Written by W.G. Duffy, FOMR Board Member
Redband trout Photo Credit: ODFW

Part II: Redband Trout Ecology

Part one of this series provided a brief review of the geologic history of the northern Great Basin and how it eventually constrained water exchange between subbasins and thus also constrained the genetic interactions of redband trout populations in the region. It also briefly introduced redband trout in the Harney Basin.

Efforts to estimate the size of redband trout populations in the subbasins of the northern Great Basin suggest that the Harney Basin is an important area for the species. During 2007 to 2012, maximum annual population sizes for these subbasins were: Catlow Valley (42,000), Chewaucan (155,000), Fort Rock (87,000), Goose Lake (125,000) Warner Lakes (160,000) and Harney (470,000).  Thus the Harney Basin or Malheur management area contains 35% to 40% of the redband trout in the seven basins of the northern Great Basin.

Redband trout in the northern Great Basin are recognized for being phenotypically diverse, having characteristics resulting from their genetic composition and in interaction with the environment.  In the Little Blitzen River and Bridge Creek they have been found to be physiologically different in swimming stamina and metabolic power from the performance reported for other rainbow trout. For example, these redband trout were found to have greater metabolic power (ability to do work) above 75^of than at 54 to 57 ^of. Further analysis found that the Bridge Creek population is very well adapted for life at high temperatures, including anaerobic metabolism or the ability to produce energy in the absence of oxygen. Performing well at 75^of would be difficult for any species of trout and could only be done where abundant energy (food) was available. This suggests that the Blitzen River is a productive river capable of supporting an abundant redband trout population.

Phenotypic diversity in redband trout may be stimulated by other factors such as habitat both physical and the structure of their biological community. In Malheur Refuge streams, redband trout spawn in the spring to early summer, as do rainbow trout throughout the world. There is, however, one example from the northern Great Basin of redband trout spawning during every month except September. Fish community structure can contribute to an adfluviall life history in redband trout, that is a life history in which adults migrate into tributary streams to spawn then migrate into lakes to take advantage of greater food resources there. There are multiple examples of redband trout following an adfluvial life history behavior to prey on tui chub in lakes. This life history follows closely the life history of anadromous salmon and steelhead and it favors more rapid growth than a fluvial (entirely stream dwelling) life history. It is not surprising that the fish community of Malheur Lake includes tui chub, along with mountain whitefish, longnose dace, speckled dace, redside shiner, Malheur mottled sculpin and that redband trout that spawn in streams migrate into Malheur Lake when conditions are favorable.

Size of redband trout varies somewhat among streams in the Harney Basin and among reaches within streams. Size differences are the result of both stream habitat conditions and the ability of stream redband to access food resources in Malheur Lake. Sampling of redband trout from the Blitzen River during 1953 to 1982 documented fish ranging from 1 to 5 years old whose length ranged from 4 to 17 inches. Redband trout in the Blitzen River were slightly larger in the lower river than in the upper river. There is, however, credible evidence the redband trout achieved greater size in the Blitzen River prior to the carp becoming abundant in the early 1950’s.

Redband trout migrate upstream to spawn in spring to early summer. Timing of migration is influenced by elevation. Most of the redband spawning are 3 or 4 years old, but larger individual fish have been documented spawning. Information on repeat spawning is conflicting with some sources reporting multiple spawning efforts and another finding only one. Two studies evaluated movement of redband trout in the Blitzen River Basin that were not spawning. One by removing fish from selected areas, then returning to sample those areas the following year.  The other study tagged redband trout and recorded the tagging location as well as the location of any fish captured a second time. Both approaches confirmed movement of redband trout, with more than 35% of the tagged fish in Bridge Creek moving and roughly 10% of those in Indian Creek moving.

Density of stream dwelling fish is often measured and reported as either number of fish per square meter (as above) or number of fish per meter of stream. Density of redband trout in two tributaries to the Blitzen River were variable during 2013 to 2015 but averaged about 2 fish/m².  In 2016, however, density in one of the tributaries, Bridge Creek, was less than 1 fish/m².  The decline in density in 2016 was attributed to severe drought conditions the previous year. Still the low value of 1 fish/m² is greater than in other northern Great Basin streams such as tributaries to Goose Lake where average density was reported to be 0.16 fish/m². 

Fish were sampled of in Malheur Lake during 2013 to assess the abundance and composition of the lake fish community. This sampling resulted in the capture of 779 individual fish of which 776 were European carp and 3 were warmwater game fish, an overwhelming result. Carp into size categories 4 to 8 inches and 8 to 12 inches long made up most of the carp measured. Carp in the 0 to 4 inch length category were least numerous and those greater than 12 inches long were intermediate in abundance. Carp became abundant in the lake in the late 1940’ and early 1950’s coinciding with a decline in redband trout, particularly the larger trout.

The Blitzen River, Silvies River and Silver Creek each originate at relatively high elevations before descending onto low gradient basins around Malheur Lake and Harney Lake. These streams and lakes represent a variety of habitats that benefit redband trout ranging from mountain streams favorable for spawning to deeper, slower flowing portions of lower streams and eventually lake/wetland systems producing abundant food resources.

Studies focused on the relationship between redband trout and habitat are limited. Descriptions of habitat frequently address water withdrawal, riparian habitat or barriers but lack substantive data on the direct response of fish to the condition described. And, aside from water temperature, little attention has been given to instream habitat.  One exception was a study involving streams in the Crooked River basin north of the Malheur watershed (Figure 2). Investigators in this study chose to look at three streams located at elevations of roughly 5,200, 4,500 and 4,000 feet having much different watershed characteristics, land-use intensity, and disturbance history.  Flow volume and water temperature increased from high to low elevation while density of cattle and timber harvest increased as elevation decreased. While these habitat differences would be expected, measures of instream habitat condition were more similar among streams than they were different. Although trout habitat conditions could be judged to decline with declining elevation, density of redband trout in the streams increased from 0.02/m² in the highest elevation stream, to 0.18/m² in the intermediate elevation stream to 0.39/m² in the lowest elevation stream.

A former manager of the Malheur National Wildlife Refuge provided personal observations on the effect of climate variability on aquatic habitats on the refuge. He recalled that Malheur Lake went dry in 1931 and 1934, and many fish died. Within a decade, however, redband trout populations in the lake were high in 1940 with the spring seeing many fish trying to negotiate various irrigation dams in the Blitzen Valley. These observations emphasize the great temporal variability in watershed conditions and habitats in the northern Great Basin. The fact that populations of the Blitzen and Silvies redband trout, likely almost decimated by drought in the early 1930’s, managed to recover within a decade supports an argument that they are both opportunistic and phenotypically flexible. Documented human impacts on Harney Basin watersheds over the last 175 years has been great as natural systems were modified to promote agriculture.  Modifications included irrigation diversions on most streams that caused de-watering and dams that blocked both upstream and downstream migrating fish. Livestock grazing sometimes resulted in stream channel degradation as well as loss of riparian habitat and other wetlands that function as protective cover for juvenile fish as well as supporting invertebrate production.