Bristol Bay sockeye salmon are returning en masse, making their way upstream to find a mate. But spawning successfully requires a total bodily transformation.
Curry Cunningham is an assistant professor working with the University of Washington's Alaska Salmon Program, which has been studying Bristol Bay sockeye for over 70 years.
“Adult sockeye will pass up into the major rivers into the lake systems and spend between two to four weeks going through that transition process from those beautiful chrome fish we tend to see out in the fishery, into the spawning form that we all know and love with the bright green head and red body, and all those secondary sexual characteristics,” Cunningham said.
Going through these bodily changes is a big expenditure of energy. One of those energy sources for salmon is their own scales. Sockeye re-absorb their scales to fuel themselves on the long swim home, and for their total transformation into spawning attire.
The most iconic part of this transformation is their bodies turning the tell-tale, deep red color.
“As Sockeye move into freshwater, they begin to liberate these carotenoid pigments, those red pigments that they've accrued, based on what they're feeding on at sea, and move those from the meat out into the external parts of the body,” he said.
The bright red color is adapted by both male and female sockeye, and shows a readiness to spawn. And as Cunningham says, it becomes a signal of a salmon’s fitness as a mate.
“It's an indication of how well they've fed out in the marine environment,” he said, “If you have a lot of carotenoid pigments, you've probably done pretty well, you're a very healthy and robust individual.”
Like many other species, it’s the males that put on the dramatic show. Male sockeye’s secondary sexual characteristics develop later in their life, and though they aren’t directly involved with the process of reproduction, they help catch the eye of female sockeye.
“Once the spawning sockeye arrive on the spawning grounds, and they develop that hump, they develop that hook jaw and also develop these very pronounced and jagged teeth,” Cunningham said.
And these features don’t just help male salmon say they’re down to spawn—a hierarchy forms based on who has the biggest jaw, deepest humped back, and the gnarliest teeth.
“Potential suitors might be driven off by the larger male or the male that has the more pronounced hooked jaw,” he said. “And then that successful male will be able to move in there potentially spawn with a female, if she so chooses.”
The male’s elongated upper jaw is called a kype, and for spawning sockeye, size does matter.
“If you're able to develop really pronounced secondary sexual characteristics, you've probably done pretty well in terms of foraging and are probably likely to be a pretty good mate,” Cunningham said.
Donning their full mating ornamentation with red bodies, green heads, and long hooked kypes for the males, Bristol Bay sockeye are ready to spawn. The female chooses her well adorned mate, buries her fertilized eggs, and the next cycle of Bristol Bay sockeye begins.
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