Fish tags: no universal tag burden percentage

A small transmitter is still a biological intervention

The ratio of tag mass to fish mass is widely used when planning telemetry. It is useful information, but it does not establish that a device will have no effect on the animal. A study of movement must also consider whether tagged fish can represent the population whose behaviour the project intends to describe.

Shorgan and colleagues’ review and meta-analysis, published in the 2026 volume of Reviews in Fish Biology and Fisheries, examines devices implanted within the coelomic cavity. Higher tag burdens were associated with lower survival and retention. The authors discuss a value around 3% as a possible compromise, while explicitly warning against treating it as a general limit.

The finding calls for more than replacing one percentage with another. Neither the historical 2% rule nor an alternative number can validate a procedure for every species, life stage and study duration. Assessment should address the particular combination of fish, equipment and operating conditions.

Specify what the ratio includes

Calculate tag burden using comparable masses and explicit units. State whether device mass is measured in air or water. Also identify which components are included, especially where a fish carries more than one device. A percentage without this information may be difficult to compare with published work.

We suggest retaining individual fish masses rather than only a group mean. The same transmitter produces different burdens in different-sized animals. A decision based solely on the mean can conceal the smallest individuals and leave unclear why particular fish were included or excluded.

Record shape, dimensions and implantation position alongside mass. Buoyancy and available internal space are not captured completely by a relative mass calculation. A preparation sheet can bring these characteristics together with life stage, reproductive condition and planned environmental conditions without claiming that every factor can be reduced to one score.

Match evidence to the intended animals and setting

Before fieldwork, look for studies involving comparable species, stages, devices and durations. Shorgan’s synthesis concerns internal implantation; it does not automatically validate an external attachment. The available evidence also represents fish groups and environments unevenly, which limits broad generalisation.

Laboratory results can inform a field decision without reproducing every constraint encountered in the wild. Likewise, a related species may provide useful information but cannot establish identical tolerance in the target species. Keep that distinction visible in the evidence record rather than treating all tagging publications as interchangeable validation.

For work conducted in France, responsibilities, permissions and applicable conditions must be checked within the project’s own framework. A value used in a scientific paper does not become a French legal requirement simply by being published. Ethical and veterinary assessment should address the procedure actually proposed.

Choose the equipment from the study question

A larger tag may provide battery life or transmission range that serves a legitimate purpose. We recommend specifying the observation period, areas to be covered and intended decision before selecting hardware. The greatest performance available is not necessarily the performance required to answer the question.

This discussion may support a shorter observation period, a different sampling design or a revised minimum fish size. It may also reveal that the question cannot be answered satisfactorily with the proposed equipment. Recording that outcome allows the campaign to be redesigned before animals are committed to it.

Describe how selection affects interpretation. If only larger fish are eligible for tagging, their movements cannot automatically represent smaller members of the population. This limitation remains even where each procedure is technically successful. It belongs in both the study plan and the final account of the population observed.

Look beyond immediate recovery

Recovery after an intervention does not answer every question about subsequent effects. Where observation is feasible, follow-up may address retention, healing, body condition and behaviours relevant to the project. Define the endpoints, timing and limits before data collection so that an apparently favourable outcome can be interpreted consistently.

A pilot and appropriate comparison groups can be discussed with responsible specialists where additional evidence is needed. Their design should address a particular uncertainty and remain proportionate. This is not a recommendation to add animals automatically or to apply an unreviewed surgical procedure.

We also suggest recording the operator, equipment, incidents and procedural differences. These details support investigation of an unexpected outcome. Their presence does not establish causation, but their absence may prevent the team from distinguishing a device question from an implementation question.

Retain tagging effects in the interpretation

A missing signal alone does not establish death. Network performance, movement and device loss are among the explanations that may require examination. Testing detection and assessing biological effects are complementary tasks: satisfactory receiver coverage cannot demonstrate that tagging left the animals unaffected.

The final report should preserve selection criteria, known losses and observation limits. Fish recaptured in good condition do not necessarily represent every tagged individual. Conclusions should remain connected to the animals actually documented, including when the available results appear reassuring.

For the next campaign, turn unresolved issues into explicit planning questions. Does the species need better evidence on a particular device? Was monitoring long enough for the intended inference? Can a smaller tag answer the same question? These decisions should follow the uncertainty rather than a routine preference for familiar equipment.

Conclusion: justify the device rather than repeat a threshold

Relative tag mass is a planning indicator, not a universal assurance. Device choice should connect fish characteristics, equipment, environmental conditions and the purpose of tracking. Appropriate evidence and transparent limits support both welfare assessment and the interpretation of telemetry data.

Vetofish can contribute through our animal welfare services. Our work with aquatic environments helps teams prepare field campaigns, discuss unresolved questions and connect practical constraints to a documented decision about the proposed tagging approach.