Data-processing artefacts in three-dimensional dive path reconstruction from geomagnetic and acceleration data

Abstract

Tri-axis magnetism and acceleration data loggers have recently been used to obtain time-series headings and, consequently, the 3-dimensional dive paths of aquatic animals. However, problems may arise in the resulting calculation process with multiple parameters. In this study, the dive paths of loggerhead turtles and emperor penguins were reconstructed. For both species, apparently unrealistic movements were found. Time-series heading data of turtles showed small regular fluctuations synchronous with stroking. In the dive paths of penguins, infrequent abrupt changes in heading were observed during stroke cycles. These were unlikely to represent true behaviours according to observations of underwater behaviour and tri-axis magnetism and acceleration data. Based on the relationship between sampling frequency and frequency of body posture change, we suggest that (1) the changes in the animals' posture concurrent with strokes and (2) the mismatched treatment (i.e. filtering and non-filtering) of the acceleration and magnetism data caused the artefacts. These inferences are supported by the results of simulations. For data sets obtained at a given sampling frequency, the error pattern in calculated dive paths is likely to differ depending on the frequency and amplitude of body posture changes and in swim speed. In order to avoid misinterpretation, it is necessary to understand the assumptions and inherent problems of the calculation methods as well as the behavioural characteristics of the study animals.

Monitoring attitude and dynamic acceleration of free-moving aquatic animals using a gyroscope

“…3) (as described by Caruso 2000, Johnson & Tyack 2003, Shiomi et al 2008) based on accelero meter and magnetometer measurements when the data logger was stationary. In the conventional method, magneto meter and acceleration mea surements were also low-pass filtered or running mean smoothed to avoid a mismatched treatment of the acceleration and magnetism data, following Shiomi et al (2010).…”

The influence of preceding dive cycles on the foraging decisions of Antarctic fur seals

“…Both 3MPD3GT and GPS loggers were attached to the dorsal fur of animals and used to reconstruct the three-dimensional dive paths and the animal's location between dives. Three-dimensional dive paths reconstructed by 3MPD3GT include some errors owing to water currents and sampling intervals [8]. The horizontal positions obtained by GPS tracking data on the surface were used to correct the three-dimensional dive path.…”

Point of no return in diving emperor penguins: is the timing of the decision to return limited by the number of strokes?

“…In addition, 3-D dive paths of one bird (CW13) in experiment A and of three birds in experiment B were calculated using a customized macro Shiomi et al, 2010) compliant with Igor Pro (WaveMetrics, Lake Oswego, OR, USA). In dive path analyses, only dives in which mean swim speed was greater than 1ms -1 were considered as reliable data.…”