Quantifying the effectiveness of fine-scale acoustic telemetry for tracking fish movements on tropical reef flats
Movements of aquatic animals are difficult to monitor, posing challenges for quantifying their relationship with the environment. Acoustic telemetry using fixed receiver stations is proving to be an effective tool to examine the ecology of marine life, including at relatively fine spatial scales. However, as with any technology, it is necessary to understand the underlying limitations to ensure that individual positions and movements are as accurate as possible, otherwise erroneous conclusions could be made about animal-environment interactions. We established a fine scale array of fixed acoustic receivers (VEMCO Positioning System, VPS) in Culebra, Puerto Rico, to track the movement of fishes in shallow fringing reef flat and adjacent habitats. The VPS system used synchronizing (sync) tags as reference points fixed in space to increase the accuracy of positions for tagged fish moving through the array of fixed receivers. For a three-month period, we modeled the probability of detection (i.e. efficiency) for sync tags by array receivers. We then used the model to examine the potential error in estimating the two-dimensional position of a tagged permit (Trachinotus falcatus) within the VPS array.
During the three-month period, we observed factors that both limit and enhance the receivers' ability to detect transmitters' signals. We first tested the receivers with the sync tags in the fixed array. We evaluated the following factors: time of day, day of the week, position of the receivers, number of days receivers were in the water, and tides. We found the number of days in the water, weekends and holidays, and irregular bathymetry were negatively correlated with probability of detection. In contrast, weekdays with less boat traffic were positively correlated with probability of detection. During this period, tides were not significantly correlated with probability of detection. By applying our model of detection efficiency to the permit detection data, we observed significant differences in habitat selection from uncorrected data. This highlights the importance of incorporating detection efficiency in estimates of animal space use, especially in dynamic environments such as shallow marine systems.