Seagrass canopies and the performance of acoustic telemetry: implications for the interpretation of fish movements

Abstract

Background: Acoustic telemetry has been used with great success to quantify the movements of marine fishes in open habitats, however research has begun to focus on patterns of movement and habitat usage within more structurally complex habitats. To date, there has been no detailed assessment of the performance of acoustic telemetry within seagrass, which forms a crucial nursery and foraging habitat for many fish species globally. Information on the detection range of acoustic receivers within seagrass is essential to guide receiver array design, particularly positioning systems. Here, we compare detection ranges for transmitters (Vemco V7) within and above the seagrass to determine impacts on the performance of a Vemco Positioning System (VPS). We also investigate the influence of environmental conditions (i.e. wind, time of day, background noise, atmospheric pressure and depth) on detection probability. Results: The performance of the VPS declined dramatically when the transmitters were positioned within the seagrass (positional accuracy = 2.69 m, precision = 0.9 m, system efficiency (i.e. the proportion of successful positions) = 5.9%) compared to above the canopy (positional accuracy = 2.21 m, precision = 0.45 m, system efficiency = 30.9%). The reduction in VPS efficiency when transmitters were within seagrass was caused by a decline in the detection range of receivers (range of 50% detections) from 85 to 40 m, as this limited the ability of the three receivers to simultaneously detect transmissions. Additionally, no detections were recorded for the transmitters within seagrass at a distance greater than 150 m from the receiver. Increasing wind speed from 0 to 50 km h −1 correlated with a 15% reduction in detections while detection probability decreased from 0.8 during the day to 0.55 at night, due to higher in-band noise (69 kHz). Conclusions: Our findings demonstrate that tagged fish ensconced within seagrass are unlikely to be detected by receivers or positioned by a VPS. Further, we demonstrate that wind conditions and the time of day create temporal variation in detection probability. These findings highlight the need for telemetry studies to perform in situ range testing and consider how fish use vegetated habitats such as seagrasses when positioning receivers and interpreting data.

Performance of a novel system for high-resolution tracking of marine fish societies

“…Previous studies using BPSK or PPM systems working at 180 kHz in freshwater environments showed even larger acoustic ranges of around 200-300 m [23,24], reduced to 80-100 m when applied in the coastal ocean [37]. In comparison, the usual 69 kHz PPM systems used in coastal marine environments usually report an acoustic range between 150 and 300 m [38,39]. Overall, the main advantage of highfrequency systems over systems working at lower frequencies resides in the smaller size of their piezoceramics and the shorter duration of their signals, which allow the production of smaller transmitters and the simultaneous tracking of a larger number of individuals, but in return, present smaller acoustic ranges.…”

“…It has been described that BPSK coding systems are less affected by environmental noise than PPM systems, because the shorter duration of the signals reduces the probability of interferences [24]. It is also known that the presence of complex habitats, such as coral reefs or dense vegetation, lower the detection and positioning efficiency of acoustic telemetry systems [14,39]. The combination between physical factors and the receiver set up also can also affect the acoustic range, for instance, by currents modifying the receiver tilt angle in receivers fixed to ropes or buoys [40].…”

Movements of marine and estuarine turtles during Hurricane Michael

“…Our results suggest turtles in SJB responded little, at least with obvious and/or large-scale movements, to the storm both before and after landfall. Our small sample sizes, paired with limitations of non-GPS capable satellite tags in identifying fine-scale movements 33 and the potential impacts of shallow and rough water on acoustic receiver detections 34 , make it difficult for us to confirm fine-scale changes in behavior. In addition, it is difficult to say if behaviors observed in this study (e.g.…”

Evaluating receiver contributions to Acoustic Positional Telemetry: A case study on Atlantic cod around wind turbines in the North Sea

“…The following assumption was made: when no receiver was removed, and thus all receivers were included in the positional analysis, the number of fish positions detected within that set-up was 100% and the triangulated positions were the 'base' fish positions. This provided the possibility to determine two metrics for each APT performance: set-up efficiency and position accuracy [1,15], for each single receiver removal option (i.e. eight or nine for F05 and C05 respectively).…”

“…Any factor influencing, or interfering with, speed-of-sound and signal propagation in the water will therefore affect APT performance [13,14]. Habitat-specific features such as vegetation type and density, bottom characteristics and presence of rocks and/or (man-made) obstacles can block signal propagation [15]. Furthermore, natural events like currents and surface waves can influence receiver detection range [12,16] and can even lead to signal propagation interference through receivers getting temporarily buried or even lost [17].…”