The fish can be seen just under a bridge on Branham Lane in San Jose.
The South Bay Clean Creeks Coalition has been tracking them in South Bay waterways for more than 10 years.
ABC7 News tagged along with Steve Holmes, the coalition's founder and executive director as he worked to track down carcasses.
He spotted one along the Guadalupe River.
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"It came in from the Pacific Ocean under the Golden Gate Bridge, it swam down to the south end of the bay," Holmes said, "For the last month, those fish were probably doing circles waiting for the right temperature and enough water so that they can move into the watershed.
As is typical, after spawning the female fish died after an estimated three-year life span.
Holmes and the coalition take the heads off of the fish and send them off to UC Davis where work is done to determine where they came from.
Holmes said it was once believed that fish in these urban waterways came from hatcheries, but the research they're helping is proving that many are not.
"We're finding that there are fish that were born in streams, and have returned to this waterway to spawn," he said.
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One thing still not determined is why the fish are so much larger than in years past.
"We suspect reasons such as that they closed the fishing season in past years," he said, "Or that because of global warming, we're starting to see a change in the food sources that are available."
The South Bay Clean Creeks Coalition has worked for years to clear more than 1.3 million pounds of trash from waterways.
It's something fisheries biologists say could contributes to the higher number of fish.
"Cleaning the stream, creating habitat for spawning fish is, is critical," said Dr. Brian Allee.
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Chinook salmon are being seen in other places.
Just down the hill from a Campbell neighborhood, Chinook salmon can be seen splashing around in the Los Gatos Creek also digging rock nests.
The East Bay Municipal Utility District reported that the Mokelumne River has seen more than 20,000 returning to spawn.
Back in the South Bay, Holmes is confident that their efforts can bring even more monster sized fish to local waterways.
"By doing this work, we're giving these fish a chance to start increasing in size and population," he said, "That's what we're all about."