Overfishing by humans.
yes
No, but they are listed as "Vulnerable".
In terms of natural predator then it will be Orca Whales. Orcas feed on vulnerable Minke whales, commonly younger ones. They also are threatened by Human activity like Whaling as they are killed in their hundreds every year.
They do interact in same way other whales may do. They may see a boat and approach it cautiously then the may swim by the boat. This is why other whales are vulnerable to being hunted due to their natural curiosity like humans.
Blue whales don't need to protect themselves because their so big. They don't have any natural predators but their calves are vulnerable to killer whales. The only threat to adults are humans.
No, it does not. Granted, they do eat nearly microscopic plankton, but they are incredibly efficient at it. The main reasons they are endangered is because of overfishing, illegal killing/poaching, and pollution.
The Minke Whale has 2 enemies. The enemies are Humans and Orca/Killer Whales. Humans hunt the Minke Whales for their blubber and around 1,000 are hunted. Orcas feed on them as food but usually targets the more younger and more vulnerable minke whale.
Large whales and some sharks have attacked orcas, not to mention people. Yes, orcas are vulnerable.
When killer whales are observed eating other whales, they typically work together in a group to hunt and feed on their prey. They may use strategic tactics and coordination to take down the larger whale, such as surrounding it and attacking vulnerable areas. This behavior is known as cooperative hunting and is a common strategy among killer whales when hunting larger prey.
overfishing, and fishes getting traped in abandoned nets
well a dolphin could get a plastic can ring stuck on his throat and die from suffocate caused by the plastic can ring.