Every year, hundreds of millions of salmon swim from the Pacific Ocean into streams and rivers up and down the West Coast from California to Alaska. They make their way, with remarkable precision and determination, to spawn in the very grounds where they were born. “It’s one of, if not the grandest migrations in the whole world,” says Professor Aaron Wirsing, who recently returned from two weeks at the Fisheries Research Institute in the village of Aleknagik, Alaska.
He was there was part of a joint project between two units in the College of the Environment—the School of Aquatic and Fishery Sciences (SAFS) and the School of Environmental and Forest Sciences (SEFS)—that launched in 2010. Led by Professors Tom Quinn and Wirsing, the research team is investigating coastal brown bear (Ursus arctos) abundance and behavior along sockeye salmon (Oncorhynchus nerka) spawning streams in Alaska’s Wood River Lakes System.
This field season, while the researchers haven’t seen as many bears, they are witnessing a record salmon run that continues to pour into the system. The latest count for just one of the streams, Hansen Creek, is already more than 50,000 salmon—which is more than double the previous record for the whole summer. Picture those fish, some 20,000 at a time, packed into a two-kilometer stretch of water only four meters wide and barely five centimeters deep. That’s a lot of fins in the water, and it makes for an unforgettable sight. “It’s like salmon soup,” says Professor Wirsing.
Before the salmon embark on that last leg to the spawning ground, they often pool at the entry point to the creek and wait days, even weeks, before venturing into the current. Why they pause at the creek mouth, and what triggers the last desperate dash, isn’t entirely clear, though it’s thought to be partly a response to predation risk, with the salmon entering in huge waves to overwhelm their predators—in this case, brown bears. The presence of fish in the creek, with silt kicked up by spawning salmon upstream, might also be a cue for others to follow.
In the best of years, salmon causalities are still fairly high as they near the end of this journey (and all Pacific salmon perish after spawning). Some lack the energy to make the final surge up the stream, or they get stranded in the shallows, sometimes just feet from their destination; others get snapped up by bears, or they provide a gruesome feast for birds that peck away at the half-exposed fish. This year, as well, the salmon are facing extreme low water levels. In many spots, the sockeye barely have a few centimeters to buoy them up the stream, and they have to muster an even more heroic effort to splash their way to the finish.
It’s too early to know precisely what has fueled this record salmon run, says Wirsing, but it could be linked to favorable oceanic conditions (e.g. lots of food at sea). One clear consequence of the high numbers, though, is higher pre-spawning mortality, due both to stranding and to low dissolved oxygen levels in the crowded streams. These salmon will also bring a huge pulse of marine-derived nutrients, which will bolster freshwater invertebrate and bear populations, and even make their way into riparian plants. One other longer-term effect, too, is that there should be another large run in four years when the offspring of these salmon have matured—provided, of course, that enough fish this year are able to spawn and oceanic conditions are again favorable.
Words and photos can’t fully capture the intensity of the annual run, but luckily Professor Wirsing got some great video (below) of the salmon scrum at the entrance to Hansen Creek. It’s like marathoners jockeying for position before the start of a race!
Photos © Aaron Wirsing/SEFS and Tom Quinn/SAFS; salmon video © Aaron Wirsing.