Mortality and long-term retention of passive integrated transponder tags by spring chinook salmon

I monitored long-term mortality and retention rates of passive integrated transponder (PIT) tags for 145,000 juvenile spring chinook salmon Oncorhynchus tshawytscha tagged as part of a large-scale tagging project. A total of 325 PIT tagged mortalities were collected during the 28-d study. Mortalities were collected, on average, 11 d after tagging, indicating that direct mortality due to tagging was rare. The mortality rate observed during the study was less than 1.0%. With the exception of the first day of tagging, 24-h retention was greater than 99.0%. A total of 113 shed tags were collected following the first day of tagging, corresponding to a 24-h retention rate of 98.1%. The overall retention rate for the study was 99.996%. I found no statistical relationship between the frequency of shedding and fish length or time of tagging. Although the relationship is not quantifiable, the frequency of sheds appeared to be linked to the experience of tagging personnel at the start of the tagging project and the continuity of personnel at a tagging station.