Data from: Seventy years of diminishing biocomplexity of California Central Valley hatchery steelhead, Oncorhynchus mykiss

The California Central Valley steelhead (Oncorhynchus mykiss) has declined
precipitously since Euro-American colonization and has been listed as
threatened under the United States Endangered Species Act since 1998.
Hatchery-origin fish now dominate the population and hatchery management
is a key listing factor. However, scant release metric information is
available. We compiled a time series of O. mykiss hatchery release data
for all four Central Valley hatcheries releasing O. mykiss between 1948
and 2017. The biocomplexity of released fish has declined since the early
1980s. Individuals have been released at increasingly similar numbers,
biomass, body sizes, times, and locations over time. Moreover, yearling
fish have been released at larger sizes, leading to the near-exclusive
release of age-1 smolts in February and March since the late 1990s and
early 2000s. Pervasive reductions in release portfolios have likely
occurred for other hatchery-supported Pacific salmonid stocks throughout
the Pacific Rim region. In an increasingly variable environment, such
reductions in intraspecific diversity could significantly affect
population stability and resilience.