Forth Coastguard were contacted by the fishing boat around 9pm on Sunday and the mine was brought closer to shore and marked with a buoy.
A coastguard spokesman said: "They were about six miles out and they dragged it in. It was dark but they think it was a mine.
"The fishing boat left it on the seabed and went back to port for the night."
Royal Navy divers assessed the mine, which was placed around a mile off the shore or Port Seton near Edinburgh, before the controlled explosion at around 1.47pm.
A coastguard spokesman said: "They were about six miles out and they dragged it in.
“It was dark but they think it was a mine. The fishing boat left it on the seabed and went back to port for the night."
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