Otoliths versus scales: evaluating the most suitable structure for ageing largemouth bass, Micropterus salmoides, in South Africa

Abstract

The suitability of scales for ageing Micropterus salmoides was determined by  comparing the precision of growth zone counts on scales with those obtained from  sectioned sagittal otoliths from a sample of 496 fish collected from Wriggleswade  and Mankazana Impoundments in the Eastern Cape, South Africa. Otoliths (1.4% rejected) were more readable than scales (41.7% and 7.5% rejected) for  Wriggleswade and Mankazana Impoundments respectively. Otolith readings were more precise (average percentage error (APE) = 13.6%; coefficient of variation (CV) = 15.8%) than scales (APE = 18.0%; CV = 21.9%) for the total sample. Growth zone counts between structures were not symmetrically distributed around  the agreed ages (otolith age = scale age) of the fish from Wriggleswade Impoundment (Bowker’s test H²: χ ² = 136, d.f. = 27, P < 0.05) and graphical  comparison showed that scales tended to underestimate age, particularly for fish older than five years. Scales are therefore not suitable structures for ageing South African M. salmoides which may attain ages of up to 13 years.

Key words: Micropterus salmoides, sagittal, otoliths, precision, growth zones, APE, comparison.