|Document Type:||Technical Memorandum|
|Title:||Status review for Oregon's Illinois River winter steelhead|
|Author/Editor:||Peggy J. Busby, Orlay W. Johnson, Thomas C. Wainwright, F. William Waknitz, Robin S. Waples|
|Tech Memo Number:||NMFS-NWFSC-10|
This report summarizes biological information gathered in conjunction with an Endangered Species Act (ESA) status review for winter steelhead Oncorhynchus mykiss from the Illinois River, a tributary of the Rogue River in southern Oregon. The National Marine Fisheries Service (NMFS) received a petition in May 1992 asking that this population be listed as a threatened or endangered species under the ESA.
The petitioners argued that geographic isolation of the Illinois River spawning grounds, the partial barrier of Illinois River Falls, life history differences between steelhead from the Illinois and Rogue Rivers, and evidence for genetic differences between steelhead from southern and northern Oregon demonstrate reproductive isolation of Illinois River winter steelhead. A tenfold decline in angler catch of Illinois River winter steelhead since 1971 was cited as evidence that the population qualifies for listing under the ESA.
In evaluating the status of Illinois River winter steelhead, NMFS focused on information for coastal steelhead populations from southern Oregon and northern California. The National Marine Fisheries Service concluded that available information does not make a strong case for reproductive isolation of Illinois River winter steelhead. Genetic data, including new data gathered for this status review, fail to show that Illinois River winter steelhead as a group are distinct from other coastal steelhead populations.
Although this does not prove that Illinois River winter steelhead are not reproductively isolated, it does mean that evidence to support reproductive isolation must be found elsewhere. Other lines of information are largely inconclusive in this regard.
Straying data for naturally spawning steelhead in the region are largely nonexistent. Geographic features such as the Illinois River Falls are potential isolating mechanisms, but evidence that they operate in this way is lacking. Although other explanations are possible, life history characteristics suggest some degree of reproductive isolation from Rogue River steelhead.
However, these same characteristics fail to show differences between Illinois River winter steelhead and most other coastal steelhead populations. With respect to the second criterion for defining an ESU, several phenotypic and life history characteristics show modest differences between Illinois River winter steelhead and steelhead from the Rogue River. These differences suggest that either a) there are some genetic differences between steelhead from the two rivers, b) the natural environments of the two rivers differ in at least some respects, or c) artificial propagation has affected life history characteristics of Rogue River steelhead, thus causing them to differ from Illinois River winter steelhead.
In any case, in the larger context of coastal steelhead from southern Oregon and northern California, it is the Rogue River fish, rather than the Illinois River winter steelhead, that appear to be somewhat anomalous. That is, steelhead from the Illinois River appear to be somewhat distinctive in comparison to Rogue River steelhead, but not in comparison to other coastal steelhead populations. We therefore conclude that although Illinois River winter steelhead may be locally distinctive within the Rogue River basin, they do not by themselves represent an evolutionarily significant unit of the biological species Oncorhynchus mykiss.