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Great Basin Redband Trout

2008 Results

 

The summer 2008 field season was the second of six sampling efforts to assess the distribution and abundance of redband trout in the six interior basins of Oregon’s high desert. A spatially representative sample of redband trout habitats was selected at the Species Management Unit (SMU) level for the Chewaucan, Fort Rock, Goose Lake, and Warner Lakes subbasins and at the population level in the Malheur Lakes subbasin. We also planned to sample the Catlow Valley SMU at the population level, however no sampling was conducted due to denied access and time constraints.

Sites were randomly selected using EPA's Generalized Random Tessellation Stratified (GRTS) design. We targeted 30 sites for each of the Chewaucan, Fort Rock, Goose Lake and Warner Lakes SMUs. We also targeted 30 sites for each of the six populations in the Malheur Lakes SMU. Actual numbers of sites sampled approached, but did not completely meet these targets (Table 1). Building on remarkable landowner cooperation in the initial year of the study, we were able to sample at 85% of selected sites, thus allowing us to maintain a representative sample across the range of each SMU. Field methods used to sample each site were the same as those used in 2007.

Length frequency analysis was used to categorize redband trout as age-0 or age-1+ fish. Separate age-class designations were made for each of five SMUs. Because age-0 fish are not always vulnerable to our gear, they are not included in our estimates. Separate age-class designations were made for each SMU. Population estimates for age-1+ redband trout in each SMU were extrapolated from estimates of fish density at the sample sites in each SMU. A calibration factor was applied to estimates for all SMUs to adjust for the negative bias associated with depletion estimates. The calibration factor was calculated as the mean ratio between depletion and mark-recapture density estimates at all sites from 2006-2008 where both methods were employed.

Depletion estimates were completed at 235 sites across the 2,340 km sample frame (potential summer distribution) of Great Basin redband trout. The final calibrated population estimates ranged from 44,000-479,000 at the SMU scale and from 29,000-130,000 at the population-scale. The precision of these estimates ranged from 15% to 42% and varied relative to the between-site variability (CV) and sample size for each sample stratum.  Densities estimated at individual sites were highly variable (density histograms).

NOTE: These results were updated in 2009 to incorporate the 2006 - 2009 calibration sites into the estimate.

 

RESULTS

SMU Population n Estimate Lower
95%
Confidence Limit
Upper
95%
Confidence Limit
Relative
95%
Confidence Interval
CV
Fort Rock   24 48,116 30,875 65,358 36 % 0.88
Chewaucan   25 135,622 104,185 167,058 23 % 0.87
Goose Lake   24 43,807 25,949 61,666 41 % 0.91
Warner Lakes   24 146,013 96,341 195,685 34 % 0.95
Malheur Lakes   138 478,170 406,391 549,949 15 % 1.02
  Blitzen 23 111,468 83,947 138,990 25 % 0.85
  East Burns 25 29,159 18,512 39,806 37 % 1.13
  McCoy 24 129,678 92,502 166,853 29 % 0.85
  Riddle 18 47,943 30,650 65,235 36 % 1.07
  Silver 25 42,957 28,336 57,578 24 % 1.21
  Silvies 23 116,965 68,112 165,818 42 % 1.25

 

MAPS & CHARTS

SMU/Population

Length
Frequency

Site Status

Site Density

Other Species

Fort Rock 1+ > 70mm Fort Rock Status Fort Rock Density Fort Rock Species
Chewaucan 1+ > 65mm Chewaucan Status Chewaucan Density Chewaucan Species
Goose Lake 1+ > 85mm Goose Status Goose Density Goose Species
Warner Lakes 1+ > 65mm Warner Status Warner Density Warner Species
Malheur Lakes
   Blitzen
1+ > 85mm Malheur Lakes Status Malheur Lakes Density Malheur Lakes Species
   East Burns 1+ > 50mm
   McCoy 1+ > 70mm
   Riddle 1+ > 80mm
   Silver 1+ > 60mm
   Silvies 1+ > 60mm
All SMUs CDF and Density Histograms

Contact Stephanie Miller or Steve Jacobs with questions or comments.

 

Send comments or questions regarding this webpage to  Shaun.Clements@oregonstate.edu