Rivers are biogeochemical processors of carbon (C), nitrogen (N), and phosphorus (P), as well as conduits of these elements to coastal waters. Rivers also host important fisheries, often providing food security for impoverished communities. In this study, we seek to merge these two perspectives by analyzing the global role of fisheries in riverine biogeochemical cycles of C, N, and P. We combine recently published models of riverine C, N, and P exports to coastal waters with estimates riverine fish catches. To convert fisheries into C, N, and P fluxes, we use median values of the body %C, %N, and %P of fish species collected on three continents.
The median fish C:N:P atomic ratio was 39:8:1, suggesting fish are N-rich and especially P-rich compared to the Redfield ratio. Based on this fish C:N:P ratio, we estimated total global riverine fisheries extractions of 704 Gg C, 172 Gg N, and 47 Gg P from rivers. Overall fishery removals of C, N, and P were small compared to total riverine fluxes of organic C or total N and P: estimated fishery removals are approximately 0.7% of riverine C and N fluxes and 1.0% of riverine P fluxes. However, fishery removals were a substantial portion of nutrient budgets in some river basins, e.g., 76% of N in the Nile River or 74% of P in the Karun River (Iran). Therefore, while fisheries play only a slight role in watershed biogeochemical budgets in most river basins, this export can be non-trivial where harvests are high. Underestimation of catches in tropical watersheds may be up to 200%, hence our results are likely conservative and actual fluxes of N and P from riverine ecosystems to local and regional food systems may be even greater.