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Do precipitation extremes drive growth and migration timing of a Pacific salmonid fish in Mediterranean-climate streams?
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Climate change is expected to increase weather extremes and variability, including more frequent weather whiplashes or extreme swings between severe drought and extraordinarily wet years. Shifts in precipitation patterns will alter stream flow regimes, affecting critical life history stages of sensitive aquatic organisms. Understanding how threatened fish species, such as steelhead/rainbow trout (Oncorhynchus mykiss), are affected by stream flows in years with contrasting environmental conditions is important for their conservation. Here, we report how extreme wet and dry years, from 2015 to 2018, affected stream flow patterns in two tributaries to the South Fork Eel River, California, USA, and aspects of O. mykiss ecology, including over-summer fish growth and body condition as well as spring out-migration timing. We found that stream flow patterns differed across years in the timing and magnitude of large winter-spring flow events and in summer low-flow levels. We were surprised to find that differences in stream flows did not impact growth, body condition, or timing of out-migration of O. mykiss. Fish growth was limited in the late summer in these streams (average of 0.02 +/- 0.05 mm/d), but was similar across dry and wet years, and so was end-of-summer body condition and pool-specific biomass loss from the beginning to the end of the summer. Similarly, O. mykiss migrated out of tributaries during the last week of March/first week of April regardless of the timing of spring flow events. We suggest that the muted response to inter-annual hydrologic variability is due to the high quality of habitat provided by these unimpaired, groundwater-fed tributaries. Similar streams that are likely to maintain cool temperatures and sufficient base flows, even in the driest years, should be a high priority for conservation and restoration efforts.