Errors in ocular index measurements in European eel Anguilla anguilla (L.)

• Published in: Ecology of freshwater fish Vol. 15; no. 4; pp. 578 - 582
• Main Authors: Acou, A., Poizat, G., Crivelli, A.-J.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01-12-2006
• Subjects: Anguilla anguilla; measurement error; ocular index; silvering
• ISSN: 0906-6691; 1600-0633
• DOI: 10.1111/j.1600-0633.2006.00189.x

–  Knowledge of maturation state of eels (Anguilla anguilla, L.) based on ocular index (OI) serves management and research functions, such as estimating how much silver eel actually could escape from a particular basin or determining more precisely the developmental biology of a particular eels. OI implies the measurements of five variables (total length and eyes diameters) in a formula which can be sensitive to measurement error. To investigate this issue, we carried out a comparative study using 13 European eels harvested in Camargue (Rhône Delta, France) and three replicated measurements by three different observers. The estimated amplitude of OI imprecision at a 95% confidence level was 1.8 whatever eel and observer. Such error may lead to classification problems particularly when OI values are close to the threshold values used to discriminate between yellow and silver stages. This study not only raised the problem of the OI imprecision but also brought up the need to develop standardised protocol for identifying silver eels. To reach this goal, two different ways are proposed in conclusion.