Researchers usually combine sonar surveys with close-up underwater video: sonar helps locate fish and promising habitat, then a remotely operated vehicle (ROV) can inspect the seabed for fish, eggs, and nest structures. Cameras and acoustic telemetry add evidence about behavior and movement, but no single method answers every question.
How researchers find a nest in deep water
- Survey a wider area. Echosounders and imaging sonar collect acoustic observations of fish and bottom habitat, helping researchers identify places to inspect more closely.
- Inspect candidate sites. An ROV carrying a camera can travel near the bottom and record visible fish, eggs, nests, and habitat. Unlike an acoustic detection, an image can provide direct visual evidence of a nest.
- Document what happens over time. Video can capture visible behavior, while acoustic telemetry can help track tagged fish and provide movement or timing information. These methods address different parts of the question rather than replacing visual inspection.
A Great Lakes study provides a direct example. Researchers surveyed Grand Traverse Bay with an ROV in December 2015, March 2017, and March 2021, at depths of 70–191 m. On 16 March 2021, they observed presumed unoccupied nests and one nest at 190 m containing a deepwater sculpin and a clump of eggs. The study authors reported this as the first evidence of where and how deepwater sculpins spawn in the Great Lakes. They also reported a surface-water temperature of 1.9 °C on that date; that measurement and the observed depths describe this field study, not general conditions for fish nests or equipment limits. Journal of Great Lakes Research study (2022).
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What each monitoring method can show
| Method | Evidence it can provide | Important limit |
|---|---|---|
| ROV with camera | Direct images of visible fish, eggs, nests, and bottom habitat. | What researchers can confirm visually depends on what falls within the camera’s view. The sculpin study demonstrates ROV observations at its surveyed sites, not the capabilities of every ROV. |
| Split-beam echosounder and imaging sonar | Acoustic observations of fish and habitat structure across survey areas. | Acoustic detections help characterize fish and habitat but are not, by themselves, visual confirmation of a nest. |
| Cameras, stereo video, and video on ROVs or AUVs | Visual monitoring of spawning aggregations; depending on the system, imagery may help assess species or size. | Animals must be visible and identifiable. A NOAA workshop report describes these as monitoring tools, not as a universal performance test. NOAA Technical Memorandum (2019). |
| Passive acoustics | Sounds produced by fish during reproductive behavior can help researchers detect, monitor, or locate species that vocalize before or during spawning. | It is not a direct method for imaging a nest and applies to species whose sounds can be detected. NOAA Technical Memorandum (2019). |
| Acoustic telemetry | Detection or fine-scale tracking of tagged fish, adding movement and timing evidence. | Results depend on tags and receiver or positioning coverage, as well as interpretation; telemetry does not directly show a nest. |
What sonar and video revealed about cod in Iceland
In a two-day cruise on 15–16 April 2009, researchers combined a split-beam echosounder, DIDSON dual-frequency imaging sonar, and an ROV at southwestern Iceland sites. Acoustic surveys showed greater Atlantic cod density and acoustic target strength at a transition between boulder/lava and sand than in either habitat alone. ROV video recorded fish rising above the bottom, behavior the researchers considered consistent with spawning. The study reports fish ascending 1–5 m above the bottom, average vertical swimming speeds of 0.20–0.25 m/s, and average spacing of 1.0–1.4 m. These are observations from that study and its surveyed sites; the paper describes spawning habitat and behavior, not documented cod nests. Atlantic cod habitat study.
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Telemetry can help researchers determine when tagged fish move through monitored areas, but its usefulness depends on the tagging approach and the coverage of receivers or positioning systems. A 2025 study of Arctic char and lake trout paired oviduct tags with acoustic transmitters in some fish, using fine-scale positioning to assess when and where tags were expelled. The authors note that spawning-telemetry approaches can depend on prior knowledge of spawning timing, location, behavior, and habitat. A tag’s movement or expulsion can contribute evidence, but it is different from seeing a fish guard eggs on video. Arctic char and lake trout telemetry study (2025).
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Choosing a method for the research question
The appropriate combination depends on what researchers need to establish. A question about whether a nest is physically present calls for visual inspection; a question about fish distribution or habitat may benefit from acoustic surveys; a question about individual movement or timing may call for telemetry. Cameras and passive acoustics can add useful monitoring evidence when fish are visible or produce detectable sounds. The cited sources do not provide a controlled comparison that ranks all these methods or establishes a universal survey protocol.
Quick Recap
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