By Julia Belden
It’s one of Mother Nature’s most spectacular events.
Mature chinook salmon, battered from a long and exhausting migration, return to the shallow tributaries of their birthplace to spawn. It’s the final act of the salmon’s short two-to-three-year life: after passing on their genes to the next generation, the fish quickly die.
This final chapter is not without drama. Large “hooknose” males, named for their distinctive hooked upper jaws, fight for access to females. However, they are not the only contenders.
A handful of males reach sexual maturity early and are considerably smaller than both the hooknose males and the females.
These smaller “jack” males practice what scientists call an “alternative reproductive tactic”: while the hooknose males jostle amongst themselves, jack males sneak in and fertilize a female’s eggs.
“They kind of just run in, deposit their sperm, and run out,” said Kevin Adeli, a researcher at the University of Western Ontario. “They’re so small they can’t fight a salmon that’s much bigger than them,” so they instead resort to stealth and deception.
Adeli is the lead author of a recently published study in the Journal of Great Lakes Research comparing dietary niches between hooknose male, jack male, and female chinook salmon in Lake Ontario.
The researchers analyzed carbon and nitrogen concentrations within the salmons’ muscle tissue. Differences in the chemical makeup of the muscle tissue amongst the salmon life-stages suggest different diets – called “isotopic niches.”
“Notably, the jack males and the hooknose males had pretty little overlap in their isotopic niche, which kind of indicates that they have separate dietary niches in the Great Lakes,” said Adeli.
While the hooknose males seem to prefer prey that occupies offshore habitats, the jacks appear to be sticking more closely to shore for their meals. The study found that female salmon occupy a niche somewhere in between.
All-natural pest control
Native to the Pacific Ocean, Chinook salmon were introduced into the Great Lakes in the 1960s in part to control another non-native species, the alewife. Alewife, a species of herring, make up a significant chunk of the Great Lakes’ biomass and are notorious for washing up on beaches during seasonal die-offs.
Lake Ontario contains more alewife than the other Great Lakes, supporting a robust salmon population.
Amber Peters, a professor of fisheries and wildlife at Michigan State University who is not affiliated with the diet study, said an adult Chinook salmon may eat four to five individual alewives daily.
Most Chinook salmon spend several years feasting and maturing in the lake, but jack males return to the spawning grounds at a mere one year of age.
What makes a jack male, Adeli said, is partly attributable to genetics, “but part of it is due to their growth rates and their energy availability.”
Therefore, more food might result in more jack males. And, as Adeli notes, the jacks are here for a good time, not a long time.
“If you do have enough energy to do a successful spawn migration, then why wait? There’s no guarantee that in a year you’ll be better off than you are now.”
While a jack salmon might be a disappointing catch compared to a 30-pound hooknose, anglers can rest assured that they only make up a tiny fraction of the population.
Lake Ontario’s Chinook salmon fishery is jointly managed by Canadian and U.S. wildlife officials. The New York State Department of Environmental Conservation stocked Lake Ontario and its tributaries with over one million Chinook salmon in 2024, according to data provided by the agency.
Of course, the salmon also reproduce without human assistance, but not enough to sustain the population. Peters estimates that only 25% to 40% of the Chinook salmon in the Great Lakes resulted from natural reproduction – the rest are raised in hatcheries.
Salmon diets and conservation implications
The dietary niche study is a step forward in solving a long-standing health problem in Great Lakes predator fish.
Alewives produce an enzyme that breaks down thiamine, an essential vitamin for cellular function.
Buildup of the enzyme within the salmons’ bodies may result in a thiamine deficiency, which is linked to lower survival and reproductive success.
Peters said thiamine deficiency is especially notable in juvenile salmon, which may display symptoms of disorientation, leaving them unable to feed properly: “They’re not able to orient in the three dimensions in the way a healthy fish would be able to.”
A well-balanced diet may be the key to salmon success – and here’s where the jack salmon figure in.
If they’re consuming different prey than hooknose males and female salmon, the jacks might hold a clue to the thiamine conundrum. Adeli wants to find out. As part of his graduate thesis, he’s doing further research to see how thiamine levels differ between jack and hooknose males.
“[Jack males’] nutritional health is different because of these dietary differences, and it opens the question to what prey species are they eating that are causing these differences,” he said.
In the meantime, these sneaky salmon will continue their epic speed runs, adding a bit of dramatic flair to the life cycle of one of the Great Lakes’ largest predators.