Fish in hot water pose tough dilemma

Spring run Chinook salmon, photographed in Butte Creek, upstream from Centerville, Calif., may become extinct in the future due to warming waters. Credit: Allen Harthorn, Friends of Butte Creek

Is it more important that the water in our rivers is available for rearing fish to eat or generating clean electricity? And how is that decision affected by rising worldwide average temperatures? A group of US scientists has now found ways to help answer these questions, inspired by the plight of spring run Chinook salmon – though the prospects for the fish remain bleak. “These fish are very vulnerable to climate change,” Lisa Thompson, director of the Center for Aquatic Biology and Aquaculture at University of California, Davis, told Simple Climate. “It is likely that these salmon will decline, and may not persist to the end of the century.”

Salmon are famous for their exhausting upstream “run” from the ocean back to the water they hatched in, where they spawn the next generation of fish, and then die. It’s estimated that at the beginning of the 20th century the waters in California’s Central Valley, running from San Joaquin to Sacramento, teemed with 1-3 million Chinook salmon making this journey every year. In the last five years this number has fallen below 100,000, contributing to ocean stocks declining to levels where no salmon fishing was allowed off the California coast in 2008 and 2009. There are many reasons for this, including overfishing, changing sea conditions and water quality.

In the Sacramento-San Joaquin River system, each season there is a run of Chinook salmon spawning. The spring run used to number approximately 1 million fish in 18 separate populations, but they have been reduced to approximately 16,000 in three groupings. Spring run Chinook salmon are therefore listed as threatened under both the California and US Endangered Species acts, Thompson said. “The adults must survive the summer in freshwater before spawning in the fall,” she noted. That means they are waiting to spawn during the state’s hottest, driest months, where raised temperatures can kill them. In trying to stay cool, the salmon can get trapped on the wrong side of stretches of warmer river water separating them from their spawning grounds.

A fishy problem

A biologist releasing a spring run Chinook salmon after rescuing it from being prevented from getting to its spawning ground by a pool of warm water in Butte Creek in July 2010. Credit: California Department of Fish and Game/Harry Morse.

Spring run Chinook salmon have to share Butte Creek, one place that they do still go to spawn, with a hydroelectric power generation plant that has drawn criticism from fishing and conservation groups. While researchers have developed ways to see how climate change might affect salmon, those tools have not been useful for deciding the best way to resolve conflict between helping the fish and other needs. Thompson and her colleagues therefore looked at how to balance the opposing sides. “We wanted to develop a model framework that would allow us to assess trade-offs between different water management objectives, including flows for fish, and flow for hydroelectric power generation,” she said. “This framework could then be used for other rivers.”

To do this the University of California, Davis, US National Center for Atmospheric Research and Stockholm Environmental Institute researchers linked three different scientific models together. They started with forecasts of the global climate through to the end of the century from six general circulation models that were used  in 2008 to assess the likely impact of climate change on California up to the end of the century. Into these, Thompson and colleagues input two scenarios for the amount of greenhouse gases humans will emit used by the UN Intergovernmental Panel on Climate Change (IPCC). These were business as usual, or scenario A2, and decreased emissions, or scenario B1. Second, the scientists fed the resulting numbers into a “watershed hydrology” model, that calculated the streamflow and temperature of waters in Butte Creek. Finally, they used that information in a salmon population model that predicted whether adult salmon would survive the summer in order to spawn later in the year.

An uncertain fate

Lisa Thompson, director of the Center for Aquatic Biology and Aquaculture at University of California, Davis. Credit: University of California, Davis

“In all cases, we predicted that summer streamflow will be lower and water temperatures will be higher,” Thompson said. “There was increased summer mortality of adult salmon and the population declined. For all the A2 combinations, and half the B1 combinations, by the end of the century no adult salmon could survive the summer. We found that by forgoing a water diversion to the hydropower plant during the critical mid-summer period, and leaving that water in the stream, the water stayed cooler and salmon persisted up to 17 years longer.”

While Thompson said that their approach, published in the Journal of Water Resources Planning and Management last week, is not precise enough to predict exactly what will happen in a given year, she underlined it can still help. As well as preventing water going to the hydroelectric plant, the scientists also used the linked-up models to look at releasing cool water from a reservoir through August and September to lower the river temperature. Though this actually seemed to make the situation worse, they now hope to test more ideas, and in the meantime have provided useful information to those controlling Butte Creek’s water flow.

“In a given year, if the water flows and temperatures are similar to what we’ve predicted, then we can say with considerable confidence that salmon will be stressed,” Thompson said. “Managers will already know that there are adaptations that they can implement that are likely to increase fish survival. We assume that there will be real people making decisions in real time, based on the most accurate weather information available.”