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ABSTRACT

The Trinity River Restoration Program has made efforts to improve juvenile salmonid rearing habitat degraded by legacy hydraulic mining and dam operations through habitat restoration and adaptive flow management. We assessed how restored off-channel habitats benefit the growth of juvenile Chinook Salmon compared to mainstem habitats in locations just downstream of Lewiston Dam and locations farther downstream affected by accretionary flows near Junction City. We hypothesized that 1) juvenile Chinook Salmon growth is greater in off-channel features during spring rearing so much that 2) those off-channel benefits would persist through the summer and fall for greater survival upon historical timing of November through December flow events that reconnect off-channel features to the mainstem and its resources. We compared temperature and juvenile growth in off-channel sites paired with mainstem sites with continuous temperature data collection and four sequential fish length and weight measurements from May to mid-June. Temperature regimes in the mainstem Trinity River are influenced by dam releases and were mostly below the optimal range (13.0–16.5°C) for juvenile growth. Temperatures in off-channel features offered warmer, more thermally diverse conditions into late spring during 2019, with some features maintaining optimal ranges, but some becoming lethally warm for extended durations (>24°C) in spring of 2020. Greater fish growth was observed in mainstem habitats compared to off-channel habitats. Greater fish growth was observed in warmer mainstem sites in downstream locations compared to cooler upstream mainstem locations. However, insufficiently sealed enclosures allowed fish to escape some experimental enclosures, compromising our study design. The loss of data at several sites did not allow for a formal analysis, resulting in a presentation of results as largely descriptive and qualitative. We recommend this study be repeated in a wetter water year to provide contrast to the findings presented herein, particularly with respect to the unexpected lower growth in thermally optimal conditions in off-channel features.