Using 2-D Hydraulic Models and Direct Observation Methods to Describe Differences in Salmonid Fry Density over a Range of Habitat Types, Trinity River, California

Juvenile rearing habitat has been identified as a limiting factor for salmon production on the Trinity River, CA.  Understanding densities at which juvenile salmon occupy a range of habitat types would help inform a large scale rehabilitation effort.  Data that could describe the distribution of fish across a range of hydraulic conditions for use in a fish production model was needed.  This study was developed using existing 2-D hydraulic models to distribute sampling throughout the range of available habitat types where direct observation was feasible.  Fish-use data, as well as physical characteristics, were intentionally collected at the same spatial resolution as the habitat estimates made by 2-D hydraulic models.  Areas of homogenous physical characteristics, determined as having similar depth, velocity, and distance to cover, were located and sampled for rearing juvenile salmonids. Simultaneous dual-diver observations were conducted at each sampling location. This sampling scheme resulted in observations of density across a broad range of habitat types.  During the emergence and rearing periods of 2013 and 2014 samples were conducted at over 4,500 locations; over 57,000 individual juvenile salmonids were observed.  Descriptive analysis show some of the differences in observed density for species, habitat types, time of year, and between years.