|
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 |
Contact Stephanie Miller or
Steve Jacobs with questions or comments.
|
