User:Doug pincock/sandbox

Overview
Acoustic tags are small sound-emitting devices that allow fisheries researchers to monitor the movements and/or behavior of free swimming fish. Similar in concept to a radio transmitter. they incorporate a piezoelectric transducer which converts electric signals to acoustic which are transmitted through the water. These are typically cylindrical as this shape provides the smallest solution for roughly omnidirectional. transmission. The necessary acoustic to electric conversion in a receiver is accomplish with a hydrophone which typially incorporates similar transducers to those used in transmitters.

Acoustic Tags are produced in many different shapes and sizes depending on the type of species being studied, or the type of environment in which the study is conducted. For oceanic environments, frequencies less than 100 kHz range are often used, while frequencies of several hundreds of kilohertz are more common in for studies in rivers and lakes.

Size, Frequency and Range Trade Offs
The choice of transmission frequency is a trade off between sound absorption losses which increase with frequency and transducer size which decreases with increasing resonant frequency. . For example, a nominal 1/2 inch (12.5 cm) diameter cylindrical transducer is resonant at around 75 kHz. As a result, almost all tags used today have a diameter on the order of 15 cm. or less and operate at frequencies over 65 kHz. On the other hand, the smallest tags incorporate much smaller transducers and hence need to operate at higher frequencies significantly reducing range potential due to higher acoustic absorption at these frequencies. The frequency dependence of ambient noise is also factor; for example, depending on wave conditions, noise level decreases with frequency up 50 to 100 kHz above which thermal noise, which increases with frequency dominates.

Depending on the battery power available, desired battery life and electrical to acoustic efficiency, The acoustic output of tags on the market ranges from little less than 1 milliwatt (141 db re 1 μPascal @ 1 metre) to 100 milliwatts (161 db re 1 μPascal @ 1 metre) or more. Using the passive sonar equation and typical values for absorption in slat and fresh water one can calculate the theoretical ranges shown in the table below to give some idea of range expectations under some typical scenarios. It should be noted that actual range can vary significantly if conditions differ from the assumptions used (See table caption) and, in particular can be significantly less when noise is greater or, as often occurs in fresh water, absorption is higher.

Background
There is the need to encode the transmitted information in some way so that individual tagged fish can be identified. Depending on the application, the requirement may be only be for a small number of unique ID codes (tens or hundreds) or for a very large number (thousands or even hundreds of thousands).This information, sometimes along with sensor data, needs to be coded into the transmissions.

Unfortunately, conditions for transmission of acoustic signals underwater are orders of magnitude worse than for radio signals in air. In particular. losses are high and distortion significant with multipath and dopller shifts being the main issues. leading to a requirement that transmissions be pulsed and use discrete modulation methods -- i.e. On-off keying, Phase-shift keying (PSK) and/or Frequency-shift keying (FSK). Pulsed transmissions are, of course, also dictated by the need to conserve battery power.

Coding Method Classification
Of the three keying methods mentioned above, On-Off Keying was universal in earlier tags and is still very common but, in many cases enhanced by the use of Phase-shift keying (PSK) or Frequency-shift keying (FSK). FSK has not often used in fish tags -- the likely reason being that it contradicts the use of a single crystal to control transmitter frequency and requires a receiver either with broader bandwidth or multi-channel capability.

Coding methods in use today can be divided into three broad classes:
 * 1) Pulse Repetition Rate (PRR) Coding. This simplest form of On-Off Keyingtransmits of a repeated series of identical pulses, each pulse containing a number of cycles of the transmission frequency, with the interval between pulses representing the information being coded -- usually a unique ID code for each fish in a study.
 * 2) Coded On-Off Keying. This approach also transmits identical pulses in a burst followed by a long silence with the interval between successive pulses representing a number of binary digits. For example, one early scheme (Vemco 4k) uses a string of seven pulses (i.e. six intervals) with three of the intervals coding four bits creating (4096 possible codes). The other three intervals are for synchronization and purposes and error detection bits. More elaborate schemes exist encoding up to 15 to 20 bits or more --  permitting tens of thousands or even millions of unique codes to be represented. In some cases, some of the available bits are allocated to sensor data -- obviously at the expense of number of IDs available.
 * 3) PSK or FSK Burst. This approach transmits a series of bits containing synchronization, data and error detection bits in a very short time -- a few milliseconds or less followed by a silence long enough for multipath echoes to die down. The idea is that all of the bits can be decoded before the first distorting echoes arrive

ID Space versus Collisions
Ideally, one would like to have a virtually unlimited number of tag ID codes so that no two tagged fish anywhere in the world would have the same ID. This is particularly important in the case of fish that might travel large distances with tags having a long battery life. This leads to a requirement for hundreds of thousands or more unique IDs. Clearly, this is not possible with Pulse Repetition Rate Coding and, to date, computing limitations in battery powered receivers make it unrealistic for PSK Burst Coding. Coded On-Off Keying, on the other hand, can allow sufficient codes but at the expense of significantly long transmission -- more than one second. As a result, this approach runs into difficulties if the number of transmitters present at a receiver at a given time is too large due to transmission interfering with each other (commonly referred to as collisions). The rate of collision is proportional to the "tag density (number of tags present times length of each transmission divided by the length of time between transmissions) and,broadly speaking, the more collisions there are the less frequently each tag is successfully detected. At the extreme, no tags will be detected at all. For example, with coding suitable for ocean environments, 95% percent of transmission are successfully decoded when tag density is 5%; 39% for a tag density of 50% and 1% for a tag density of 200% . With a typical coding scheme and transmission rate of one code per 60 seconds, the aforementioned tag densities correspond to the simultaneous presence of roughly 2, 8 and 30 tags respectively.

With Pulse Repetition Rate of PSK Burst Coding, a far greater number of tags can be simultaneously present but at the expense of far fewer available ID Codes.

Examples of Currently Available Tags (Non Sensor)
The following table shows some examples of currently available tags of illustrating frequency, power trade offs discussed above. Information is taken from manufacturers' data sheets and tags are listed in order of increasing transmission frequency. In almost all cases for each entry, the vendor offer other tags with the same transmission frequency and coding but providing various trade offs on size (including diameter), power and battery life.

Sensor Tags
Many tags include environmental and physiological sensors. Currently, there are commercial products containing one or more of the following sensors:
 * Depth (using a pressure sensor)
 * Temperature
 * Acceleration (allowing various types of activity to be monitored)
 * Conductivity/Salinity
 * Heart rate
 * Muscle activity
 * Tilt

The table below, provides examples of some of these commercial products. For each vendor, the smallest tags offered are shown. In most, if not all, cases larger models offering increased power and or battery life are also offered. Listings are in alphabetical order by vendor and specifiications are those shown on vendors' data sheets.

Tag Attachment
Acoustic tags can be attached to, gastrically inserted in or surgically implanted into fish (or almost any aquatic life).

Several different types of methods are used to attach the tag to the fish. The tag may be embedded in the fish by cutting a small incision in the abdominal cavity of the fish (surgical implantation), or put down the gullet to embed the Acoustic Tag in the stomach (gastric implantation). While less common, external attachment is also used when, for example, the tag contains sensors that neeed to be exposed to the environment or the fish is so large that other attachment methods are difficult.

Methods and Applications
The following summarizes the methods typically used as well as some typical applications using each.

Mobile Tracking
This involves following tagged fish from a boat using a directional receiver -- usually incorporating an directional hydrophone or an Ultra-short baseline array so that the operator or firmware can monitor the direction from the tracking vehicle to the tagged fish. Examples of portable receivers used for this type of application include the Vemco VR100 and the Sonotronics MANTRAK. Currently, there are no commercial products incoprorating an ultra short baseline array

In the early days, when equipment was more limited and less was known of fish behaviour, this was the predominant approach. However, these studies were very limited by the requirement to follow the fish from a boat with the result that few individual fish were tracked for more than a few days. Since the 1990s, the so-called Passive Tracking approached described below have been by far the most common.

More recently, there has been a growing interest in the use of mobile tracking using autonomous vehicles and even marine animals as platforms for receivers to detect fish in the open ocean where no receivers are installed.

Passive Tracking Receivers and Data Recovery
Basically, all receivers used in passive tracking are similar in that they employ omnidirectional hydrophones. The differences lie in how they process detections and how the data is accessed by the user. The following summarizes the approaches used:


 * Autonomous Receiver with detections logged in internal data storage. Examples include the Vemco VR2W and VR2Tx, the HTI Model 300, the L0tek VHS2000, the Technologics JSATS Autonomous Receiver and the Sonotronics MinSUR . The advantage of this approach is that receivers can be deployed anywhere and the disadvantages that the receiver needs to be recovered to access the data and, depending on location, this can be difficult especially in locations where it is impractical to attach them to buoys.. Receiver recovery is often facilitated through the use of an Acoustic release. One commercial product (the Vemco VR2AR has an integrated acoustic release.
 * Wired Receivers where the area being monitored is relatively small making it feasible to run cables from the installed receivers to a central site. The advantage of this approach is immediate access to the data but this can be outweighed by cabling costs which can be substantial. A number of cables receivers intended for use in fine positioning applications (See below) allow for the connection of a number of hydrophones to a single receiver/processor on the surface. Examples include the HTI Model 290, the Lotek MAP 600 and the JSATS Cabled Receiver. Others, for example the Vemco VR2c and the Thelma Biotel TBR 700 Realtimeconsist of a single hydrophone and integrated receiver underwater with processed detections transmitted over a cable.
 * Autonomous Receivers with Remote Communication Capability. These function similarly to the Autonomous Receivers described above with the added feature that data can be accessed remotely, fby various methods, for example through an Acoustic Modem communicating to a surface vessel (e.g the Vemco VR4-UWM) or, for receivers attached to buoys on the surface, by satellite or by radio or cell phone (e.g. the Lotek WHS3000. Also of note are a class of receivers which use remote communication for status reporting and limited control functions, for example the Vemco VR2TX and the Vemco VR2AR

Migration Monitoring
This approach uses lines of receivers to monitor movements of tagged fish along a coastline or in a river. The idea is to provide complete receiver coverage at various points along the anticipated migration path and log individual fish as they progrees through the system.

Typical migration monitoring applications and citations of representative studies are listed below:
 * The Pacific Ocean Shelf Tracking (POST) network, one of the earlier systems using acoustic tracking to monitor large scale migrations
 * Various studies on salmon smolt survival as they descend the Columbia River (e.g. Colotelo et al )

Aggregation Monitoring
This approach comes into place where the bahaviour of fish (presence/absence, sensor readings, etc.) aggregating in a particular area is being studied. In many cases each aggregation point of interest would only have a single receiver.

Typical aggregation monitoring applications and citations of representative studies are listed below:
 * Monitoring fish behaviour and response to Fish Aggregation Devices (FAD) in stationary locations or drifting in the open ocean.
 * Monitoring spawning behaviour in coral reefs.
 * Monitoring habitat use around seamounts.

Fine Positioning
This approach is used when one needs to know detailed movements of tagged fish in a articular location (lake, around an obstacle such as a dam, etc.). It involves the installation of an array of receivers in known locations with spacing such that each transmission form each fish is detected by a number f receivers. Fish position is then calculated by triangulation based on time of arrival of each transmission at each receiver. With well designed and installed systems, 2 or 3 dimensional positioning accuracy on the order of one metre (or better in some cases) can be achieved.

Typical fine positioning applications and citations of representative studies are listed below:
 * Monitoring tagged fish approaching diversion and guidance structures at hydropower dams so that operators and regulators can evaluate specific migration pathways used by the fish (most often salmon smolts), identify where fish mortality occurs and assess fish behavior in relation to hydrodynamic conditions and/or any other environmental parameters.
 * Habitat use, movements and site fidelity in estuaries, in Marine Protected Areas , around wind farms , etc.
 * Study of individual fish interactions (for example, spawning behaviour )