Return of the King Salmon: Dispersal Patterns for Spawning Cedar River Chinook Salmon after Circumvention of a Century-Old Migration Barrier

In rivers where anthropogenic migration barriers have been removed or fish passage facilities have been constructed after extensive periods of obstruction, very little research and monitoring has been conducted to determine the relative abundance, origin (hatchery or wild), or dispersal characteristics of salmonid colonizers and their source populations.  In September 2003, Seattle Public Utilities (SPU) completed construction of a fish passage facility that circumvented Landsburg Diversion Dam on the Cedar River, WA.  Chinook salmon (O. tshawytscha) were given voluntarily access to approximately 33 km of mainstem and tributary habitats for the first time in over a century.  Chinook used the fish passage facilities and spawned above the dam in all years of the study (2003-2010).  The percentage of total basinwide redds that were observed above the dam generally increased over eight years of study.  Males consistently outnumbered females with a mean sex ratio of 3.1 to 1.0 for survey years 2003 to 2009 (range = 1.3:1 – 4.7:1).  Redd counts indicated that the majority of Chinook females passed above the dam, spawned in mainstem habitats within 4 kms above the dam.  However, in most years, some Chinook redds were spawned at the upstream extent of available spawning habitats (rkms 53-55) and, in 2007 and 2010, a small number of redds were identified in tributary habitats above the dam.   Chinook redd observations below the dam consistently occurred before Chinook had initiated spawning above the dam and, in most years, before the first Chinook had been passed upstream of the dam.   Annual spawn timing distributions and peak spawning activity below the dam were noticeably earlier than those above the dam with the exception of 2007.  Typically, a small contingent of Chinook continued to spawn below the dam for up to two weeks after Chinook completed spawning above the dam.  The percentage of hatchery origin fish passed upstream of the dam exceeded the percentage of hatchery origin carcasses collected below the dam in all survey years.  In the first two years after fish access was restored (2003 and 2004), upstream hatchery migrants outnumbered wild upstream migrants by more than a factor of two.  However, wild fish outnumbered hatchery fish in the remaining survey years (2005-2010).  We believe that the results of this study will help support and inform future studies and management decisions related to barrier removal or circumvention in river systems inhabited by salmonid populations.