Little effort has been made to conserve the East Asian finless porpoise, which is hindered by a lack of robust baseline data on their population status and threats. We conducted a local ecological knowledge survey around south Bohai Sea to collect information about the spatio-temporal distribution, population trends, and threats to East Asian finless porpoises. Based on the local ecological knowledge of 186 fishermen, April and September were the two months when porpoises were most frequently sighted, while February, November and December were when sightings were rare. Western coastal of Laizhou Bay and southern regions around Yellow River Estuary were identified as hotspots for sightings. More than half of the fishermen reported an overall decline in population through increasing water pollution, lower food resources and sustained fishery bycatch. These results provide insights into the distribution pattern and seasonal trends of the East Asian finless porpoises living in south Bohai Sea, thereby providing useful information for local authorities tasked with protecting this endangered cetacean.