Dendritic biodiversity in a large watershed and portfolio effects in First Nation fisheries

Background/Question/Methods

Biological variation can stabilize ecosystem function by buffering the effects of environmental change. Indeed, studies from multiple systems have found positive correlations between species richness and ecosystem function and stability. The result of this increased diversity on ecological stability has been termed the ‘portfolio effect’ and is analogous to financial theory. Riverine ecosystems are characterized by dendritic structures, whereby smaller tributaries flow into larger segments. In large and relatively intact watersheds of western North America, anadromous salmon migrate upstream from the ocean to spawning habitats located throughout these watersheds. We posit that these river networks naturally aggregate diversity based on their branching, dendritic structure, where upstream reaches are exposed to less salmon diversity than their downstream counterparts. This study examines portfolio effects of fisheries located within the Fraser River, a large watershed (220,000 km²) in British Columbia that hosts six species of anadromous Pacific salmon. We examined the stability of catches from First Nation fisheries, compiled by Fisheries and Oceans Canada, located throughout this large watershed. We hypothesized that fisheries lower in the watershed would integrate more salmon diversity, both among species and populations, and thus have more stable catches through time.

Results/Conclusions

First Nation fisheries were located through the Fraser watershed, from 30 km to 1000 km from the ocean. Fisheries at these different locations thus aggregated different levels of salmon biodiversity. Specifically, fisheries close to the mouth of the Fraser caught all six species of anadromous Pacific salmon, while fisheries high in the watershed only caught only one or two species. The stability of catches depended on salmon species identity and the location of the fishery. On short time frames, sockeye salmon and pink salmon did not exhibit a significant relationship between watershed location and CV; variation appeared to be driven by their 4 and 2 year cycles, respectively. In contrast, the stability of Chinook catches increased closer to the ocean. Higher in the watershed, the CV of Chinook catch was 1.25, while low in the watershed the CV was five times lower. Thus, population- and species-level biodiversity can increase fishery stability. This provides a rare example of diversity driving portfolio effects at large spatial scales. More generally, large and intact watersheds may be unique integrators of diversity.