A humpback whale has been spotted in the Sacramento River, nearly 97 kilometers inland from the Pacific Ocean, according to multiple reports. The whale entered San Francisco Bay earlier this week and is now moving eastward into the Sacramento-San Joaquin Delta. This unusual sighting has raised concerns among scientists and researchers, as humpback whales are typically found in oceanic environments and not in freshwater systems. The whale was first spotted by Denny Glover, a fisherman on the Sacramento River, who was fishing for salmon and rockfish when he noticed the animal. Initially, Glover and his friends thought it might be a dolphin or porpoise. However, after seeing the whale spout water again, they realized it was a humpback whale. Glover told the Associated Press that the whale was much larger than his boat, which was about 6.4 meters long. According to the Marine Mammal Center, the whale has been identified through a photographic catalog and has been documented 15 times in the past 24 years between California and Mexico. The last documented sighting was in February in Baja California, Mexico, but the whale’s sex remains unknown. Humpback whales are among the largest animals on Earth, weighing up to 35 tonnes. They are marine mammals that typically migrate between polar and tropical regions, following seasonal food sources. They generally move to temperate and polar regions in warmer months to feed and return to warmer waters to reproduce and give birth. Occasionally, they may end up in river branches or deltas due to accidental navigation or distress behavior. Large whales are not adapted to live in freshwater for long periods, as their bodies are designed for the saltwater environment. Rivers lack the appropriate food resources for these animals, making them unsuitable long-term habitats. In 1985, a humpback whale named Humphrey spent several weeks in the Rio Vista area of the Sacramento River before returning to the ocean. Similarly, in 2007, a mother and calf, Delta and Dawn, spent nearly three weeks in the Sacramento Delta before returning to the ocean. The reason for this whale’s presence in the river remains unclear. Biologists at the Marine Mammal Center advise the public to avoid interacting with the whale or attempting to guide it back to the ocean until more information is available. "We do not know if this whale is simply passing through or if it is suffering from an underlying health issue. The best people can do is observe this adult whale from a respectful distance," said Kathi George, director of cetacean conservation biology at the Marine Mammal Center.