What acoustic fish tags do

Acoustic tags are small transmitters implanted in or attached to fish that send out sound signals underwater. Receivers stationed in the water pick up these signals and record when and where each tagged fish passes by. Unlike satellite tags that track fish at the surface, acoustic tags work entirely underwater and can follow fish movements in lakes, rivers, and coastal areas for months or years.

The tag itself is typically the size of a large vitamin pill — usually between 5 and 20 millimeters long — and weighs less than a gram. It runs on a battery and emits a unique acoustic code at regular intervals, often every 30 to 180 seconds depending on how long the study needs to run. A network of fixed receivers spread across a study area listens for these signals and stores the data on an internal memory card or transmits it wirelessly to researchers.

This method has become the standard for tracking fish movement in rivers during migration, monitoring how fish use different habitats, and studying survival rates through dams or other obstacles. Because the signal travels through water rather than air, acoustic tracking works in murky water, at night, and in conditions where visual observation is impossible.

Key Takeaways

  • Acoustic tags transmit unique sound signals that receivers detect and record, creating a detailed map of where a fish travels and when.
  • Tags are surgically implanted in the fish's body cavity or attached externally, and the fish typically recovers within hours and behaves normally.
  • A single receiver can detect tags from hundreds of fish simultaneously, and networks of receivers across a river or lake create continuous tracking data.
  • Battery life and tag size limit how long a fish can be tracked — smaller tags last weeks to months, while larger tags can transmit for a year or more.
  • Researchers use acoustic data to understand migration timing, survival through dams, habitat use, and how environmental changes affect fish populations.

How the tag gets into the fish

Acoustic tags are placed inside a fish through a surgical procedure performed by trained technicians, usually at a hatchery or research facility. The fish is first anesthetized with a chemical solution — typically tricaine methanesulfonate (MS-222) — which renders it unconscious and immobile for 10 to 20 minutes. Once the fish is sedated, a small incision (usually 8 to 12 millimeters) is made along the fish's belly, and the tag is inserted into the body cavity near the stomach.

The incision is then closed with a few stitches or surgical glue, and the fish is placed in clean, cool water to recover. Most fish wake up within an hour and resume normal behavior within a day. The entire procedure takes 5 to 10 minutes per fish. Survival rates after tagging are typically 95 percent or higher, though this depends on the fish species, tag size, and the skill of the technician.

For some studies, researchers attach tags externally using a harness or adhesive, which avoids surgery but usually results in shorter tag life because the tag can fall off or become fouled with algae. External attachment is more common in short-term studies lasting a few weeks.

How receivers detect and store the signal

Acoustic receivers are stationary devices anchored to the river or lake bottom or mounted on docks and bridges. Each receiver has a hydrophone — an underwater microphone — that listens for acoustic signals in a specific frequency range, typically 69 kilohertz. When a tagged fish swims within range (usually 300 to 500 meters, depending on water conditions and receiver sensitivity), the receiver detects the tag's signal and records the time, the tag's unique ID code, and the signal strength.

A single receiver can detect signals from hundreds of different tags at the same time because each tag has its own unique code, similar to how a radio station broadcasts on a specific frequency. The receiver stores this data on an internal memory card or, in newer systems, transmits it wirelessly to a base station on shore. Researchers download the data periodically — sometimes daily, sometimes weekly — and analyze it to track each fish's movement.

The range and sensitivity of a receiver depend on water clarity, temperature, and the presence of noise from boat traffic or industrial activity. In clear, quiet water, a receiver might detect a tag from 500 meters away; in murky or noisy water, the range might drop to 200 meters. Researchers account for this variation when placing receivers and interpreting the data.

What data acoustic tags reveal about fish behavior

Acoustic tracking data shows researchers exactly when a fish passed a specific location and how long it stayed in a particular area. By combining data from multiple receivers, researchers can reconstruct a fish's entire journey — for example, tracking a salmon's migration upstream through a river system, noting when it stops to rest, and recording whether it successfully passes through a dam or gets delayed.

This level of detail reveals patterns that other methods cannot capture. Researchers can see which fish survive a dangerous passage and which do not, how water temperature or flow affects movement speed, whether fish return to the same spawning grounds year after year, and how habitat restoration projects change fish behavior. In coastal areas, acoustic arrays have tracked fish movements between rivers and the ocean, showing how environmental changes in one location affect populations in another.

The data also helps managers make decisions about dam removal, water release schedules, and habitat protection. For example, if acoustic data shows that most juvenile salmon delay their migration when water temperature rises above a certain threshold, managers can adjust dam operations to maintain cooler water during critical periods.

Tag size, battery life, and tracking duration

The size of an acoustic tag determines how long its battery lasts and how much data it can store. Small tags (5 to 7 millimeters) typically transmit for 30 to 90 days before the battery dies. Medium tags (10 to 12 millimeters) can transmit for 6 to 12 months. Large tags (16 to 20 millimeters) may transmit for 18 months or longer. The trade-off is that larger tags are heavier and more invasive to implant, so researchers must choose a tag size that matches both the fish species and the length of the study.

Transmission interval also affects battery life. A tag that transmits every 30 seconds will drain its battery faster than one that transmits every 180 seconds. Researchers adjust the transmission interval based on how much detail they need — a study tracking salmon migration through a river system might use a 60-second interval to catch every movement, while a long-term coastal study might use a 180-second interval to extend battery life.

Once a tag's battery dies, it stops transmitting and the fish can no longer be tracked. The tag itself remains in the fish's body cavity and causes no harm. Researchers cannot retrieve tags from wild fish, so the tag is essentially a one-time investment per fish.

Acoustic arrays and receiver networks

A single receiver can track fish in a limited area, but researchers studying large rivers or lakes deploy acoustic arrays — networks of dozens or even hundreds of receivers spread across the study area. These arrays create a continuous detection zone that captures every tagged fish passing through, regardless of time of day or water conditions.

The Columbia River system, for example, has a network of over 60 acoustic receivers maintained by multiple agencies and universities. This array allows researchers to track juvenile salmon from the moment they enter the river until they reach the ocean, a journey of several hundred kilometers. Similar arrays exist in the Great Lakes, the Mississippi River, and many coastal regions.

Data from multiple receivers also allows researchers to estimate a fish's position more precisely. If two nearby receivers detect a tag at slightly different times, researchers can calculate approximately where the fish was when it transmitted. This triangulation method is less precise than GPS but provides far more detail than a single receiver could offer.

Limitations and challenges of acoustic tracking

Acoustic tags work only in water, so they cannot track fish on land or in air. They also require a network of receivers to be useful — a single tag in a river with no receivers provides no data. This makes acoustic tracking expensive for large-scale studies, which is why most projects focus on specific river systems or coastal areas where receiver networks already exist or can be built with grant funding.

Tag implantation requires trained personnel and specialized equipment, so acoustic tracking is typically done at hatcheries or research facilities rather than in the field. This means researchers can only tag fish that are already in captivity or that can be captured and held temporarily. Wild fish that are never captured cannot be tagged.

Water conditions also affect detection range. In very turbid water or areas with high noise levels, receivers may miss some signals, creating gaps in the tracking data. Researchers account for this by using statistical models to estimate where a fish likely traveled during periods when no signal was detected, but this introduces uncertainty into the results.

Frequently Asked Questions

Does the tag harm the fish?

The surgical implantation causes temporary stress, but most fish recover fully within 24 hours and show no long-term effects. Survival rates after tagging are typically 95 percent or higher. The tag itself is inert and does not move around inside the fish or cause infection if the surgery is performed correctly.

How far can a receiver detect a tag?

Detection range typically varies from 200 to 500 meters depending on water clarity, temperature, noise levels, and receiver sensitivity. In clear, quiet water, range may extend to 600 meters or more. In murky or noisy water, range may drop to 100 meters. Researchers test detection range in their study area before deploying receivers.

Can acoustic tags be used in saltwater?

Yes. Acoustic tags work in both freshwater and saltwater, though saltwater is more conductive and can affect signal propagation. Coastal researchers use acoustic arrays to track fish movements between rivers and the ocean, and some tags are designed specifically for marine environments.

What happens to the tag after the fish dies?

The tag remains in the fish's body and stops transmitting once the battery dies. If a dead fish is recovered and the tag is still transmitting, researchers may detect it in one location for an extended period, which signals to them that the fish is no longer moving. The tag itself does not decompose or cause environmental harm.

How much does it cost to tag a fish with an acoustic tag?

Tag cost varies widely depending on size and battery life, ranging from roughly $300 to $2,000 per tag. Add the cost of receivers (typically $3,000 to $5,000 each), installation, and personnel, and a multi-year acoustic tracking study can cost hundreds of thousands of dollars. This is why most acoustic research is funded by government agencies or large universities.