Landscape-Level Sampling for Status Review of Great Basin Redband Trout

—In response to a petition to list Great Basin redband trout (subspecies of rainbow trout Oncorhynchus mykiss) as threatened or endangered, the U.S. Fish and Wildlife Service conducted a status review in 1998. To support that review, we conducted a survey of the abundances of redband trout in each of six subbasins of the Great Basin that included the states of Oregon, California, and Nevada. We used a generalized random-tessellation stratified algorithm to select a target sample size of 35 sites/subbasin. Out of a target number of 210 sites, 185 were visited by three-person crews that surveyed stream habitat and estimated the abundance of fish populations in sample reaches with lengths that were nearly 20 times the respective channel widths. A minimal sampling intensity was based on previously encountered levels of between-site variance in estimates of redband trout abundance. The total population estimate of age-1 and older (age-1þ) redband trout in the Great Basin was 971,313 fish, with a 95% confidence interval equaling 615% of the mean estimate; 95% confidence limits ranged from 15% to 31% for population estimates in individual subbasins. Age-1þ fish abundance in terms of numerical density showed no significant differences between any subbasins. However, there were significant differences in terms of biomass: Catlow Valley subbasin biomass was significantly higher than the Great Basin mean, whereas Goose Lake subbasin biomass was significantly lower than the basinwide mean. These comparisons were supported by like differences in average weight. Analysis of stream habitat characteristics and fish abundance revealed no relationships that were generally consistent throughout the Great Basin, although spatial patterns were evident within some stream systems where sampling intensity was sufficiently high. Redband trout (subspecies of rainbow trout Oncorhynchus mykiss) occur in inland drainages of the Pacific Northwest, USA. Currens (1997; 2007) suggests that separate groups of redband trout evolved in large river systems, such as the Columbia, Klamath, and Sacramento rivers. Great Basin populations of redband trout (Figure 1) occur in basin-and-range geology and persist in fragmented habitats that are peripheral to and isolated from riverine core groups; these populations probably constitute unique evolutionary lineages. Redband trout populations connected to perennial lake systems have evolved adfluvial life histories. Such populations may have adaptations to unique habitats, and their importance as units of conservation could likely equal or exceed that of large riverine core populations (Li et al. 1995; 2007). Great Basin populations of redband trout are found in arid forest and desert environments characterized by extreme fluctuations in streamflow and temperature. Information collected after droughts in 1992 and 1994 suggested that some populations exhibited depressed abundance. A 1997 Endangered Species Act (ESA) petition to list Great Basin redband trout as a threatened or endangered species prompted a population status review by the U.S. Fish and Wildlife Service (USFWS) in 1998. Redband trout have little commercial value and historically have supported only a small sport fishery. Hence, they have attracted less attention from managers, they have not been well researched, and their status has been less adequately documented compared with other salmonids in the Pacific Northwest. Although the distribution of Great Basin redband trout was generally known (Flitcroft and Dambacher 1999), particularly lacking were reliable estimates of population abundance and an understanding of critical habitat. The objective of this study was to help fill these information gaps.