Setting up interpretation criteria for ageing juvenile european anchovy otoliths

Abstract

A standardisation of daily age determination in juveniles for this species is proposed by examining a wide range in length of juveniles. Sagitta otoliths and the saggital plane were selected for the analysis and daily increments were read in the postrostrum and antirostrum axes. Certain regions of the otolith show wide, rhythmic double growth patterns. These 2 axes were read with two different objectives (x10 and x20) and using two different interpretation criteria: group band reading (GBR), by which a repetitive cyclic set of growth bands were taken as single daily increments and individual mark reading (IMR), by which each microincrement, regardless of its appearance, constituted a daily count. Hatching dates, relative error and coefficient of variation were applied in order to find the most consistent reading method, and were compared with known growth rates in this and other genera of anchovy. Though the hatching dates produced by the two ageing methods are compatible with the spawning period, we consider the GBR method to be the most reliable ageing procedure because: a) it is the most precise and robust (not being affected by the examination procedure); b) it allows the rhythmic pattern of alternating growth bands to be interpreted as sub-daily microincrements generated in the later phases of larvae and during metamorphosis; and c) it produces high rates of growth compatible with larva growth increments and with other studies for the same genus at similar temperatures.

Keywords

• engraulis encrasicolus
• otolith
• daily increments
• sub-daily
• juvenile
• metamorphosis
• growth