A fate assessment key for acoustically tagged tagged fish

Acoustic telemetry allows for automatic tracking of undisturbed aquatic animals for long time periods with high spatiotemporal resolution. Critical to the analyses of such telemetry data is to separate signals representing the tagged animal from signals that were sent after the animal suffered predation, fishing or other mortality, or simply expelled a tag that then became stationary on the bottom. In field studies this is not a trivial task, as the true fate is not known unless the tagged animal is re-sighted. The researcher therefore typically has to judge the fate on the basis of the patterns in the signal detections, a process that can easily become very subjective. In this study a dichotomous fate assessment key for 310 acoustically tagged sea trout (Salmo trutta) was developed, allowing for replication of the fate assignment by independent investigators. The key has ten fate categories when used with tags containing depth sensors, the highest resolution that can be deduced from the data. The key helps identifying how the data from each individual fish can and cannot be used in subsequent analyses, for example survival analysis. The logic behind allows the key to be tailored to studies of other aquatic species as well.