Napans strolling along the river Sunday may have caught a rare sight — a harbor porpoise surfacing in the Oxbow. “Been 10 years since there was a sighting in the Napa River,” he told The Press Democrat. Historical records show a harbor porpoise swam several miles up the Napa River in 2007 — one of the first sightings in decades. Though often mistaken for dolphins, porpoises are a separate species. Since 2007, The Marine Mammal Center has collected public reports of whale, dolphin and porpoise sightings to help researchers track marine life across the region.