are they at any threat?
They were under threat during the historic times, for example, Blue whale takes peaked in 1931 when over 29,000 blue whales were killed.
Blue whale
Blue whale takes peaked in 1931 when over 29,000 blue whales were killed. 
how long does it take for a blue whale to give birth Gestation lasts 10–11 months. where are these type of whales mostly found? Monterey Bay, San Diego, Baja California Sur, Mexico; Gulf of St. Lawrence to name some. where is this bay located? On the coast of the U.S. state of California. can you name some rivers or lakes found here? Sacramento River, San Joaquin River, Pit River, Feather River and Tuolumne River. are there any protected areas here? Yes, Soquel Canyon State Marine Conservation Area, Portuguese Ledge State Marine Conservation Area, Pacific Grove Marine Gardens State Marine Conservation Area, etc. how long can the former whale live for? 73 years do these whales migrate? They are not as specific in their movement patterns, and there is substantial evidence of alternative strategies, such as year-round residency, partial or differential migration, and anomalous habits such as feeding on breeding grounds. what is the diet like of these whales? Its diet consists almost exclusively of euphausiids (krill) except off Sri Lanka. what is krill? Small crustaceans of the order Euphausiacea. how does plastic effect them? Preliminary research indicates krill can digest microplastics under in diameter, breaking them down and excreting them back into the environment in smaller form. can we eat them? Yes what kind of hearing power do the former whale have? Its inner ears appear well adapted for detecting low-frequency sounds.
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Blue whale [SEP] Historic threats
Blue whales were initially tough to hunt because of their size and speed. Large-scale takes did not begin until 1864, when the Norwegian Svend Foyn invented the exploding harpoon gun and by using them with steam and diesel-powered ships. From 1866–1978, more than 380,000 blue whales were taken, mostly from Antarctic waters. Blue whale takes peaked in 1931 when over 29,000 blue whales were killed. The International Whaling Commission banned all hunting of blue whales in 1966 and gave them worldwide protection. However, the Soviet Union continued illegal whaling into the 1970s, and the last catch was taken by a Spanish expedition in 1978.