Fish telemetry: checking what the receiver array can detect

A detection record is not a census

An acoustic receiver records signals it can recognise. It does not observe every fish around it, nor does its position alone define the full area an animal can use. A gap in the record may reflect departure, failure to receive a signal or an interruption in recording.

That distinction matters when tracking informs environmental management. Before interpreting a gap as disappearance, a failed passage or lack of habitat use, examine what the array could detect during the relevant period. A map showing receiver locations is necessary information, but it is not evidence that coverage was continuous or sufficient for every proposed inference.

What the open literature establishes

Kessel and colleagues’ 2014 review describes spatial and temporal variation in detection range in passive acoustic studies. It recommends range testing and assessment of changing conditions. A nominal radius taken from another site or a product description therefore cannot establish constant coverage in a field deployment.

Matley and colleagues’ 2024 review addresses the long-term effects of electronic tracking devices on fishes. Evidence is unevenly distributed across species and situations. Assessing receiver coverage does not remove that separate uncertainty: the behaviour being recorded comes from tagged animals. These two reviews address different parts of the evidence chain and should not be made to answer each other’s questions.

Write the decision question first

Vetofish proposes starting the array plan with a defined question. Is the study intended to document passage, presence within an area or a sequence of movements? These objectives may require different receiver positions and different ways of handling missing observations.

For each question, state what will count as evidence and what will remain unresolved. Detection near a passage does not necessarily establish that it was crossed. A single record may be insufficient to reconstruct a route. Equally, many detections elsewhere cannot fill a gap at the specific point needed to answer the management question.

This separates an exploratory movement study from a deployment designed around a particular decision. If that decision question emerges only after collection, assess it against the coverage that was actually documented. Theoretical equipment capabilities cannot retrospectively supply evidence that the deployment did not record.

Test the conditions that matter

Describe a range test through its equipment, positions, timing and site conditions. Signals from a known transmitter can provide a basis for comparing expected emissions with recorded detections. That comparison requires information about how the transmitter emits. A detection total without a defensible denominator has limited meaning.

The useful test is not necessarily the greatest distance achieved on a favourable day. A narrow passage, a change in water level or a location essential to the study question may warrant specific assessment. Retain the place and period of the test with its results, rather than turning the result into a guaranteed radius for the whole study.

Fixed reference transmitters can reveal variation through time, as discussed by Kessel and colleagues. They do not replace every other test or reproduce every position occupied by a moving fish. Their value is that part of the array’s detection capacity remains observable while the study proceeds.

Preserve the array’s operating history

Keep installation, confirmed operation, maintenance, removal and data retrieval distinct in the technical record. A map containing every receiver does not identify which were operating on a particular date. An interval without data should remain visible even if the hardware was later recovered successfully.

Relevant environmental measurements, such as temperature or salinity where collected for the site, may support investigation of changing detection performance. Their presence in a spreadsheet does not establish that they explain a gap. Dates, units and measurement locations are needed before that relationship can be assessed.

An availability timeline may communicate the deployment more clearly than an equipment list. It can identify confirmed intervals and periods whose status remains uncertain. This is a suggested reporting arrangement, not a universal standard for an acceptable percentage of coverage. Adequacy depends on the question and the study design.

Keep several explanations for silence

When a fish stops appearing in the record, first review array availability and the coverage relevant to that animal’s possible position. Then consider other explanations consistent with the evidence. Some cases will remain unresolved. Stating that outcome is preferable to assigning every silence to emigration or mortality.

Tagging effects also belong in the interpretation. Behaviour recorded from tagged fish cannot simply be assumed to represent all untagged individuals. Matley’s review does not establish that electronic devices have no effects in every situation. It supports examining the scope and limitations of the available evidence for the planned study.

Retain observations, analysis rules and conclusions as distinguishable parts of the record. If a rule changes, explain why and which results depend on it. A clear final map can hide variation in detection effort unless it is accompanied by the information needed to understand that variation.

Give managers a usable account

An operational report can state the question, the period actually covered, tests completed, interruptions and findings that support a defensible interpretation. Areas and periods with inadequate information should be equally visible. The manager needs to know which decisions the data support and which remain open.

Use unresolved points from one campaign as design questions for the next. A passage may need a more suitable test, reference monitoring may need extending or data recovery arrangements may need clarification. Each proposed change should respond to an identified uncertainty. Adding receivers is not automatically an answer to every weakness.

Conclusion: show what the array can establish

Acoustic telemetry becomes more useful when detection limits are assessed and retained with movement observations. Tracking the animal and evaluating the array are connected parts of the same argument. Neither a transmitter nor a receiver map alone guarantees a sound environmental conclusion.

Vetofish’s advice and support service can contribute to that planning. Our aquatic environment expertise helps connect the field question, relevant health information and the limits of the monitoring system to the intended management decision.