Just as a deck of cards has four different kings, the state of California has four different types of King Salmon. Each run is creatively named for the season when the adults return to the rivers – we have fall, late-fall, winter, and spring runs. King salmon, also known as Chinook salmon, are the topic of recent news in major California newspapers and science publications alike. One type of Chinook salmon, the winter-run Chinook, is only found in Northern California and is now staring extinction in the face. So I thought I’d take a few paragraphs to introduce you to this fascinating critter, and try to explain why this fish, which requires cold water during California’s hot summer, exists here in the first place.
In addition to returning in different seasons, the four runs of Chinook salmon have evolved quite different strategies to cope with the stress of being a cold-water fish in the hot, dry, California summer. Well, technically, fall-run fish avoid the heat all together by arriving in the fall, spawning quickly in the lower river reaches, and moving back to the ocean as very young fish. The spring-run fish do stick it out through the summer, but have evolved a reliable coping mechanism. These fish use the high spring flows to enter the rivers and migrate up to the high tributary headwaters. In these mountain habitats they found cool snow-melt, plentiful shade, and deep cold pools that let the adults survive the stressful summer months until cooler fall temperatures make the spawning gravels suitable for eggs and young fish. Both of these strategies are common to many runs in the Pacific Northwest. But when we look at the timing of the winter-run, things become a little more confusing.
Schematic contrasting life-history stages of three of the four runs of Chinook salmon in California. The yellow box indicates the hottest period of summer. Data compiled from TNC and S.O.S. report by CalTrout.
Even though winter-run adults move into the rivers in the cool months of November or December, they actually spawn at the very beginning of the hot stressful summer. Their eggs and sensitive juvenile stages are subjected to the hottest weather of July and August. We rarely see this reproductive timing in any anadromous salmon runs in the lower forty-eight states. How can this be a successful strategy? The clue lies in the historical spawning areas in the northern tributaries of the Sacramento River system. This area around Mt. Shasta has unique geologic history of volcanism, which allows the snow melt from the tall peaks to percolate into regional aquifers and emerge into a network of spring-fed creeks. Incredibly, these creeks flow year-round at a constant water level and a cool temperature. Not only do these creeks let spawning salmon avoid the stress of warm summer temperatures, but the ecosystems also produce prodigious amounts of fish-food, likely peaking in summer when the juvenile winter-run salmon are emerging as tiny hungry fish. In this context, it is not a paradox for fish as adaptable as salmon to have evolved to spawn in the summer. In these spring-fed creeks, the young are not threatened by high water temperatures and are born at the exact time when their food is most abundant. Brilliant!
Yet this strategic timing of the winter-run, so well adapted to their historical habitats, is mismatched with today’s world. Shasta Dam, completed in 1945, blocked all access to those miraculous spring-fed creeks that previously composed the winter-run spawning grounds. It compels winter-run adults to spawn in the warm mainstem of the Sacramento River. Ironically, this run persists because of cool-water releases from Shasta dam. The dam may eliminate access to habitat, but as water is released from the cool layers at the bottom of the reservoir it also creates a single, small area in the Sacramento River with (generally) cool water temperatures through the summer.
In our current drought, the water in the reservoir is no longer enough to supply the myriad of water needs within California, let alone maintain enough deep, cool water to be released for fish. In 2014 managers estimated that unavoidable spikes in the temperature of dam releases in September killed an estimated 95% of the eggs and fry. To put this in context, average estimates of mortality between 2002-2012 were 74%. And this mortality is occurring to a species that has already declined to 1-2% of the historical population estimates.
Water temperatures measured just downstream of Shasta Dam. The target water temperature under the Endangered Species Act is 56°F, shown by the dashed grey line. The critical period of cold water for winter-run eggs and juveniles extends through October. Data collected by CA Dept Water Resources at the KWK station, shared online via the California Data Exchange Center.
In 2015 the agencies have used a more cautious approach for releasing water throughout the year, trying to save more of the limited cool water for late season releases. It appears that this has paid off by successfully keeping temperatures below or near the 56°F threshold that has been set as the legal target. However, preliminary counts of juveniles beginning to migrate to the ocean are suggesting this year may be even worse than 2014. If future climate change leads to another string of years with hot water and unsuccessful spawning, this unique evolutionary lineage will rely upon a small captive population maintained at Livingston Stone Hatchery. To avoid this precarious future, there are plans in place for reuniting these fish with their spring-fed creeks, so see my next post for more!
Further reading on the status and natural history of the winter-run:
Google chrome users: click here to download a RSS extension