Portfolio simplification arising from a century of change in salmon population diversity and artificial production

1. Population and life-history diversity can buffer species from
environmental variability and contribute to long-term stability through
differing responses to varying conditions akin to the stabilizing effect
of asset diversity on financial portfolios. While it is well known that
many salmon populations have declined in abundance over the last century,
we understand less about how different dimensions of diversity may have
shifted. Specifically, how has diminished wild abundance and increased
artificial production (i.e., enhancement) changed portfolios of salmon
populations, and how might such change influence fisheries and ecosystems?
2. We apply modern genetic tools to century-old sockeye salmon
(Oncorhynchus nerka) scales from Canada’s Skeena River watershed to (i)
reconstruct historical abundance and age-trait data for 1913–1947 to
compare with recent information, (ii) quantify changes in population and
life-history diversity and the role of enhancement in population dynamics,
and (iii) quantify the risk to fisheries and local ecosystems resulting
from observed changes in diversity and enhancement. 3. The total number of
wild sockeye returning to the Skeena River during the modern era is 69%
lower than during the historical era; all wild populations have declined,
several by more than 90%. However, enhancement of a single population has
offset declines in wild populations such that aggregate abundances now are
similar to historical levels. 4. Population diversity has declined by 70%,
and life-history diversity has shifted: populations are migrating from
freshwater at an earlier age, and spending more time in the ocean. There
also has been a contraction in abundance throughout the watershed, which
likely has decreased the spatial extent of salmon provisions to Indigenous
fisheries and local ecosystems. Despite the erosion of portfolio strength
that this salmon complex hosted a century ago, total returns now are no
more variable than they were historically perhaps in part due to the
stabilizing effect of artificial production. 5. Policy implications. Our
study provides a rare example of the extent of erosion of within-species
biodiversity over the last century of human influence. Rebuilding a
diversity of abundant wild populations – that is, maintaining functioning
portfolios - may help ensure that watershed complexes like the Skeena are
robust to global change.