Evaluating the behavioural consequences of external pop-off biologging packages on two freshwater fishes

Quantifying the behaviour of animals in the wild can provide important information on their ecology and welfare. External biologgers that 'pop-off' to enable tag retrieval have been widely used in marine systems; however, they have only recently been applied to freshwater fishes. As a result, questions remain about the impact that external biologgers have on the behaviour of freshwater fishes. Here, two species of freshwater fish that were previously implanted with acoustic transmitters were recaptured with rod-and-reel and fitted with custom pop-off biologging packages (50 × 30 × 15 mm; 14.7 g) in a small (23 ha) lake. Habitat use and movement behaviour (e.g., cruising, exploratory, stationary) were assessed for each individual for 72 h when pop-off biologging packages were affixed to the fish, and once they were detached from the fish. Acoustically tagged fish in the system that were never equipped with a pop-off biologging package were included as controls. Behavioural impacts of pop-off biologging packages were more evident for Largemouth Bass Micropterus nigricans than for Northern Pike Esox lucius in water temperatures between 20 and 27°C. Largemouth Bass spent less time in the limnetic habitat and more time close to refuge when pop-off biologging packages were attached compared to controls or when they were detached. Northern Pike spent more time in the limnetic habitat and less time in the semi-complex habitat when pop-off biologging packages were attached compared to controls. Pop-off biologging packages had no impact on the movement behaviour of Northern Pike. Largemouth Bass attached with pop-off biologging packages were stationary for longer compared to controls, or once the pop-off biologging packages were detached. Once the pop-off biologging packages detached, Largemouth Bass movement behaviour resembled the controls, which spent more time cruising and exploring. These results provide important information on behavioural artefacts that can manifest when wild fish are equipped with short-term external biologging packages.