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Applying long-term redd datasets to estimate historic salmon abundance

Date: July 22, 2020

New research is helping understand the past and more accurately estimate future salmon recovery potential in Central Idaho’s Middle Fork Salmon River.

Two boats on the rocky bank of the Middle Fork Salmon River, surrounded by trees and tall mountains.
Because of its wilderness condition, the Middle Fork Salmon River offers a glimpse of historical Chinook salmon natal habitat. The processes that have shaped this landscape for eons remain intact, and human influences are minimal. (Jim Brock)
Research Fisheries Scientist Russ Thurow has worked in the Middle Fork Salmon River (MFSR) river basin since the 1980s. Since 1957, the Idaho Department of Fish and Game (IDFG) has consistently counted salmon nests, or redds, within index areas of six MFSR tributaries. In 1995, Thurow began a continuous redd survey of all remaining MFSR spawning areas not counted by collaborating biologists with the IDFG, Shoshone-Bannock and Nez-Perce Tribes, and Payette, Boise, and Salmon-Challis National Forests.

The invaluable index and continuous redd datasets enabled Thurow and his team to estimate potential Chinook salmon production in the 1950s–1960s.

The team used contemporary redd census and spawn timing data to calculate how many redds would have been counted historically by expanding the timeframes and spatial extent of archival index counts. Their analysis revealed the MFSR supported a conservative estimate of 48,000 adult Chinook salmon during the 1950s–1960s. For the past 25 years, salmon populations in the MFSR have averaged < 1,500 adults, about 3 percent of the historical estimate. In 2019, 322 salmon returned to spawn, about 0.7 percent of 1950s–1960s abundances.

Notably, several contemporary management goals underestimate historical potential. These low goals suggested to the authors that managers were influenced by the shifting baseline syndrome (SBS), where recent species abundances and environmental conditions are erroneously accepted as reflecting historical conditions. “It’s something we humans tend to do,” Thurow says. “We believe what we are experiencing today is close to what is possible, when it may actually be a long way from true potential. This research helps suppress the SBS by providing more accurate reference points for managers to consider.”

Thurow considers it biologically feasible to rebuild MFSR Chinook salmon to 1950s–1960s abundances, provided effective actions are taken to address outside-basin factors and improve survival of salmon after they migrate from the MFSR.

Key Findings

● By combining archival “index” redd counts with contemporary redd census and spawn timing data, researchers conservatively estimated that the Middle Fork Salmon River historically supported a potential 24,000 redds or 48,000 adult Chinook salmon.

● Several contemporary management goals are fractions of the 1950s–1960s estimated potential. For example, the National Marine Fisheries Service “minimum viable abundance” goal is 10.4 percent of the estimated 48,000 salmon potential. Such underestimated goals suggest that managers were influenced by the shifting baseline syndrome (SBS).

● This work addresses the SBS by improving estimates of salmon population potential and by providing more accurate reference points for managers to consider in planning recovery efforts.

● Chinook salmon production in the Middle Fork Salmon River was historically high and it remains high because the basin retains critical building blocks for salmon recovery. The primary factors limiting recovery of its Chinook salmon populations occur outside natal habitats. Recovery is biologically feasible, provided effective actions are implemented to address outside-basin factors.

Principal Investigators: 
Principal Investigators - External: 
Timothy Copeland - Idaho Department of Fish and Game
Bryce Oldemeyer - Henry’s Fork Foundation
Research Location: 
Middle Fork Salmon River