Coded Wire Tag Placement Affects Homing Ability of Pink Salmon

Abstract

Coded wire tags (CWTs) are routinely injected into the snouts of Pacific salmon Oncorhynchus spp. as fry to estimate contributions of tagged populations to spawning escapements and near-shore fisheries and to assess straying. Because not all fish are tagged, tag recoveries are extrapolated to include contributions of nontagged fish released at the same time and location. A key assumption in the extrapolations is that CWTs do not affect homing ability and therefore migratory pathways. We studied tag position within the heads of homing and straying adult pink salmon O. gorbuscha returning to Prince William Sound, Alaska, to test the hypothesis that poor tag position induces straying. Heads from straying and homing tagged adults were recovered in 1992 and 1994 and X-rayed. Tag locations were categorized into critical and noncritical areas, based on tag position relative to olfactory organs and nerves. In 1992, the 37 analyzed pink salmon that strayed were more likely to have tags in critical positions than the 154 analyzed fish that homed. In 1994, no such correlation was detected among the 66 fish tested, and we hypothesize that other factors influencing straying might have obscured a relationship. We interpret these results to indicate that tag position can affect adult homing ability in pink salmon tagged as fry.