Editorial Feature

Shark Migration Monitored with Underwater Robots

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An Oceanographic Telemetry Identification Sensor (OTIS) is a remote-controlled device and is at the forefront of research in marine and robotic engineering for being able to detect multiple sand tiger sharks (Carcharias taurus) off the Maryland coast. The device is designed with an array of acoustic receivers that are capable of recognizing the signals transmitted by the shark transmitters as they move along the coastal waters. This technology involves the use of an acoustic transmitter, 34 pop-off satellite archival tags for storing data about the shark’s journey and automatically releasing the data for transmitting the location signal and a VEMCO mobile transceiver (VMT) for transmitting and receiving information to inform its location and checking for the sounds of other marine mammals with acoustic tags.

Researchers can determine the water conditions preferred by sharks for swimming during their migrations with the help of OTIS. This glider can also travel farther from the locations of the static receivers to gather information on oxygen levels, water clarity, and temperature. The following video by SMRU Instrumentation animates the application of an animal-borne oceanographic sensor to track animal migration patterns and provides a general idea of how this underwater technology works to study marine biology.

Shark-Inspired Aquatic Robot - Design and Development

Sharks are known for their tendency to display unpredictable behaviors. However, having said that, their behavior is still not completely understood because it is difficult to observe shark movements in a field setting. This gap in our knowledge brought the idea of developing bio-inspired aquatic technology that could help provide more insight into shark migration and shark behavior. A study by Long J.H. et al (2011) examined the mechanical properties and anatomy of sharks by focusing on the vertebral columns (also known as the backbone) and developed two types of biomimetic vertebral columns (BVC) that include:

  • The vertebral column in which only the shape of the vertebrae was changed
  • A vertebral column designed with an invertebral joint that has a varying axial length

In the following video, Dr. Long, a Professor at Vassar College and his team introduces the study of vertebral columns of a shark to help build an underwater robot that will help monitor shark behavior and migration.

 

During the testing period in this study, at physiological bending frequencies, the viscoelastic properties of the BVCs were compared with that of the sharks. These BVCs were then used to design a propulsive tail that comprises a rigid caudal fin and a vertical septum. A surface-swimming robot that was designed in this study was based on a biological system (e.g., a shark) that uses the propulsive tail as a propeller. From a functional perspective, the swimming speed of the robot is increased when the BVCs become stiff. Moreover, the robot can acquire a longer stride length with the help of stiffer BVCs.

It has been observed from the developed BVCs that their mechanical properties measured by the loss of moduli at bending frequencies and curvatures can be varied by changing the following parameters:

  • Shape of the vertebrae
  • Length of the intervertebral joint
  • Material properties of hydrogel used for making the intervertebral joint

BVCs have the ability to act as propulsive elements in swimming robots like MARMT and Tadro4. Simple pitch oscillation obtained from a servo motor can be converted into a bending wave, which drives the caudal fin along a lateral direction, by using BVCs.

Further observations with respect to the identified variables affecting the mechanical properties of BVCs include the following:

  • Design of biomimetic systems – The use of systems that are simpler than the targeted biological system can be extended over the targeted range of mechanical behaviors.
  • Control of mechanical properties- In a flexible matrix, the shape of the rigid elements is more significant than the properties of the flexible material or the shape of the rigid elements.
  • Control of reconfiguration- The flexible propulsive system is not capable of producing constant motions with respect to motor inputs due to the dependence of viscoelastic materials on strain and strain rate.

Tracking a Shark

Tracking sharks also helps with the sound management and conservation efforts for these animals. Forney C et al (2012) proposed a method for estimating the velocity, orientation, and position of a tagged shark in real-time using an autonomous underwater vehicle (AUV), Oceanserver IVER2 AUV, that has a receiver and stereo-hydrophone. The AUV, having three degrees of freedom, is a torpedo-shaped robot that is actuated using two fins for controlling the pitch, two rear fins for controlling yaw and a rear propeller for realizing locomotion. The antenna of the AUV consists of a built-in GPS receiver for providing latitude and longitude measurements at a rate of 1 Hz.

The study by Forney C et al (2012) involved experiments performed at a large pier in Avila Beach and Sea Plane Lagoon in Los Angeles. Avila Beach pier experiments include AUV tracking of a moving tag with a stationary tag and sensor characterization. The starting position of AUV was varied relative to the initial distance to the tag in order to ensure tracking during stationary and moving tag experiments. The AUV would automatically track the position estimates of tags, which are produced by a particle filter.

The experimental procedure involved fitting a leopard shark with acoustic transmitters as a basis for tracking much larger marine fish. The AUV was allowed to track and follow the tagged shark after it had been released. These experiments have been important in forming a foundation to demonstrate the ability of AUVs to track and monitor marine life on a long-term scale.

Future Research

The latest developments in the monitoring of shark life have involved the development of a state estimator to track and follow a tagged shark using a particle filtering algorithm. This algorithm includes correction and propagation steps for controlling the particle movement. Future research may involve the calibration of tag signal strength using both the Lotek system (a monitoring system that is used to record the vertical and horizontal movement of marine life) and an external sensor system. Valuable range measurements, required for tracking swimming sharks, can be obtained from this calibration. In addition, the reduction or streamlining of a hydrophone profile would improve the service life of the battery of an AUV. It would also be interesting to observe and understand how the introduction of underwater robotic technology is affecting the movement patterns of marine life, and in particular sharks. Furthermore, only through trying shall we understand how to integrate this technology into marine life without disrupting any behavioral patterns.

Sources and Further Reading

  • Long.J.H, Koob.T, Schaefer.J, Summers.A, Bantilan.K, Department of Biology, Vassar College, Friday Harbor Labs, University of Washington, David Geffen School of Medicine, University of California, Marine Technology Society Journal,2011,Volume 45,pp.119-128
  • Shark Migrations Studied with Underwater Robot
  • Forney C et al. Tracking of a Tagged Leopard Shark with an AUV: Sensor Calibration and State Estimation. 2012. Published in Robotics and Automation (ICRA), 2012 IEEE International Conference.

This article was updated on 6th February, 2020.

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