Double-tagged fish

From PTAGISWiki

Identifying double-tagged fish

A double-tagged fish contains two (or more) PIT tags. Rarely, a researcher might intentionally insert both a half-duplex (HDX) and full-duplex (FDX) tag into the same host animal, but in most cases fish are double-tagged accidentally, with either two FDX or two HDX tags. Two similar tags in close proximity (such as the body cavity of a fish) generally prevent either tag from being detected. Occasionally, one or the other tag is detected; rarely are both tags detected. To properly evaluate these detection events, either as a passive interrogation or the recovery of the tagged host, researchers must be aware that a single host fish was marked with two unique PIT tags.

Cross-referencing double-tagged fish

If a researcher double-tags a fish, s/he must flag both tag data event records with a "DB" conditional comment. In the rare case where the two tags are intentionally implanted in the fish, then each record should also reference the the other tag code type and ID, e.g.,

  23  000.00000AC723    1240   6500.0  15U  |RF MT AT DB|Also tagged with FDX code 3D9.1C2D413E0C 
  24  3D9.1C2D413E0C    1240   6500.0  15U  |RF MT AT DB|Also tagged with HDX code 000.00000AC723 

In the typical case where a second PIT tag is inadvertently implanted in a previously-tagged fish, then the tag record containing the "original" tag code must be identified as a "recapture" event with an "RE" flag, and both records should cross-reference the other tag code ID, e.g.,

  23  3D9.1BF3A927E1    1240   6500.0  15U  |RF MT AT DB RE|Accidentally double-tagged with 3D9.1C2D413E0C 
  24  3D9.1C2D413E0C    1240   6500.0  15U  |RF MT AT DB|Accidentally re-tagged with 3D9.1BF3A927E1

Preventing the inadvertent double-tagging of fish

Researchers are often unaware they've double-tagged a fish until they submit their data file to PTAGIS and are informed that the file contains one or more duplicate tags. The duplicate tag error occurs when the "original" tag code is detected in the host fish. In such a case, the number of duplicated records is likely only half of the number of fish double-tagged; in the other half of the cases, the "new" tag is preferentially detected and the original tag is suppressed. If the "original" tag is preferentially detected and reported to PTAGIS, then the researcher will likely lose any information and correlation with the "new" tag s/he has inserted into that same host animal. Similarly, if the "new" tag is preferentially detected, then the correlation with the "original" tag is lost, potentially compromising another researcher's study. It's critical that PIT tag researchers take all appropriate precautions to prevent double-tagging their own or other researchers' fish.