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 analysis.

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.