Background

Acoustic 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.

Results

Perfect 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, confidence intervals on steelhead survival (64%) were 1.5x larger for the 180 kHz tags (47-85% vs. 51-77% for 69 kHz) because of the reduced DE.

Conclusions

The 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.

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. 

File list:

Tags_metadata.csv
Detections_data.csv
Detection_efficiency_69kHz.r
Detection_efficiency_180kHz.r
Count_detections.r
Survival_using_180kHz_tags.r
generate_capture_histories.r
util.r

Instructions:

It is simplest to put all files into a single working directory for all R scripts to work. In addition to the packages listed at the top of the R scripts, Program Mark must be installed to run Survival_using_180kHz_tags.r (http://www.phidot.org/software/mark/rmark/). See publication for detailed analytical methods.

File descriptions:

Tags_metadata.csv-- each row represents a steelhead smolt from Seymour River hatchery that was acoustic-tagged in 2015. fish_id=concatenation of the 69 kHz and 180 kHz codes transmitted by the acoustic tags implanted into this inidividual; code_label_69 and tag_id_69=identification codes transmitted by the V9 69 kHz tags; code_label_180 and tag_id_180=identification codes transmitted by the V4 180 kHz tags; fl=fork length in mm; weight=fish weight in air in g; treatment_type=not populated; utc_release_datetime=date and time the tagged fish were released in universal coordinated time yyyy-dd-mm hh:mm:ss.

Detections_data.csv-- each row represents a recorded transmission from an acoustic tag implanted into a steelhead smolt from Seymour River hatchery in 2015. tag_id and code_label= unique identification  codes transmitted by the tag; receiver_sn= serial number of the receiver that recorded the transmission; detection_datetime= date and time the detection was recorded in universal coordinated time yyyy-mm-dd hh:mm:ss; subarray= name for the group of receivers deployed at the same general location (see Figure 2 in publication for map); subsubarray= adds detail to subarray name; position= for subarrays deployed in lines, this is a numerical position from east to west or south to north; kintama_nickname= a shortened name the receiver subarray and position; final= direction of migration; tag_type= tag implated (V9-1H or V4-1H); utc_release_datetime=date and time the tagged fish were released in universal coordinated time yyyy-dd-mm hh:mm:ss; tag_id2= unique identification  code transmitted by the other tag implanted into this individual; fish_id=concatenation of the 69 kHz and 180 kHz codes transmitted by the acoustic tags implanted into this inidividual.

Detection_efficiency_69kHz.r-- R script that calculates the detection efficiency of the 69 kHz V9 tags implanted into steelhead smolts from Seymour River hatchery in 2015.

Detection_efficiency_180kHz.r-- R script that calculates the detection efficiency of the 180 kHz V4 tags implanted into steelhead smolts from Seymour River hatchery in 2015.

Count_detections.r-- R script that counts the number of detections of each tag recorded at the Discovery Islands subarray and its component south and north subsubarrays.

Survival_using_180kHz_tags.r-- R script that provides empicial estimates of the conficence intervals on the survival estimates that would have been obtained had fish been tagged only with V4 180 kHz tags and had exited the study area prior to tag expiry (i.e. as in a regular study).

generate_capture_histories.r-- R function that generates a simulated frequency distribution for all possible capture histories and returns a data frame that can be passed to RMark.

util.r-- R function called from generate_capture_histories.r