Experts say it is unclear whether the sudden surge is the result of low mackerel catches or if other factors have caused the phenomenon.
Noel Hawkins, who works on the Summer Queen Cruises out of Ullapool, has been monitoring and photographing changes to the marine environment.
He said: "Huge numbers of jellyfish have been shoaling at Ullapool Harbour and Loch Broom. Numbers not seen in years have been accumulating around the sea, coast and washing ashore on the beaches."
He said there was concern that the jellyfish may be having an impact on fish numbers in the area, with little sign of the expected annual mackerel shoals.
Mr Hawkins explained: "Certain fishing zones in the world have reported 'jellification', where jellyfish have reached such levels that other species have been displaced. There are scientific teams looking into this, but we just don't know.
"The most common species are the harmless blue-ringed moon jellyfish but there are also blue jellyfish and red lion's mane that can cause swimmers and fishermen problems if touched. There are also continued reports of barrel jellyfish, the largest jellyfish encountered in UK waters."
Faserburgh-based Andrew Tait, of the Scottish Pelagic Fishermen's Association, has spent 50 years fishing.
He said: "I am on the board of the west coast inshore group and they are making a big effort to catch more mackerel because of the increased allocation, but the uptake has been poor this year. I wouldn't think the jellyfish would have anything to do with it."
He said fishermen had noticed climate change was having an effect on water and tidal streams. "Our concern is that it doesn't affect the spawning cycle of the fish. Water to the fish is like air to us. If they don't like the water they could spawn in a place different to the last 100 years."
Calum Duncan, Scotland Programme Manager at the Marine Conservation Society, said: "It is very difficult to link cause and effect with certainty. That is why we are very keen that the public report their sightings of jellyfish to us so we can get a fuller picture. It appears that proliferations of jellyfish blooms across the world are on the rise.
"Jellyfish are just large plankton, so you get naturally occuring blooms caused by the surface of the sea getting warmer in spring and summer which, combined with current and wind, can lead to localised aggregation.
"That may be the explanation of what has happened in Loch Broom."
But he said climate change and pollution from agricultural run-off are taking nutrients into the sea which artificially boost the blooms.
Mr Duncan said: "However, globally there is also speculation that over-fishing may be contributing to the blooms. Fish eat plankton. If you have fewer fish to eat the plankton, then there may well be bigger blooms.
"So it could also be that there are more jellyfish in Loch Broom because there are fewer mackerel, rather than the other way round."