The girl, described as aged between 15 and 18, initially approached the fish counter and engaged the victim in a discussion about various types of fish. But her interest in seafood was far from innocent, as the 52-year-old shop worker soon found out.
As they talked, the teen assailant suddenly seized a bream from the display counter and slapped the unwitting ASDA employee with the freshwater fish.
Nearby, a teenage boy appeared to be filming the assault and fled the scene along with the girl, sparking fears that the incident may have been an unusual twist on the 'happy-slapping' craze that has made headlines several times in the last decade.
"Without warning, the woman has picked up a large bream from the fish stall and slapped the worker across the face before running out of the store," Lancashire Constabulary explained in a statement which described the assault as 'completely unacceptable'.
"The victim believes the incident was being filmed on a mobile phone by a man stood nearby who also ran from the scene when challenged."
The police have released CCTV images of the pair in the supermarket. The girl was described as between 15 and 18 years old, with untidy shoulder-length blonde hair and an Irish accent. Her male accomplice was said to be around the same age, wearing a dark coat and dark grey trousers.
PC Graham Hartley said: "I would appeal to anybody that witnessed this incident or recognises either the man or the woman pictured in the CCTV images to come forward."