High Mortality of Largemouth Bass Implanted with Transmitters at Colder Water Temperatures

Biotelemetry via surgical implantation of an electronic tag is a common way to examine fish behavior and movement. Previous studies suggest higher post-operative survival should be expected when implanting tags at colder water temperatures. However, during the initial part of our study, all 26 adult largemouth bass (Micropterus salmoides) we implanted with transmitters at water temperatures from 14 to 17 C at Toledo Bend Reservoir, Texas, died within 4 wk post-surgery. To further investigate this phenomenon, we conducted two tagging trials at 13 C, observing post-operative mortality of 100% (n = 5) and 58% (n = 12); all fish that died developed external fungal infections (i.e., saprolegniasis). Post-operative survival was 100% in a third trial at 24 C (n = 6) and no fungal infections were observed. Subsequently, tagging mortality was ≤ 20% when 81 largemouth bass were tagged at water temperatures from 22 to 30 C at Toledo Bend and Lake Fork reservoirs. Our results suggest that immunosuppression of largemouth bass coupled with greater fungal virulence in colder water likely caused the complete mortality of 31 largemouth bass initially implanted with transmitters at Toledo Bend Reservoir and in our first trial. Investigators conducting surgeries during similar conditions should be aware that high post-surgical mortality could result if fungal virulence is high during tagging.