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Sara Faggion, Pierre Sanchez, Marc Vandeputte, Frédéric Clota, Alain Vergnet, Marie-Odile Blanc, François Allal

Individual tagging is key to a better understanding of early life stages in fish. Very small RFID transponder
microchips (500×500×100 μm, 82 μg) are now available. The aim of this study was to develop a protocol to
tag European sea bass (Dicentrarchus labrax L.) larvae from 61 days post-hatching (dph; standard length
~10 mm) to 96 dph (standard length ~28 mm) through intra-coelomic implantation of microchips. The suitability
of such a tagging procedure was evaluated, with the purpose of determining the minimal fish age and
body size for microchip tagging without adverse effects on survival and growth performance.
We produced an experimental population composed by 50:50 normally pigmented larvae and albino larvae
through artificial fertilization. Five tagging trials were performed over 35 days, in fish aged 61, 75, 83, 89 and 96
dph. Each time, 50 normally pigmented fish were tagged, while 50 albino fish were used as controls. Mortality
was recorded daily, while biometric measurements were performed at 75, 83, 89, 96, 103 and 110 dph via image
Microchip tagging was possible in larvae from an age of 75 dph (standard length ~20 mm), with satisfactory
performance in terms of survival rate (between 84 and 98% 24 h after tagging) and growth rate, and without
significant differences in comparison with the untagged controls. In contrast, tagging before 75 dph is not to be
recommended, as the age group 61 dph was the most affected in terms of survival (only 62% of fish survived 24 h
after tagging) and growth rate, showing significant differences compared to the untagged controls. The overall
microchip reading success rate for the age groups throughout the experiment was 51.4%, the overall reading
success rate at each biometric measurement was 48.2%, probably due to the change in orientation of the microchip
inside the fish body cavity.
The tagging protocol developed was then overall successful, albeit with a moderate reading success.
Precocious tagging could allow the collection of new types of data (individual, longitudinal) related to larval
development, behavioral studies, physiological and immunological investigations. Future tests could focus on
the effects of tagging on baseline locomotion and behavior, as well as the suitability and the efficiency of
intramuscular microchip tagging on larger fish.

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