Portfolio simplification arising from a century of change in salmon population diversity and artificial production.
Abstract
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? We apply modern genetic tools to century-old sockeye salmon Oncorhynchus nerka scales from Canada's Skeena River watershed to (a) reconstruct historical abundance and age-trait data for 1913-1947 to compare with recent information, (b) quantify changes in population and life-history diversity and the role of enhancement in population dynamics, and (c) quantify the risk to fisheries and local ecosystems resulting from observed changes in diversity and enhancement. 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. 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. 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.