Laboratory and Field Experimental Validation of Two Different Predation Sensors for Instrumenting Acoustic Transmitters in Fisheries Research

Abstract

We validated the performance of two types of predation sensors on acoustic telemetry transmitters in a combined field and laboratory study design using juvenile Atlantic Salmon Salmo salar and a predator large Brown Trout Salmo trutta. One tag model was outfitted with an acid‐sensitive predation sensor, and the other was equipped with a tilt‐based orientation sensor. We found similar response times for the two tag types to identify a predation event (time between known ingestion of a tagged fish and triggering of the predation sensor). However, the orientation sensor model was more accurate in laboratory trials, correctly identifying 100% of events in which predators were fed the tagged fish, whereas correct identification of predation events was 50% for the acid‐sensitive sensor in similar trials. The acid‐sensitive tags were smaller (0.65 g in air), and all 10 were eventually expelled by predators in the laboratory (2–22 d after tag ingestion). By contrast, only two of five tags with orientation sensors (1.4 g in air) were expelled in a similar period (13 and 16 d after tag ingestion). Both tags had distinct benefits: acid‐sensitive sensor tags were very small, and orientation sensor tags were seemingly more accurate and transmitted raw orientation data to receivers for postprocessing.

Acoustic telemetry predation sensors reveal the tribulations of Atlantic salmon (Salmo salar) smolts migrating through lakes

“…In our study, six of the twelve smolts that died, died without the predation sensor indicating predation. This might be due to mortality caused by surgery, tag burden, or natural mortality, but during a parallel laboratory validation study of the sensor, a false‐negative rate of 50% was observed (Lennox et al 2021), corresponding to the 50% unknown mortalities in our field study. False positives (i.e.…”

“…Accounting for possible alternative causes of mortality will be important, although identifying avian predation requires further tag validation (i.e. for aquatic birds such as ducks and cormorants) and may be impossible for fully terrestrial species such as raptors and corvids (Lennox et al, 2021). Factors such as water temperature, conspecific density, predator density, and water flow will affect smolt migration patterns (Thorstad, Whoriskey, et al, 2012), contributing to interannual variation in survival that must be accounted for in a multi‐year study.…”

“…If a tag disappeared or was detected by a single receiver for the remainder of the study period, the mortality was assumed to result from other mortality, although it may have been a false negative. Indeed, laboratory evaluation of these tags found that only 50% of the predation sensors recognised predation when euthanised smolts were force fed to trout (Lennox et al, 2021). The location of tag switch was defined as the site of mortality for predated fish, whereas the location of the last receiver to register a tag that was subsequently lost or stopped moving was designated as site of non-predator mortality.…”

“…Accounting for possible alternative causes of mortality will be important, although identifying avian predation requires further tag validation (i.e. for aquatic birds such as ducks and cormorants) and may be impossible for fully terrestrial species such as raptors and corvids (Lennox et al, 2021).…”

Design of an Optimal Testbed for Tracking of Tagged Marine Megafauna

“…We consider the specification of an actual acoustic tag manufactured by Thelma-Biotel Inc., Trondheim, Norway [10]. These acoustic tags are used globally in applications to monitor fauna in marine environments and to track migration patterns [35], [36]. The mobile node is an acoustic tag (model: ID-HP16) emitting 69kHz single-tone signals of intensity 158dB re 1µPa @ 1m.…”