Rick Beattie (32), from Holywood, bought the sea bass from Tesco at Knocknagoney last week.
"I was preparing it for dinner and noticed something hanging from its mouth.
"When I pulled it out, I saw that it was a small creature with an egg-sac attached to it. It was absolutely disgusting," he said
A Tesco spokeswoman said its customer service team was now dealing with the matter.
Source: Belfast Telegraph (print edition)