Cramer Fish Sciences

     After the pulse of fry on March 1^st, catches of juvenile Chinook dropped off rapidly (see Rotary Screw Trap Summary #4 for more details), but we continued to mark all available juvenile Chinook (Figure 1).  A total of 161 Chinook salmon were tagged, and 3 fish had immediate mortality due to tagging (1.8%).  Of these tagged fish, 149 were held overnight to evaluate 24-hr mortality and tag retention.  Five fish failed to retain their tags giving us a 96.6% tag retention, an improvement compared to the last sampling period.  However, we observed an increased mortality after overnight holding.  Of the 149 fish held overnight, there were 14 mortalities (9.4%).  The fish were held in-river with livecars and net pens in a variety of configurations.  Mortality seemed to be unaffected by holding configurations, but further studies are needed to fully determine problems with in-river holding.

     Figure 1. Total Catch and Total Tagged Chinook Released at Caswell including mortalities (most after overnight holding).

     We contacted Northwest Marine Technologies (NMT), providers of the CWT equipment, to inquire about mortality due to tagging.  NMT assured us the mortality we had observed was not due to tagging.  Mortality associated with these types of CWT tagging operations is around 1%, they said.  However, we were interested in testing our tagging operations, so we used our equipment and taggers, and performed an experiment with 100 Feather River hatchery fish (60-73 mm FL) (Figure 2).

     Figure 2. Feather River hatchery fish in holding tank awaiting experimental coded wire tagging.

     We marked 50 individuals with full length sequential tags and compared with a control group (anesthetized, handled, untagged) of 50.  Upper caudal and lower caudal fin clips were used to distinguish between groups and fish were held together in a river-water fed external holding tank.  After 24- and 48-hours, no mortality was recorded in either group, which assured us tagging was an improbable cause of the observed mortality.  (We thoroughly cleaned our equipment before reinitiating coded wire tagging on the Stanislaus River to prevent cross contamination.)

     We have sent specimens to the regional U. S. Fish and Wildlife Service Fish Health Laboratory for analysis.  Further details will be available in upcoming updates.   

     Please review the final Detailed Study Plan for Coded Wire Tagging of Chinook Salmon in the Stanislaus River for more information on the CWT study.

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