They were fishing for haddock and ling off the North East coast when their net became worryingly heavy.
The crew of the Franchise were sailing near Nigg Bay, just a mile off the coast, when they saw something far out of the ordinary swim towards them.
Retired fisherman Robert Warden, 75, was one of those on board at the time and described the moment the unidentifiable dark mass swam into the Franchise’s net.
Mr Warden, now of Montrose, said: “We just saw this thing, this big brown thing come into the net. We got a scare as we couldn’t think what it was in the water. It was that big.
“We had some job getting it up on the boat.”
Mr Warden, whose crew included skipper Jackie Reid and the late John Ritchie, worked to keep the turtle wet with sea water while the alarm was raised, back in 1977.
The turtle measured around 5ft long and took up the entire area where the boat’s catches would normally be held.
Mr Warden added: “We radioed to the shore and at the time there was a zoo in Aberdeen. We were told to bring it ashore so people from the zoo could come and have a look.
“We just kept hosing the turtle down on the boat but it just lay there and we just tied to keep it wet.”
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Staff from Aberdeen Zoo, which closed in 1977, arrived at the harbour and boarded the Franchise to take a look at the turtle.
Mr Warden added: “The people from the zoo said it was too big for them to take and they didn’t want it. They said to take it back to where we found it.
“It had been hard getting it on the boat - and it was even harder getting it back over the side.”
Mr Warden, a fisherman who spent almost 50 years at sea, said: “It was just nae normal, for something like that to happen.”
Later, family of Mr Ritchie were told the turtle had weighed a tonne and may have come from the Caribbean. Records on the catch were held at the Fisheries Research Service at Torry, which has now closed.