Evaluation of double acoustic tagging techniques to track American shad Alosa sapidissima movements at multiple spatial scales

Abstract

Enhancements to acoustic telemetry technology now allow for tracking aquatic animal movements at multiple spatial and temporal scales. While large acoustic arrays allow for broad-scale tracking of movement and migrations at scales of thousands of km across multiple years, more focused smaller arrays offer the potential to quantify fine-scale movement across shorter time frames. Investigators seeking to quantify movement at multiple scales may require a double-tagging approach if their telemetry systems and tag capabilities are different (e.g., 69 kHz for broad-scale, and 307 kHz for sub-meter fine-scale). We tested multiple double-tagging approaches (InnovaSea-Vemco V9 and HTI-495LY tags) for American shad Alosa sapidissima during three individual tank survival experiments including: 1) two free-floating acoustic tags and a dart tag, 2) two acoustic tags attached to one another and a dart tag, and 3) two acoustic tags attached to one another with no dart tag and revised handling and tagging techniques to reduce stress. In each experiment, we used a mixed effects Cox’s proportional hazard model to test for differences in relative survival between groups (double acoustic tagged and control fish). At the completion of each experiment, all fish were X-rayed to evaluate tag placement in the body cavity. During the first experiment, American shad that received two free-floating tags and a dart tag had significantly lower relative survival compared to control fish after three weeks. The second experiment with treatment fish receiving two attached tags and a dart tag still had lower survival compared to control fish, but the magnitude of survival differences between treatment and control was lower compared to experiment one. In the third experiment, when two attached tags were used, dart tags were eliminated, and changes to fish handling and tagging techniques were implemented (new tagging apparatus, addition of lubricant, and different fish orientation) survival was equal among treatment and control fish. X-ray images showed distinct differences in post-mortem tag positioning between the first experiment (misaligned tags) compared to the second and third experiments (tags horizontally aligned within intestinal tract). American shad are generally difficult to study experimentally and sensitive to collection, handling, and surgical implantation. Our study showed strong evidence that attaching two acoustic tags to one another to create a single, larger tag improved survival and reduced internal tag misalignment. Fine- and broad-scale acoustic telemetry studies can be achieved with aquatic animals, but care must be taken with all species, and particularly those susceptible to handing stress to minimize tagging effects (e.g., mortality, altered movement behaviors, and tag burden).