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Conversion FactorIn the context of fishery statistics the term "conversion factor" is used principally when converting the volume or mass (more commonly referred to as the "weight") of a product at one stage in the production chain to its volume or mass at another stage in the chain.

Conversion factors from landed to nominal weight

Conversion of Landed Weight to Live WeightPerhaps the most common use of conversion factors is for the conversion of the landed weight of a product to its live weight equivalent (i.e. its mass, or weight, when it was removed from the water). In most situations it would be impracticable (largely because of the restraints imposed by the working conditions) to accurately record the weight of fish at the time of capture. The accuracy of measures recorded on fishing log-sheets thus depends heavily on the experience of the observer in making a visual assessment of the volume and composition of a catch (see Section O. LOGBOOKS*). Except on factory vessels which have facilities for processing (e.g. filleting, freezing) the catch, the first occasion on which weight of the fish may be accurately recorded is often at the time of landing, either in a port prior to auction or in a processing plant. The landed weight may differ significantly from the weight of the product as it was taken from the sea, due partly to the physiological changes associated with death and, often more importantly, to processing of the catch on board the vessel (e.g. gutting, beheading, filleting).The landed weight is more an indicator of the mass at the time of landing than of the mass of an identifiable product. For example, landed fish may be whole fish, fish which have been simply gutted, or gutted and headed, or filleted fish and thus the landed weight in many circumstances has to be considered as a measure of a very heterogenous group of products. Aggregation of such data for further evaluation might well produce some ambiguous results. Thus it is normal, in further processing the data, to apply conversion factors to the individual products which express the mass in a more homogenous way. The most common approach is to convert the mass to the mass of the product as it was taken from the water, often referred to as the nominal catch or the live weight equivalent of the product, see Section B. CATCH AND LANDINGS**.

*Link to Section O. LOGBOOKS.

**Link to Section B. CATCH AND LANDINGS.

The establishing of conversion factors is a task which cannot be undertaken lightly and requires careful consideration of the elements likely to affect it. Taking the example of the factor converting the landed weight to the live weight equivalent, the following are among the elements which would have to be taken into consideration when undertaking such an investigation:

a) the zone of capture of the product. An experienced eye can often identify the zone of capture from the physical characteristics of the fish (e.g. ratio of the length to the depth of the body) and these differences may well affect the conversion factor.

b) the size of the product. As a fish grows the proportion of skeletal material to flesh may well change, such a change could well affect the conversion factor.

c) the season of capture. Certain species in certain waters are subject to significant changes in physical characteristics in different seasons of the year (e.g. due to state of maturity and fat content) which may affect the resulting conversion factor.

d) the extent and method of processing on board the fishing vessels. There could be considerable differences in the method of presentation of a nominally identical product at the time of landing. For example, machine gutting, heading and filleting could result in very different products (in terms of the proportion of the product removed) compared with hand processing.

It is obvious from the above that a "simple" conversion factor should be the result of a lengthy, thorough, and therefore expensive investigation. It is not surprising that, once a factor has been established, in many situations it is not revised on a regular basis. It is known that many factors employed by national authorities have been in use for 20 years or more without any attempt to revise them and, while there is no evidence to suggest that these factors are no longer applicable, the CWP has expressed the view that national authorities should regularly review the factors that are in use. The accuracy of such factors is important because most subsequent analyses of the landings (e.g. for stock assessment and management) and the resultant stock management measures, require that the quantities are expressed in a uniform unit, the live weight equivalent.

The CWP has requested FAO to maintain records of the conversion factors used by the national authorities and accordingly FAO has introduced 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. However, FAO requests national authorities that introduce significantly different conversion factors in the intervening period, to submit them to FAO.

Caution is advised when studying the conversion factors used by the different national authorities. While most countries use conversion factors for an identified products 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. An example of this is to be found in Section P. 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 "rival" 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 in, for example, Kg 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.

FAO. "Quantity conversion factors: Atlantic fish species - landed or product weight to live weight." FAO Fisheries Circular No.725. 1980 online version
FAO. "Conversion factors - landed weight to live weight."FAO Fisheries Circular No.847. 1992 online version
FAO. "Conversion factors - landed weight to live weight."FAO Fisheries CircularNo.847. Revision 1. 2000.  online version
This Revision 1 is the result of close collaboration between FAO's and the "Fisheries" Sector, Directorate for Agriculture, Environment and Energy Statistics, EUROSTAT. The information on conversion factors presented was obtained from four sources:

a) A questionnaire circulated to national authorities by FAO in 1992-93, requesting the submission of conversion factors used nationally in compiling catch data reported to FAO and other international organizations.

b) A study commissioned by the European Commission and conducted by COFREPÊCHE in December 1996. This study, whose main object was to review the status of the work on conversion factors in the countries bordering the North-east Atlantic, resulted in the compilation of factors, many of which had not been submitted to FAO on the above-mentioned questionnaire. The factors were obtained from various sources within the countries contacted: fishery administrations, statistical units and technical organizations.

c) Results from discussions between the European Commission and the fishery inspectorates of the Member States of the European Union. The factors used in compiling submissions to the EU's catch quota monitoring system were found identical to those used in the compilation of catch statistics for other purposes.

d) Additional information on other conversion factors also submitted by the national statistical bodies to EUROSTAT.

The inquiry conducted by FAO in 2001 did not yield sufficient results to justify a second revision of the above publication (FAO Fishery Circular No. 847).
Torry Research Station, Aberdeen (UK).Yield and nutritional value of the commercially more important fish species.FAO Fisheries Technical Paper. No. 309.Rome, FAO. 1989.187p.  online version
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