The coastguard said the search cost tens of thousands of pounds
Parents are being urged by Shetland coastguard to supervise their children more closely to prevent hoax mayday calls being made.
It comes after a large-scale rescue operation was mounted when a child's voice was heard calling for help on the shipping radio distress channel.
A helicopter, rescue craft and other boats searched the area near Yell Sound for two hours but found nothing.
The call, made at 1500 BST on 4 July, was later deemed to be a hoax.
Shetland Coastguard Watch Manager Katrina Hampson said the incident began when a female voice was heard on the distress and urgency channel, channel 16 saying, "We need immediate assistance".
She said: "We have listened to the tape of the call several times, slowed it right down and increased the volume. In the background a couple of seconds before the part that we initially heard you can hear a child's voice saying 'mayday, mayday'.
"It now seems likely that this was child playing around with a radio, rather than a real mayday call."
Ms Hampson added: "Please can we urge parents to closely supervise their children on board boats and around marine radios. This search cost tens of thousands of pounds and could have taken valuable resources away from someone who really needed them."