The huge sea creature was seen earlier this week just off the Kinghorn coast in Fife along with a smaller whale, thought to be its calf.
Members of the British Divers Marine Life Rescue (BDMLR) – an internationally recognised voluntary marine mammal organisation – have been searching for the mammal since a fisherman at sea told them about the incident.
Paul Smith, a member of the BDMLR’s “large whale specialist entanglement team”, said they were working with local fishermen, sailors and the UK Civil Aviation Patrol to find the whale.
He said: “We’ve got about eight people who are going out on the boats and various team members spotting on land. As soon as we get a sighting, we will zip out in the boat.
“But it could go on for months because the Forth is a huge body of water. It’s about being in the right place at the right time. We are on standby and ready to go if any sightings come in.”
Mr Smith also highlighted that the work they do, which requires specialist training, can be “very dangerous”. He said: “We have had people around the world dying trying to free whales. We are riding on its back trying to free the gear with ultra sharp knives, so it’s very, very dangerous.”
Mr Smith said they were first notified about the snagged whale by a fisherman heading east to the North Sea last Tuesday.
The team was mobilised, but they were unable to trace the creature as the whale had a 12-mile head start.
On Wednesday, the whale made its way into the Firth and was spotted just a few hundred yards from Pettycur Harbour.
The specialist team was mobilised again, but sea conditions deteriorated and visibility was too poor to have any chance of finding it.
Humpback whales can grow to more than 40ft in size. Pictures of the distressed whale, posted on the Forth Marine Mammals group on social media, have drawn several concerned comments.
Claire Sandilands posted: “Wow – someone did think there was something unusual in the water behind the whale when we saw it in Kinghorn!”