Fish Facts is a regular segment this season on the Bristol Bay Fisheries Report - where we take a deep dive into fish science, ecology, and research, and swim the salmon life cycle, from the open ocean to home streams and rivers.
After one to two years of munching on zooplankton in freshwater, young sockeye - or smolt - head for salt water to begin the next chapter of their anadromous lives, where there’s a whole different menu available.
Dr. Katie Howard studies salmon diets in the open ocean, and is an ocean fisheries scientist with the Alaska Department of Fish and Game.
“The sockeye, pink and chum, eat a pretty diverse array of invertebrates and fish,” she said. “And the amount that their diets overlap or don't overlap, is really dependent on where they are.”
Howard calls these three species of salmon generalists; for the most part, they’ll eat whatever is available to them. But she says chinook, or king salmon, choose different marine meals.
“Chinook tend to focus more on fish and squid. But, which fish and which squid really depends on where they are and what's available,” she said.
Howard and her team study salmon in their first year in the ocean, including how their diets vary. She says one of the biggest drivers of diet change for salmon are marine heat waves, periods of abnormally high ocean temperatures, like the infamous “blob” of the Gulf of Alaska that first appeared in 2013. These heat waves can have significant impacts on salmon, and the marine creatures they feed on.
“What we saw on these marine heatwaves years is that their diets shifted,” Howard said. “For all species, their diets seem to shift. But at least for chinook and chum salmon, their stored energy, how much energy they had stored up before winter in their bodies, declined a lot from previous years that weren't the marine heat years.”
Howard says in these warmer periods, jellyfish become more abundant. And chum salmon don’t hesitate to chow down .
“They have evolved to have this specialized gut that allows them to eat gelatinous animals like jellyfish. And that's something other salmon don't do,” she said. “So that's weird.”
But not all food is created equal, and Howard says gelatinous animals like jellyfish are missing key nutrients that salmon need.
“Chum salmon typically have really diverse diets, they're eating lots of different things,” she said. “But during these really warm marine heatwave years, they shifted to a much less diverse diet and a higher proportion of gelatinous animals in their diet, which tend not to have a very high caloric content. They're not necessarily a super good food to be eating.”
For the final chapter of a salmon’s life, they return to their homes in freshwater streams and lakes to spawn. But Howard says that saltwater sustenance is the last they’ll get on the long swim home.
“They're basically doing the equivalent of running an ultra marathon every day for a month,” she said. “And they're doing it without eating.”
While salmon struggle upstream to spawn, they’re also fighting to not get eaten.
And in the Nushagak, that’s by bears. Daniel Schindler is a professor in the University of Washington's Alaska salmon program, part of the team monitoring the Nushagak runs, where he studies the freshwater portion of salmon’s lives.
“If you take a sample of hair from a bear here,” he said. “And you ask, with isotopes of the nitrogen in that hair, and you ask how much of that bear was supported by eating salmon, you'd get somewhere between 80 and 90% of that bear was produced by eating salmon.”
Between jellyfish, squid, and zooplankton, the phrase, “you are what you eat” rings true for salmon, as well as the creatures that depend on them.
To learn more about what salmon eat in freshwater lakes, see Fish Facts: Salmon’s freshwater feeding frenzy
