Abstract
BackgroundAcoustic telemetry is now a key research tool used to quantify juvenile salmon survival, but transmitter size has limited past studies to larger smolts (> 130 mm fork length). New, smaller, higher-frequency transmitters (“tags”) allow studies on a larger fraction of the smolt size spectrum (> 95 mm); however, detection range and study duration are also reduced, introducing new challenges. The potential cost implications are not trivial. With these new transmitters in mind, we designed, deployed, and tested the performance of a dual-frequency receiver array design in the Discovery Islands region of British Columbia, Canada. We double-tagged 50 juvenile steelhead (Oncorhynchus mykiss) with large 69-kHz tags (VEMCO model V9-1H) and small 180-kHz tags (model V4-1H). The more powerful 69-kHz tags were used to determine fish presence in order to estimate the detection efficiency (DE) of the 180-kHz tags. We then compared the standard error of the survival estimate produced from the tracking data using the two tag types which has important implications for array performance and hypothesis testing in the sea.ResultsPerfect detection of the 69-kHz tags allowed us to determine the DE of the 180-kHz tags. Although the 180-kHz tags began to expire during the study, the estimated DE was acceptable at 76% (SE = 9%) when we include single detections. However, 95% confidence intervals on steelhead survival (64%) were 1.5 × larger for the 180-kHz tags (47–85% vs. 51–77% for 69 kHz) because of the reduced DE.ConclusionsThe array design performed well; however, single detections of the 180-kHz tags indicates that under slightly different circumstances the DE could have been compromised, emphasizing the need to carefully consider the interaction of animal migration characteristics, study design, and tag programming when designing telemetry arrays. To increase DE and improve the precision of 180 kHz-based survival estimates presented here requires either an increase in receiver density, an increase in tag sample size (and modified transmitter programming), or both. The optimal solution depends on transmitter costs, array infrastructure costs, annual maintenance costs, and array use (i.e., contributors). Importantly, the use of smaller tags reduces potential tag burden effects and allows early marine migration studies to be extended to Pacific salmon populations that have been previously impossible to study.
Highlights
Acoustic telemetry is a key research tool used to quantify juvenile salmon survival, but transmitter size has limited past studies to larger smolts (> 130 mm fork length)
Our study demonstrates that 180-kHz tags can be used in the coastal ocean with reasonable increases in array cost relative to 69-kHz tags, given careful attention to array design and tag programming
We found that the confidence limits on estimated survival were approximately 1.5 times larger for the 180kHz data relative to the 69-kHz data because of reduced detection efficiency (DE)
Summary
Acoustic telemetry is a key research tool used to quantify juvenile salmon survival, but transmitter size has limited past studies to larger smolts (> 130 mm fork length). Acoustic telemetry is a key research tool used to study where, and in some cases how, juvenile salmon die during the early phases of their migration and to estimate survival, but transmitter size has limited past studies to larger smolts [1]. As a consequence of the use of smaller tags, transmitter life, signal detection range, and (often) acoustic power output are reduced, limiting study duration and reducing the probability that a tag will be detected in the vicinity of a receiver.
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