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

Ryszard Traczyk, Victor Benno Meyer-Rochow*, Robert M. Hughes

*Corresponding author:

ABSTRACT: Ageing 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 in Pseudochaenichthys georgianus. Changes of the otolith’s morphology and mass correlate with the fish’s size and age group. The otolith micro-increment analysis is capable of establishing an icefish’s age 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 fish life’s history are reflected by measurable changes of the otolith’s micro-increment data as seen in our analyses of age groups 0-VI. The first drastic changes in daily micro-increment shapes and periodicities occur in connection with the hatching period of the icefish. The second drastic change affecting daily micro-increments is related to the different environments that the approximately 7-cm long young fish encounter when changing from a pelagic life to that of a bottom dweller. 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.