Southern California Steelhead Spawning Observations in Two Dammed Rivers

The ecology of spawning Oncorhynchus mykiss (both anadromous and resident forms) is a topic of uncertainty in the southern California steelhead Distinct Population Segment.  The National Marine Fisheries Service (NMFS) initiated the use of a standard spawning ground survey protocol in 2009/2010 to conduct redd counts in southern California coastal drainages where endangered steelhead populations exist.  Surveys were conducted in the Malibu Creek and Ventura River watersheds after the first measurable precipitation on Dec 19, 2009, through May 28, 2010.  Index reaches in the Ventura River watershed (6.6-km) below Matilija Dam and the entire anadromous reach of Malibu Creek downstream of Rindge Dam (4.8-km) were surveyed bimonthly.  Spawning activity was collectively observed from Jan 25, 2010, through May 14, 2010, in the study rivers.  Stream temperature at redd locations ranged from 11.5° – 18.0° C at time of observation.  Spawn timing was distributed throughout the survey season, with approximately 46 percent of spawning activity observed 1 – 2 weeks after freshets.  The mainstem Ventura River streamflow (USGS 11118500) range was 36 – 68 cfs at the time of spawning based on estimated redd age, and Malibu Creek streamflow (USGS 11105510) range was estimated to be 18 – 39 cfs.  Redd size in the mainstem Ventura River (n = 20) ranged from 0.22 – 2.47m², while the redd size in Malibu Creek (n = 6) ranged from 0.48 – 2.36m².  Redd size in San Antonio Creek (n = 17), tributary of the Ventura River, displayed a narrower size range than what was observed in the mainstem Ventura River (0.35 – 0.86m²).  Season total spawning density (i.e., redds per kilometer) in the Ventura River watershed ranged from zero to 23.0 redd/km, while spawning density was 1.3 redd/km in Malibu Creek.  The findings from these early studies indicate that the amount of spawning is patchily distributed throughout both study watersheds and that the timing of redd construction is related to periods of elevated streamflow.  Redd size and direct observations of spawning O. mykiss suggest that anadromous and resident O. mykiss spawn concurrently in southern California watersheds.  Future studies are planned to extend the current understanding of steelhead spawning ecology at the southern geographic extent of the species' range.