Proposed Population Structure of Puget Sound Steelhead

Jim Myers, Northwest Fisheries Science Center of NOAA Fisheries

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One of the tasks given to the Technical Recovery Team (TRT) has been the identification of demographically independent populations (DIPs) within the Puget Sound Steelhead Distinct Population Segment (DPS), a geographic area including tributaries draining into Puget Sound and the Strait of Juan de Fuca from Boundary Bay to the Elwha River. The definition of a DIP does not diverge greatly from Ricker’s definition of a stock, except that the level of exchange between populations is limited to levels that would not affect the demographic trajectory or extinction risk of either population.

In identifying DIPs, the TRT considered a number of factors that would produce demographically distinct units: geographic distance, river structure (barriers, fluvial morphology), ecology, and life history characteristics (habitat preference, run timing, spawn timing. For example, summer-run (stream maturing) and winter-run (ocean maturing) steelhead existed in the same river as DIPs where temporal migration barriers were identified. In addition, population genetics, straying estimates, population demographics provided measures of independence. Finally, the TRT has addressed population structure from a historical perspective, because the historical template is the only example we have of a viable DPS. The review of historical data also provided the TRT with a perspective of the diversity of steelhead habitat utilization and life history diversity.

DIPs effectively become the basic unit for viability modeling in the DPS, while major population groups (MPGs) identify broader scale geographic or life history diversity. Effectively, an MPG meets the definition of a “significant portion of the range” under the U.S. ESA; where the loss of any one MPG puts the entire DPS at risk.

The TRT identified 3 MPGs containing 32 DIPs in the Puget Sound Steelhead DPS. Within the Northern Cascades MPG, 16 DIPs (8 winter run, 3 summer/winter run, 5 summer run) were identified as historically present. In the Central and South Puget Sound MPG, 8 winter-run DIPs were historically present. Within the Hood Canal and Strait of Juan de Fuca MPG historically contained 8 DIPs (1 summer/winter run, and 7 winter run with 2 of these winter runs possibly historically including summer-run components). In many cases both historical and contemporary population-specific information was limited or lacking. Given the diverse biological characteristics of steelhead, the TRT considered that there were a number of additional populations, but that the available information was insufficient for such designations at this time.