After nearly a century of people building dams on most of the world’s major rivers, artificial reservoirs now represent an immense freshwater footprint across the landscape. Yet, these reservoirs are understudied and overlooked for their fisheries production and management potential, indicates a study from the University of California, Davis. 

The study, published in the journal Scientific Reports, estimates that U.S. reservoirs hold 3.5 billion kilograms (7.7 billion pounds) of fish. Properly managed, these existing reservoir ecosystems could play major roles in food security and fisheries conservation.

“There is a large amount of fish mass in U.S. reservoirs that are being overlooked, despite the value being comparable to fish harvest from fisheries around the world,” said lead author Christine Parisek, a Ph.D. candidate in the UC Davis Ecology Graduate Group and the Department of Wildlife, Fish and Conservation Biology.

States with most reservoir fish

For the study, the authors analyzed, digitized, ranked and classified reservoir data collected by the U.S. Army Corps of Engineers between the 1970s and 1990s, after dam construction had tapered off from its heyday of the 1940s to 1960s. The data include fish biomass and production rates from 301 reservoirs in the United States.

Southern U.S. reservoirs contained 1.92 billion kilograms (4.2 billion pounds) of fish. Reservoirs across the entire U.S. were estimated to contain 3.43 billion kilograms (7.6 billion pounds) of fish.

Most states show reservoir stock of at least 100 million kilograms (220 million pounds). The top five states with the most standing stock, or total weight, of reservoir fish are Texas, Arkansas, Oklahoma, Florida and South Dakota. 

When total weight is adjusted for how much reservoir surface area is available in the state — similar to a per capita measure — Louisiana, Indiana, Alabama, Maryland and Illinois ranked highest. 

The study also said the large mass of fish in U.S. reservoirs is significant for the global carbon cycle, as fish play important roles in carbon flux, food webs, nutrient cycling and energy transfer. 

Managing amid challenging realities

The authors emphasize the study is not making an argument for prioritizing building reservoirs over protecting and restoring natural-flowing rivers. The study states: “Ecological effects of dams have been overwhelmingly negative and represent one of the principal drivers of freshwater biodiversity loss at all scales.” 

The study does suggest unrealized opportunities to better manage both natural and built ecosystems, given the realities of reservoirs’ continued existence, climate change, and the dire challenges facing native fish.

“We should be able to walk and chew gum,” said fish ecologist and senior author Andrew Rypel, director of the UC Davis Center for Watershed Sciences and professor in the Department of Wildlife, Fish and Conservation Biology. “We should be able to decommission and remove some dams, and manage others for food and as important habitats. 

“In a worst-case scenario where salmon go extinct and native fishes go away, these fisheries may be all we have left. It’s worth having some foresight about how to make them well managed and how to use these ecosystems to deliver value for the environment and for people.”

Additional authors include co-first author Francine De Castro, Jordan Colby and Steven Sadro of UC Davis, and George Leidy of consulting firm AECOM and Stillwater Science.

The study was funded by the National Science Foundation, UC Davis Center for Watershed Sciences’ Bechtel Next Generation Funds, Agricultural Experiment Station of the University of California, and the California Trout and Peter B. Moyle Endowment for Coldwater Fish Conservation.

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