The Influence of Holding Time and Mark Type on the Trap Efficiency Estimates for Juvenile Spring Chinook in the Chiwawa River, WA

The Chiwawa River rotary screw trap has been in operation since 1992.  Mark recapture trials are frequently carried out to estimate trap efficiency in order to calculate abundance estimates.  In 1997, Spring Chinook salmon Oncorhynchus tshawytscha in the Chiwawa River were listed as endangered under the Endangered Species Act in response to extremely low numbers of returning wild adult fish.  Low abundance of juvenile spring Chinook also increased the logistics in estimating juvenile fish abundance.   In order to obtain a sufficient sample size to conduct mark-recapture trials, fish are collected over a period of 1-3 days prior to marking and release upstream of the trap. A potential bias may exist if the number of days fish are held influences the probability of recapture.  Prior to 2007, mark-recapture efficiency trials were carried out using a combination of external marks (e.g. caudal fin clips or Bismarck brown dye).  Beginning in 2007, passive integrated transponder tags (PIT) were used as a mark and has the advantage of an individual mark versus the traditional batch mark type.  However, another potential bias may exist if the mark type (caudal fin clip versus PIT tag) influences the probability of recapture.  Paired groups of juvenile spring Chinook were PIT tagged based on the number of days held.  In addition, paired groups of fish were release with different mark types (e.g. caudal clips vs. PIT tags).  Results would suggest if biases in trap efficiency estimates are sensitive to the duration fish are held prior to release or mark type.