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Sausalito fisher Christian Cavanaugh returned from a recent trip 50 miles offshore with a prize: a nearly 200-pound bluefin tuna. Cavanaugh, a boat captain and the owner of Big C Charters, described seeing an uptick in the number of bluefin in Northern California waters over the past six weeks. Typically, the fish stay south of Point Conception in Santa Barbara County. Advertisement Article continues below this ad “I’ve caught a lot of tuna fishing long-range boats out of San Diego, so we do have a little bit of experience doing it, but seeing one that’s [around] 200 pounds come up here out of Sausalito is a pretty unique thing,” Cavanaugh told SFGATE. “It’s exhilarating.” The northward shift of bluefin — a massive fish species known for its athletic swims across the Pacific and cherished as a sushi delicacy worldwide — has also appeared in California’s commercial and recreational fishing data in recent months. The geographic shift is part of a slow trend over the past decade, due to factors like prey availability and international conservation efforts after overfishing, according to Elizabeth Hellmers, a senior environmental scientist for the California Department of Fish and Wildlife. But this year in particular, Hellmers said, there’s another factor: Ocean temperatures are warmer than usual due to California’s marine heat wave and an unusually strong El Niño developing. Advertisement Article continues below this ad “Usually, bluefin are below Point Conception generally, but with these El Niño conditions and the warming water, they are moving farther north,” Hellmers told SFGATE. She added, “They prefer waters between 68 to 78 degrees as their ideal range. As the waters warm farther north, they are following that water.” Scientists from the Scripps Institution of Oceanography at UC San Diego have documented an extreme marine heat wave forming in California waters since 2025. And the oceanographers anticipate California’s water will only get warmer in the coming months. Another massive warm blob that first bloomed along the equator in the Pacific Ocean — part of the rapidly strengthening El Niño event — is spreading from South America north toward California. "Warmer waters in the eastern Pacific can allow for big open ocean fish like tuna to expand their range closer to the California coast," Colleen Petrik, an assistant professor of biological oceanography at Scripps, said in a recent statement. "This range expansion also occurs vertically in the water column. Normally, deeper waters that are high in nutrients can be low in oxygen. But when upwelling is reduced by El Niño, oxygen levels in the middle depths can increase, which allows species like tuna (that need a lot of oxygen) to move into those depths and find more food." The fingerprints of El Niño are not on Bay Area waters yet, but oceanographers are on high alert. Advertisement Article continues below this ad “So far we have not seen evidence that El Niño’s influence has reached the West Coast, so the heat we have seen so far is largely left from marine heatwaves,” Michael Milstein, a spokesperson for NOAA Fisheries, wrote to SFGATE. “There are currently warm and cold patches off much of the California Coast, although waters remain warm off Southern California. We are watching indicators such as salinity and sea level where changes would signal El Nino’s arrival, but we have not seen those yet.” BECOME A FRIEND. GET OUR AD-FREE APP. Additional warm water fish species and sharks are shifting northward, too. Reports from Northern California and the Pacific Northwest have been streaming in, Milstein said. This week, a large striped marlin was caught off Washington state. California fishers have also recently seen dorado, wahoo and tripletail, according to Hellmers. “It’s an overall shift, and I think we are going to continue seeing a lot more of the less common species than we usually do,” Hellmers said. “I do think it’s an even bigger shift for the bluefin, since it’s on a larger scale than maybe with some of these other species and they are so important recreationally and commercially.” Advertisement Article continues below this ad If you've seen an unusual fish in California waters, you can report it here: https://scripps.ucsd.edu/marine-vertebrate-collection/el-nino-fish-records-form — Super rare tropical bird swoops over California. Experts fear a trend. — New fault capable of massive earthquakes discovered in California — Deadly parasite is flourishing in a California river — NASA satellite spots El Niño driving higher seas toward California Sign up for daily SFGATE breaking news alerts here.