Acoustic biotelemetry is generally used to monitor those creatures living or moving in deep water or salt water habitats. Lotek offers acoustic systems to track a wide variety of marine inhabitants, such as salmon, trout, plaice, cod, crabs, sea turtles, and many others.
Seawater covers more than 70% of the Earth’s surface and is home to many of its creatures. Salt water presents unique challenges to those wishing to track marine wildlife. Radio waves are highly absorbed by salt water, making Radio technology a poor choice for sending messages through the ocean.
Fortunately sound waves (also known as acoustic waves) are not similarly impeded by seawater. Sound can also travel more than 4 times faster in water than in air allowing near real-time listening over long distances with the right tools. For these reasons, acoustic signals are the preferred communication tool for researchers who wish to track fish and wildlife in marine habitats.
As with radio technology, acoustic telemetry requires transmitters to send signals and receivers to hear them. In the case of acoustic telemetry an additional piece of equipment (hydrophone) is used as an intermediate between the transmitter and receiver. A hydrophone is a type of underwater microphone that receives acoustic signals, which are then either stored or converted into radio signals for rapid transmission through the air to a receiver.
Lotek has successfully expanded the boundaries of acoustic telemetry. Our powerful MAP System employs CDMA coding (similar to that found in cell phones) that allow the simultaneous detection of countless unique individual signals on a single acoustic frequency. Equally innovative is the MAP System’s ability to discern transmitter tags reliably, even in the acoustically noisy ocean environment.
Why so many acoustic telemetry systems?
Specific research questions, data requirements and environmental constraints have resulted in three different types of acoustic telemetry systems evolving for fisheries monitoring.
So what’s the long and short of acoustic telemetry?
Each system came about to address a specific application space. Outside the respective application space, each system will be less effective than the system designed for a particular environment. No one system performs optimally in all application spaces.
Tracking in rivers or estuaries characterized by high velocity, high noise or high traffic requires a more robust receiving system and corresponding encoding method. This results in relatively high equipment power requirement and equipment cost than a simple analog system. Simultaneous detection and tracking of multiple tagged individuals at high rates, potentially outfitted with depth, temperature or motion sensors under high noise conditions requires a more sophisticated digital receiving system similar to those used to manage cell phone traffic. Digital systems can pack a huge amount of data within a very short pulse, but require more power to detect and decode that information in real time. A three pulse MAP code is transmitted in less than ¼ second and allows for over 80,000 unique IDs.
In order to track the smallest fish and minimize the impact of tagging on survival, a system was developed to produce the smallest possible tag. The Juvenile Salmon Acoustic Telemetry System (JSATS) was originally developed to track thousands of salmon smolts in the Columbia River for the U.S. Army Corps. of Engineers. In an environment also characterized by high velocity, high noise and high traffic, the JSATS system uses a single 31-bit digital pulse emitted in less than 1 millisecond and provides over 65,000 unique IDs.
Tracking coastal migration patterns, characterized by relatively low noise and low traffic requires low cost receiving systems that use very little energy. Attributes of low cost and low power also make deployment of acoustic “gates” or “curtains” practical. A single gate can consist of upwards of 100 receivers, each requiring ships and crew for deployment and retrieval for data download or battery replacement. Under these conditions, a simple, single frequency analog receiving system is optimum. Individual transmitter identities are generated through a unique sequence of pulses in time. The total number of unique IDs is a function of the number of pulses that make up the ID and the number of available/allowable time delays between pulses. For example, one R64K codeset ID is made up of a unique sequence of eight pulses with the entire set consisting of over 64,000 unique IDs. In order to mitigate the impact of signal reflection or reverberation, the eight pulses are spread out in time requiring over two seconds for transmission of a single ID.
Unique Identity Codes
(The Long & Short of Acoustic Telemetry)
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