PIT Tags in the Stream: Tracking Genuine Fish Movement or Rogue Tags?

Passive integrated transponder (PIT) tags are increasingly used to study fish movement, growth rates, behavior, and migration patterns.  This information is critical to further understanding population dynamics and movement patterns of salmonids with complex life histories.  Although retention of PIT tags in live fish is generally high, tags can sometimes be expelled by live fish or released after death.  Shed tags can affect analysis, especially when tag detection cannot be definitely linked to a live fish or if tags are transported by flows, mimicking fish behavior such as downstream movement by anadromous salmonids.  To understand how shed tags act in the environment and how they affect analysis, we examined the fate of 148 PIT tags (48 in 2007 and 100 in 2008) intentionally deposited in a small study stream in central California.  We use mobile readers to survey the stream annually for three years to determine the movement and persistence of tags.  Tag survival was a function of time and distance traveled and was dependent upon flow and habitat type of initial tag location.  As the number of PIT tags used in a particular stream increases over time, shed tags will increasingly complicate analysis and interpretation of true fish movement.