The incident took place on the intervening night of Thursday and Friday.
"In a minor incident off the West Coast of India, a fishing vessel hit the periscope of the submarine which was involved in an exercise. The exercise was being conducted in pitch dark conditions as required..." Navy spokesperson D K Sharma said.
He said that there was no major damage to the submarine and it has since returned to the Mumbai dock where repairs are on.
Meanwhile, asked how could a fishing vessel hit the periscope when an exercise was on, defence sources said the exercise was about getting divers out without being noticed.
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"It required the submarine to be under water near the coast with no lights or sound. Since there are many fishing vessels around, one of them hit the periscope," they said.
The submarine was immediately brought back to the dock and the "misalignment" of the periscope is being rectified after which it will be back in the waters, the sources said.