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This report presents a limiting factors analysis (LFA) developed to determine why natural-origin Trinity River Chinook salmon Oncorhynchus tshawytscha are not meeting adult escapement goals and to identify which life stages and processes most strongly constrain population performance. The analysis uses a life cycle model to compare survival and production across the Trinity River, Klamath River, estuary, early marine period, and ocean, and to distinguish constraints that the Trinity River Restoration Program (TRRP) can manage directly from those requiring coordination or improved information. The results place TRRP actions within the full Chinook salmon life cycle and clarify how conditions within and beyond the Trinity River interact to influence adult returns.

 

Ocean harvest and survival after juveniles leave the Trinity River produced the largest modeled effects on naturally produced adult escapement. Reducing ocean harvest substantially increased modeled escapement and, in some scenarios, allowed the recovery target to be exceeded. Survival through the Klamath River, estuary, early marine period, and ocean also strongly influenced whether gains from Trinity River restoration translated into increased adult returns. Within the Trinity River, juvenile rearing habitat, survival, growth, and temperature were the strongest constraints directly managed by TRRP, although their effects on adult returns depended on downstream and marine survival. Spawning and incubation conditions affected production but did not explain most of the adult escapement gap under the scenarios tested; these factors may remain important in specific reaches, years, or disturbance conditions (e.g., fires, floods, etc.).

 

The nested limiting-factor scenario illustrates the relative magnitude and sequence of these constraints. Modeled mean pre-spawn adult escapement increased from 9,581 adults under baseline conditions to 37,520 after the tested ocean harvest and survival changes, to 53,908 after additional Klamath River disease and temperature improvements, and to 68,035 after additional Trinity River rearing survival, habitat, and temperature improvements. These cumulative results are diagnostic comparisons, not forecasts. Their apparent numerical precision reflects the specific model inputs and assumptions and should not be interpreted as precise predictions of future escapement. Instead, the pattern indicates that recovery likely depends on improvements across multiple life stages and spatial scales.

 

Report w Appendices: CFS 2026, Limiting Factors Analysis Technical Memorandum w Appendices

 

Data package: Trinity LCM R Code