"We arrested 40 Indian fishermen yesterday and they were booked today," MSA spokesman Commander Mohammad Farooq said.
At the time of the arrests, there were about 30 other fishermen on the boats stopped by the MSA, he said.
"They were allowed to go back on humanitarian grounds since they were old and minors," he told PTI.
The detentions came a month after Pakistan held 58 Indian fishermen in similar circumstances, and two months after more than 300 were released from Pakistani jails as a goodwill gesture.
The fishermen say they do not realise that they have crossed the maritime boundary.
In late August, 337 Indian fishermen, including seven juveniles, were freed from jails in Karachi.
