Age determination in the icefish Pseudochaenichthys georgianus (Channichthyidae) based on multiple methods using otoliths

R Traczyk,Vb Meyer-Rochow,Rm Hughes

doi:10.3354/ab00736

R Traczyk, Vb Meyer-Rochow + Show 1 more

Open Access

https://doi.org/10.3354/ab00736

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Abstract

Aging Antarctic icefish is difficult because of their lack of scales and poorly calcified bones. Icefish ages must therefore be estimated from otoliths. We describe a method of reading daily micro-increments in connection with shape, size and mass analyses of the otoliths of the South Georgia icefish Pseudochaenichthys georgianus. Changes in otolith morphology and mass correlate with fish size and age group. The otolith micro-increment analysis is capable of establishing the age of an icefish by relating the daily micro-increment count to the life history of the fish. Micro-increment measurements and analyses are relatively simple to do by light and scanning electron microscopy and by using micro-densitometer and digitizing equipment. Drastic changes in the life history of an individual are reflected by measurable changes in its otolith micro-increment data as seen in our analyses of age groups 0-VI. The initial drastic change in daily micro-increment shapes and periodicities occur in connection with the hatching period of the icefish. The next drastic change in otolith shape and daily micro-increments occurs when ~7 cm long fish shift from pelagic to benthic habitats. As the fish age beyond group III, individual otolith variability lessens until they begin spawning. Our results indicate a single population of P. georgianus between the Antarctic Peninsula and South Georgia.

Highlights

Age assessments provide essential information about population structure and population growth resulting from environmental changes
There are 8 advantages of the methods presented in this paper: (1) Acetate replicas of otoliths viewed by light microscope (LM) reveal the regularity of daily micro-increments, and the scanning electron microscopy (SEM) demonstrates the larval microincrements of the entire otolith nucleus as well as the daily micro-increment variability with age (Fig. 3)
(2) SEM images and high-resolution LM facilitate visualization and automated daily micro-increment counting in icefish larvae and adults that cannot be accurately or precisely achieved by LM images of poor resolution (Fig. 3) that result in considerable observer subjectivity and uncertainty (Ralston & Williams 1988)

Summary

Introduction

Age assessments provide essential information about population structure and population growth resulting from environmental changes. The age of a fish affects its body length and the size, mass and shape of its otoliths because changes in body shape and mass are reflected in similar changes in its otoliths to retain its ability to remain in neutral balance. Different age groups can be identified on the basis of daily otolith micro-increments, otolith diameter, otolith shape and otolith mass, and by the presence or absence of a secondary primordium and/or an anterior colliculum. These age-related changes reflect the role of otoliths as gravity and swimming monitors and the varied sensory functions that otoliths play during the different growth stages and environments of the fish

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