85
instruments that record and store data on-board. This holds
true not only for standard time-depth recorders (TDRs), but
also for cameras, accelerometers and magnetometers. As
they only store the data on-board, one is usually required to
recover the instrument and download the data. In the next
section we introduce location-only satellite tags. Collecting
animal locations is straightforward for Argos satellite tag
users, as once they are deployed, the data is relayed and provided online to the user, which enables near real-time tracking. However, this fundamentally differs from global
positioning system (GPS) devices, since the position determination works differently. With Argos, the satellite system
determines the device’s position, while with GPS, the device
determines its own position. The latter has the disadvantage
that one usually needs to recover the GPS tags, or couple
them with satellite or mobile phone networks. Finally, there
are tags, which consist of both an archival tag (e.g., TDR,
accelerometer, magnetometer) and a satellite tag (e.g.,
Argos, GPS). These satellite-linked data loggers are the
most sophisticated ones and also the ones most commonly
used. A ‘Quick guide to bio-telemetry terminology’ can be
found in Box 1.
Archival Loggers
Archival loggers are deployed on animals to record, in general, movements, specific behaviors, physiological processes or environmental conditions. It is necessary to
retrieve these instruments to download data, which can be
challenging in highly mobile marine predators. This disadvantage is offset by deploying archival loggers on species
with high site fidelity that haul out or breed on land (e.g.,
elephant seals, seabirds during breeding season), or by combining them with a radio transmitter (Very High Frequency –
VHF, or Ultra High Frequency – UHF) to relocate the tag
when it falls off (Wilson et al. 2002; Dragon et al. 2012;
Villegas-Amtmann et al. 2013). Since data transmission is
not a constraint for archival loggers, these devices are able
to collect data in high-resolution, which are otherwise
impossible to obtain and extremely valuable to study finescale processes.
Per Scholander is generally regarded as the pioneer of
bio-logging; he deployed the first archival loggers on marine
animals in the 1940s (Ropert-Coudert and Wilson 2005;
Kooyman 2007; McIntyre 2014). In his famous monograph
on diving physiology, he recorded the maximum dive depths
of whales, dolphins, and seals by using a capillary tube depth
gauge attached to the animal (Scholander 1940; Ponganis
2013). Later, maximum dive depth recorders were used on
Weddell seals Leptonychotes weddellii in Antarctica, to
study their maximum diving and breath-holding capacities
(DeVries and Wohlschlag 1964). A major step in the history
of bio-logging was the development of the first TDR by
Gerald Kooyman, which was able to record full time-depth
profiles of a dive (Kooyman 1965, 1966). This invention led
to fascinating opportunities to study the diving behavior of
marine top predators and the related physiological adaptations (Kooyman 1973). At the time, these devices were still
quite heavy and, for instance, too large for most bird species.
However, due to rapid technological advances, their dimensions have decreased quickly, which means that they can
presently be used on a variety of animals (Ropert-Coudert
and Wilson 2005; McIntyre 2014; Hussey et al. 2015).
At the same time, improved and additional sensors were
included in the classic TDR deployment, enabling diving
behavior to be studied in even greater detail. For instance,
magnetometers were incorporated, which measure the orientation towards the earth’s magnetic field in three axes (heading/yaw, pitch, and roll) and are especially sensitive to record
angular rotations (Fig. 1b). Tri-axial magnetometry enables
researchers to track three-dimensional movements of diving
animals via dead-reckoning, and thus to reconstruct threedimensional profiles of a dive (Davis et al. 1999; Mitani et al.
2003; Wilson et al. 2007; Williams et al. 2017). This was an
important improvement, as marine animals inhabit a threedimensional space and respond to environmental cues in all
three dimensions. For diving predators both the horizontal
and vertical distribution of prey patches is important.
Foraging chinstrap penguins Pygoscelis antarctica, for
instance, choose to pass by shallow and dense prey aggregations and reach for deeper and more homogenously distributed prey fields with higher encounter probabilities (Zamon
et al. 1996)—a finding which would be overlooked in a conventional, two-dimensional analysis of predator-prey distribution. Therefore, taking all three dimensions into
consideration is essential to understand fine-scale habitat use
or foraging behavior.
At present, it is also common to incorporate accelerometers into archival loggers. These devices measure acceleration, which is caused by earth’s gravity (static component)
and a change in the animal’s speed (dynamic component). It
usually records accelerations in three dimensions, the x-, y-,
and z-axes or surge, sway, and heave (Fig. 1a). When positioned on the head and/or jaw of a marine predator, accelerometers can provide information about rapid head
movements, indicating prey capture attempts (Naito et al.
2010; Kokubun et al. 2011; Gallon et al. 2013). However,
accelerometers are often deployed close to the animal’s center of gravity, i.e., in the center of the trunk, which is more
useful to record overall movement patterns (e.g., swimming,
resting, and flying). As such, tri-axial accelerometry can be
used to identify and quantify different behaviors and activity
patterns, and subsequently, put them in relation to energy
Bio-telemetry as an Essential Tool in Movement Ecology and Marine Conservation
instruments that record and store data on-board. This holds
true not only for standard time-depth recorders (TDRs), but
also for cameras, accelerometers and magnetometers. As
they only store the data on-board, one is usually required to
recover the instrument and download the data. In the next
section we introduce location-only satellite tags. Collecting
animal locations is straightforward for Argos satellite tag
users, as once they are deployed, the data is relayed and provided online to the user, which enables near real-time tracking. However, this fundamentally differs from global
positioning system (GPS) devices, since the position determination works differently. With Argos, the satellite system
determines the device’s position, while with GPS, the device
determines its own position. The latter has the disadvantage
that one usually needs to recover the GPS tags, or couple
them with satellite or mobile phone networks. Finally, there
are tags, which consist of both an archival tag (e.g., TDR,
accelerometer, magnetometer) and a satellite tag (e.g.,
Argos, GPS). These satellite-linked data loggers are the
most sophisticated ones and also the ones most commonly
used. A ‘Quick guide to bio-telemetry terminology’ can be
found in Box 1.
Archival Loggers
Archival loggers are deployed on animals to record, in general, movements, specific behaviors, physiological processes or environmental conditions. It is necessary to
retrieve these instruments to download data, which can be
challenging in highly mobile marine predators. This disadvantage is offset by deploying archival loggers on species
with high site fidelity that haul out or breed on land (e.g.,
elephant seals, seabirds during breeding season), or by combining them with a radio transmitter (Very High Frequency –
VHF, or Ultra High Frequency – UHF) to relocate the tag
when it falls off (Wilson et al. 2002; Dragon et al. 2012;
Villegas-Amtmann et al. 2013). Since data transmission is
not a constraint for archival loggers, these devices are able
to collect data in high-resolution, which are otherwise
impossible to obtain and extremely valuable to study finescale processes.
Per Scholander is generally regarded as the pioneer of
bio-logging; he deployed the first archival loggers on marine
animals in the 1940s (Ropert-Coudert and Wilson 2005;
Kooyman 2007; McIntyre 2014). In his famous monograph
on diving physiology, he recorded the maximum dive depths
of whales, dolphins, and seals by using a capillary tube depth
gauge attached to the animal (Scholander 1940; Ponganis
2013). Later, maximum dive depth recorders were used on
Weddell seals Leptonychotes weddellii in Antarctica, to
study their maximum diving and breath-holding capacities
(DeVries and Wohlschlag 1964). A major step in the history
of bio-logging was the development of the first TDR by
Gerald Kooyman, which was able to record full time-depth
profiles of a dive (Kooyman 1965, 1966). This invention led
to fascinating opportunities to study the diving behavior of
marine top predators and the related physiological adaptations (Kooyman 1973). At the time, these devices were still
quite heavy and, for instance, too large for most bird species.
However, due to rapid technological advances, their dimensions have decreased quickly, which means that they can
presently be used on a variety of animals (Ropert-Coudert
and Wilson 2005; McIntyre 2014; Hussey et al. 2015).
At the same time, improved and additional sensors were
included in the classic TDR deployment, enabling diving
behavior to be studied in even greater detail. For instance,
magnetometers were incorporated, which measure the orientation towards the earth’s magnetic field in three axes (heading/yaw, pitch, and roll) and are especially sensitive to record
angular rotations (Fig. 1b). Tri-axial magnetometry enables
researchers to track three-dimensional movements of diving
animals via dead-reckoning, and thus to reconstruct threedimensional profiles of a dive (Davis et al. 1999; Mitani et al.
2003; Wilson et al. 2007; Williams et al. 2017). This was an
important improvement, as marine animals inhabit a threedimensional space and respond to environmental cues in all
three dimensions. For diving predators both the horizontal
and vertical distribution of prey patches is important.
Foraging chinstrap penguins Pygoscelis antarctica, for
instance, choose to pass by shallow and dense prey aggregations and reach for deeper and more homogenously distributed prey fields with higher encounter probabilities (Zamon
et al. 1996)—a finding which would be overlooked in a conventional, two-dimensional analysis of predator-prey distribution. Therefore, taking all three dimensions into
consideration is essential to understand fine-scale habitat use
or foraging behavior.
At present, it is also common to incorporate accelerometers into archival loggers. These devices measure acceleration, which is caused by earth’s gravity (static component)
and a change in the animal’s speed (dynamic component). It
usually records accelerations in three dimensions, the x-, y-,
and z-axes or surge, sway, and heave (Fig. 1a). When positioned on the head and/or jaw of a marine predator, accelerometers can provide information about rapid head
movements, indicating prey capture attempts (Naito et al.
2010; Kokubun et al. 2011; Gallon et al. 2013). However,
accelerometers are often deployed close to the animal’s center of gravity, i.e., in the center of the trunk, which is more
useful to record overall movement patterns (e.g., swimming,
resting, and flying). As such, tri-axial accelerometry can be
used to identify and quantify different behaviors and activity
patterns, and subsequently, put them in relation to energy
Bio-telemetry as an Essential Tool in Movement Ecology and Marine Conservation