In capture fishery and aquaculture statistics,  conversion factors are often used 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 (e.g. Indicative factors for converting product weight to live weight for a selection of major fishery commodities). Other conversion factors may be used such as converting the mean length of a fish or other aquatic organism to its mean live weight equivalent.

Conversion factors depend primarily on the species of aquatic organism 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 fileting 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.

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. filet weight 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 close agreement (e.g. ±10%) there are instances where the differences are significantly greater. Experience has shown that such differences in factors may be justifiable because the brief descriptors often used to identify products may mask significant differences in the cut and/or presentation of each 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 which is typically 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 or other aquatic organisms to the diet and compare this 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, other aquatic organisms and fishery products.

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).