Rotary Screw Trap


Smolt trapping using a rotary screw trap 

RCD monitors steelhead and salmon populations by collecting data on the number of smolts migrating to the ocean each year.  A smolt is a young steelhead or salmon that is migrating downstream to begin the saltwater phase of its life.  We use an 8-foot rotating fish trap located in the Napa River to catch, measure, and release these fish in order to generate population estimates and collect genetic samples.  In addition to salmonids, we also collect data on other Napa River fishes.


  • Trap is installed in the Napa River in spring and remains in place until flows diminish 
  • RCD biologists and volunteers check the trap daily to remove debris and process the catch
  • Salmonid smolts are measured, weighed, fin clipped for genetic analysis, then released
  • Other fish species are counted and released
  • Some smolts are marked and released upstream to calculate trapping efficiency 

Smolt Monitoring Survey Reports