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.
When salmon first hatch from eggs, fighting their way out of freshwater gravel beds and into the world, they’re hungry. The young salmon are on the hunt for tiny, microscopic animals called zooplankton.
“So these little crustaceans are the major food source for juvenile sockeye and also for their competitors things like sticklebacks, which these lakes are full of,” said Daniel Schindler, a fisheries biologist with the University of Washington, standing in the research lab at Lake Aleknagik.
Looking through a microscope, the zooplankton look like a cross between a shrimp and an alien with a translucent, shelled body and a variable number of legs and antennae. And, Schindler says, they’re booming in the Nushagak watershed.
“The amount of zooplankton has almost doubled over the last 70 years,” he said. “And we think that's a response to climate warming, which is warming the lakes up. Ice is breaking up earlier in the spring, so there's a longer growing season. And this translates into faster growth rates for the juvenile sockeye.”
Schindler is part of a team of researchers that has been monitoring the Nushagak and Wood river salmon runs since the 1950s, which includes keeping tabs on their food sources
Young salmon feed on zooplankton for the first year of their lives.
“There's five major species of zooplankton we find in these lakes and over this 70 year time period that we've been studying these lakes, a couple of those species have really exploded, and seem to be very sensitive to warming water temperatures. And we're seeing both bigger sockeye and more sockeye fry,” he said.
Sockeye hunt zooplankton in a delicate diurnal dance, lurking in the deep cold waters of the lakes during the day, and stealthily rising up to feed on zooplankton at night, hidden by darkness.
“It's only at night to come up to the surface to feed. And then they go back down the following morning and hang out in that cold water, basically hiding from their predators,” he said.
Zooplankton themselves feed on other kinds of plankton, like phytoplankton which convert the sun’s rays to energy.
Schindler says that the boom in zooplankton may be the leading cause of growing salmon runs, and last year’s bay-wide record run of 79 million sockeye.
“That's because the rate at which the zooplankton can reproduce depends on water temperature,” he said. “And climate warming has increased lake temperatures, extending the growing season, and as a result, more zooplankton are grown in these lakes, which means more food for juvenile sockeye.”
That’s a good thing for hungry young salmon. But after a year or two in the lakes, fish migrate downstream to the open ocean, where competition for food and survival is fierce.
On the next Fish Facts, we dive into what we know - and don’t know - about what salmon eat in the open ocean, before returning to Bristol Bay rivers to spawn.
Get in touch at fish@kdlg.org.
