion factors - landed weight to live weight. FAO Fisheries Circular No.847, Revision 1. Rome, FAO. 176 pp. (also available at http://www.fao.org/3/x9144t/x9144t.pdf).