Occurrence, fate, and confounding influence of ghost passive integrated transponder tags in an intensively monitored watershed

Abstract

Over the last three decades, passive integrated transponder (PIT) tags have been widely used to study fish populations. Interpretation of PIT tag detections, however, can be confounded by the presence of ghost tags, tags liberated when a fish dies. We used a combination of mobile antenna surveys, stationary antenna detections, and multistate mark–recapture modeling to assess the abundance and fate of ghost tags in a coastal California watershed. Accumulation of ghost tags from released hatchery-origin coho salmon (Oncorhynchus kisutch) smolts was substantial during California’s recent drought, with 2224 ghost tags identified during mobile reader surveys. Between surveys, PIT tags moved downstream a median distance of 346 m and a maximum distance of 1982 m. Stationary antenna array detections indicated that these movements occurred during high-flow events, concurrent with live fish movement. The multistate model estimated that, during winter, approximately 40% of tags were buried in the substrate beyond the read range of mobile readers. Failure to account for transport and burial dynamics of ghost tags can lead to biased estimates of fish abundance, survival, and movement.

Negative feedback concept in tagging: Ghost tags imperil the long-term monitoring of fishes

“…We infer that long-term research monitoring using passive telemetry often does not allocate funds for removing PIT tags from the environment, which may decrease the efficiency of the research and imperil the continuity of monitoring programmes, forcing scientists and stakeholders to change their monitoring schemes. The presence of ghost tags can be attributed to shedding and fish mortality [23,26,40,41]. Reproductive expulsion of PIT tags is common and seems to be a very important contributor to the observed accumulation of ghost tags.…”

“…We recorded a retention rate of 85 to 98% in asps, while in salmonids the retention rate during reproduction is often lower (65-92%) [27,[41][42][43][44][45]. Some salmonid fishes are semelparous and PIT tags used for tracking end up in the environment after their deaths [23]. Given that this technology is more frequently used to monitor…”

“…PIT tags are released into the environment by dead individuals due to their natural mortality. These PIT tags are retained in rivers for long periods and may represent a substantial source of false positive detections of fish that are already dead [23][24][25]. Fish reproduction can result in PIT tag loss from an individual due to expulsion of gametes [26,27] and fish can also shed them through insertion site [28].…”

“…Fish reproduction can result in PIT tag loss from an individual due to expulsion of gametes [26,27] and fish can also shed them through insertion site [28]. Both expelled and mortality related PIT tags are referred to in the scientific literature as "ghost tags" [23,29]. Ghost tags that accumulate at a study site within antenna range may interfere with effective detection of migrating individuals due to the inability of the system to read more than one PIT tag at a time-so called "PIT tag collisions" [30][31][32].…”

“…Ghost tags that accumulate at a study site within antenna range may interfere with effective detection of migrating individuals due to the inability of the system to read more than one PIT tag at a time-so called "PIT tag collisions" [30][31][32]. Although indefinite lifetime is one of the major advantages of PIT tags, in the case of ghost tags their long life can become disadvantageous and a source of scientific waste in the environment [23]. If the goal of a study is long-term monitoring, the presence of ghost tags may cause inefficient monitoring and antenna underperformance.…”