Tagging of Atlantic cod (Gadus morhua) with intragastric transmitters: effects of forced insertion and voluntary ingestion on retention, food consumption and survival

The effects of intragastric tagging on transmitter retention, food consumption and survival were investigated in Atlantic cod ( Gadus morhua L.) under laboratory conditions. Cod (60–84 cm) were intragastrically tagged with two different sizes of dummy acoustic transmitters (16 × 45 and 16 × 82 mm) using the methods of forced insertion and voluntary ingestion. Cod that were tagged by forced insertion began to regurgitate transmitters the first day after tagging and continued to do so throughout the period of study. The percentage of fish retaining transmitters in this group dropped to 50% after 32 days. By comparison, cod that were tagged by voluntary ingestion exhibited a 44-day delay in the onset of initial regurgitation. This was followed by a rapid rate of regurgitation, with the percentage of fish retaining transmitters in this group dropping to 50% after 60 days. These retention times are prolonged in comparison to previous studies and have been attributed to the lower temperature regime used in this study. Transmitter size did not significantly affect the duration of retention. During the first 20 days following tagging, the average food consumption per fish was higher in the voluntary ingestion group compared with the forced insertion and sham groups. Mortality levels were 13, 44 and 56% for the voluntary ingestion, forced insertion and sham groups, respectively, over the period of study. We associate low food consumption and high mortality rates in the first 20 days of the experiment with high levels of stress due to handling. The findings indicate that voluntary ingestion is a more viable technique for the intragastric tagging of cod in field studies.