Data from: Do precipitation extremes drive growth and migration timing of a Pacific salmonid fish in Mediterranean‐climate streams?

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.