Should total landings be used to correct estimated catch in numbers or mean weight-at-age?

• Published in: ICES journal of marine science Vol. 54; no. 3; pp. 462 - 470
• Main Authors: Lewy, Peter, Lassen, Hans
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01-06-1997
• Subjects: fish stock assessment; Marine; sampling; sampling error; Sum Of Product (SOP) correction; fish stock assessment; Sum Of Product (SOP) correction; sampling error; sampling
• ISSN: 1054-3139; 1095-9289
• DOI: 10.1006/jmsc.1996.0189

Many ICES fish stock assessment working groups have practised Sum Of Products, SOP, correction. This correction stems from a comparison of total weights of the known landings and the SOP over age of catch in number and mean weight-at-age, which ideally should be identical. In case of SOP discrepancies some countries correct catch in numbers while others correct mean weight-at-age by a common factor, the ratio between landing and SOP. The paper shows that for three sampling schemes the SOP corrections are statistically incorrect and should not be made since the SOP is an unbiased estimate of the total landings. Calculation of the bias of estimated catch in numbers and mean weight-at-age shows that SOP corrections of either of these estimates may increase the bias. Furthermore, for five demersal and one pelagic North Sea species it is shown that SOP discrepancies greater than 2% from the landings are very unlikely. Larger discrepancies probably are indications of problems with the sampling design. The proper action is to reexamine the sampling programme and to revise it where needed.