Acoustic Tagging Project

Frequently Asked Questions

Q: Do the receivers constantly listen?
A: Yes, the receivers are constantly listening for the transmitters. Anytime a fish implanted with an acoustic transmitter (or "tag") enters the detection range of a it will be recorded. The detection range in Gray's Reef National Marine Sanctuary is generally estimated to be 200 meters in any direction.

Q: Do the transmitters constantly ping?
A: The transmitters do not constantly ping, but they do have a consistent system. They will send out a signal every 120 seconds, whether or not they are within the 200 meter detection range of a receiver.

Q: Do the tags harm the fish?
A: This is a very important question that specific studies have been designed to answer. When fish injured, or stressed, they behave differently. They may refuse to eat, swim, school, mate, or otherwise behave as we would expect them to. Studies suggest that fish that survive the initial tagging recovery period of about 48 hours will return to normal behavior. These studies have also shown that tagged and untagged fish do not exhibit any difference in behavior.

Q: Does the fish feel anything when the tag transmits the "ping"?
A: Several studies have found no evidence that a fish feels anything from the transmissions. Since fish cannot tell us the answer to this question, we have to infer it from their behavior. Observations made during these studies have shown no alteration in behavior or any response to the ping. Also, these pings occur at a frequency of 69 kHz and have a very small power output, suggesting that the signal is too weak to be felt by the fish.

Q: Does the surgery harm the fish?
A: Surgical implantation of the acoustic tags is not without impact to fish. Surgery requires longer fish handling times, more invasive procedures, long initial recovery periods, and the risk of infection. However, attaching the tags externally, also impacts fish. External tagging can disrupt the fish's balance, cause abrasions or the tag can snag on objects. Also, external tags can affect swimming speed and energy expenditure by causing drag. Comparing the different attachment alternatives, we decided that surgically implanting tags was the preferred option.

Q: How far away can we monitor a tagged fish?
A: The distance from a receiver that a tag can be detected depends upon a number of variables. Weather, visibility, and currents can all dramatically alter the receiver detection range by changing the acoustic environment. Waves, boat motors and noise from other organisms can also interfere with a tag's acoustic signal. Finally, underwater features such as reef ledges, wrecks, artificial reefs, and other underwater structures can block a signal. To determine what detection range can be expected in a particular area, a range test must be done. Our range tests determined that we can reliably detect fish about 200 meters away.

Q: Can we hear the transmitters?
A: You cannot hear it from inside the fish. The tags emit a signal at a frequency is barely detectable by human ears. If a transmitter is held close to your ear with no other sound around, then you might faintly here a clicking noise.

Q: How long do the transmitters last?
A: The life span on a transmitter varies with the ping rate, frequency, storage time, and power output. The tags used at Gray's Reef have an estimated life span of about two years.

Q: How do we collect the data?
A: The receivers collect the "pings" from individual fish when they are located within the detection range of a receiver. This data point is recorded in the receiver's flash memory software, along with the date and time the event occurred. The receivers must be brought to the surface to retrieve the data. For this study, the receivers are collected by divers about every 3 months. The data is then downloaded from the receiver to a computer and then the receiver is placed back into the field. Once the data is on the computer, it is ready to be analyzed.

Q: How do you identify the fish on the data?
A: Each fish's tag sends out a unique signal. When the signal is picked up by the receiver, the receiver "recognizes" the signal and reports the corresponding tag id, along with the date and time of event.