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. 2021 Feb 10;11(6):2717-2730.
doi: 10.1002/ece3.7226. eCollection 2021 Mar.

Analysing detection gaps in acoustic telemetry data to infer differential movement patterns in fish

Affiliations

Analysing detection gaps in acoustic telemetry data to infer differential movement patterns in fish

Michael J Williamson et al. Ecol Evol. .

Abstract

A wide array of technologies are available for gaining insight into the movement of wild aquatic animals. Although acoustic telemetry can lack the fine-scale spatial resolution of some satellite tracking technologies, the substantially longer battery life can yield important long-term data on individual behavior and movement for low per-unit cost. Typically, however, receiver arrays are designed to maximize spatial coverage at the cost of positional accuracy leading to potentially longer detection gaps as individuals move out of range between monitored locations. This is particularly true when these technologies are deployed to monitor species in hard-to-access locations.Here, we develop a novel approach to analyzing acoustic telemetry data, using the timing and duration of gaps between animal detections to infer different behaviors. Using the durations between detections at the same and different receiver locations (i.e., detection gaps), we classify behaviors into "restricted" or potential wider "out-of-range" movements synonymous with longer distance dispersal. We apply this method to investigate spatial and temporal segregation of inferred movement patterns in two sympatric species of reef shark within a large, remote, marine protected area (MPA). Response variables were generated using network analysis, and drivers of these movements were identified using generalized linear mixed models and multimodel inference.Species, diel period, and season were significant predictors of "out-of-range" movements. Silvertip sharks were overall more likely to undertake "out-of-range" movements, compared with gray reef sharks, indicating spatial segregation, and corroborating previous stable isotope work between these two species. High individual variability in "out-of-range" movements in both species was also identified.We present a novel gap analysis of telemetry data to help infer differential movement and space use patterns where acoustic coverage is imperfect and other tracking methods are impractical at scale. In remote locations, inference may be the best available tool and this approach shows that acoustic telemetry gap analysis can be used for comparative studies in fish ecology, or combined with other research techniques to better understand functional mechanisms driving behavior.

Keywords: animal movement; biotelemetry; elasmobranchs; marine protected areas; network analysis; sharks; spatial and temporal segregation; sympatry.

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Conflict of interest statement

None declared.

Figures

Figure 1
Figure 1
Acoustic array in the BIOT MPA with the locations of 93 acoustic receivers shown in red, adapted from Carlisle et al. (2019). Insert shows the location of the BIOT MPA in the Indian Ocean, with the exclusive economic zone (EEZ) and MPA boundary indicated by the dotted line. Gray lines show the contours of major submerged geographic features. Shallow reefs are <20 m in depth, with deep reefs between 20 and 100 m in depth
Figure 2
Figure 2
Schematic describing designation of “restricted” and “out‐of‐range” movements. Black and gray arrows indicate a movement either to and from the same point (recursion), or between two points (transition). Time between detections for recursions, and relative deviation from expected time (RDET) for transitions, is represented by length, curvature, and color of the arrow. As time and RDET increase, length and curvature increase, and color gets lighter indicating less‐directed movement. Red dashed line indicates our cutoff detection gap (91 min for gray reef sharks and 64 min) for silvertip sharks for recursions and RDET (0.128 for gray reef sharks and 0.164 for silvertip sharks) for transitions
Figure 3
Figure 3
(a) Frequency density plots of gray reef shark (gray) and silvertip shark (blue) diel variance (a) and seasonal variance (b) in percentage “out‐of‐range” movements, with the sun, moon, cactus, and rain cloud indicating daytime, nighttime, dry season, and wet season “out‐of‐range” movements, respectively
Figure 4
Figure 4
Plots of condition modes of random effects for individual gray reef and silvertip sharks. Departures from global intercept are plotted with 95% CIs (black bars). Individuals where CIs do not cross zero indicate average “out‐of‐range” movements significantly different than the average. Individuals conducting less than the average “out‐of‐range” movements have negative global intercept values, and those that conduct more have positive intercept values. Individuals are identified by species and sex

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