(CBS News) Deep-sea fishermen are not usually looking to land a whale. But the fishing line and nets they leave behind are often blamed for a huge loss of ocean life.
According to one estimate, abandoned fishing gear kills more than 300,000 whales and dolphins every year.
However, one man is trying to turn that around. On Captain Dave Anderson's whale watching boat, the mission goes beyond giving tourists an adventure. It also might be called "mission nearly impossible." Anderson hunts for whales tangled in old fishing nets.
Anderson told CBS News, "The problem with fishing gear is that it is really hard to see."
Though almost invisible, fishing nets can injure or even kill a 150-ton whale.
Anderson explained to his whale watching group, "The gill nets are like a curtain - a curtain of death."
Photographs of humpback whales analyzed for a government report found half had signs of injuries from fishing gear. Anderson says other studies indicate the problem is even bigger.
"We have 1,000 whales and dolphin dying in nets every single day," Anderson said. "This is not sustainable.
So Anderson now rescues whales wrapped in fishing gear. To do that, he has special authorization under the Marine Mammal Act to get close to whales that need help.
Monic Deangelis, director of the National Oceanic and Atmospheric Administration's Mammals Division, said, "There is a real safety issue. It is definitely not for the lighthearted."
The recent rescue of a gray whale near Dana Point, Calif., showed what a tough job it can be. The whale had fishing gear all around its tail.
Anderson said, "Every time (the whale) is moving...with his tale flukes, the netting is just chafing against his skin."
The first task for Anderson's team was to attach markers to the whale. A two-day high-speed chase followed. Finally, the rescuers got close enough to start cutting the net away.
The fishing gear had caught a lot more than the gray whale. Anderson explained there was a seal in the netting, two leopard sharks, an angel shark, crabs and squid eggs. "A whole ecosystem was in that netting," Anderson said.
But the rescues don't always have a happy ending. Anderson has just written a book about the long, unsuccessful struggle to save a gray whale he called Lilly.
The whales Anderson loves to see are the ones that don't need to be rescued.
For Blackstone's full report, watch the video in the player above.