This is actually based off of a particular section of the Kalevala.
The hero of the poem is a man named väinämöinen, he was engaged to marry a woman named Aino against her will, she killed herself by drowning in a lake. Her spirit then returns as a large fish, väinämöinen goes fishing on this lake after hearing the story of mermaids and plentiful fish. He catches her and tries to gut the fish at which point she reveals her true nature and scared and upset flees back into the lake again.
This is actually based off of a particular section of the Kalevala.
The hero of the poem is a man named väinämöinen, he was engaged to marry a woman named Aino against her will, she killed herself by drowning in a lake. Her spirit then returns as a large fish, väinämöinen goes fishing on this lake after hearing the story of mermaids and plentiful fish. He catches her and tries to gut the fish at which point she reveals her true nature and scared and upset flees back into the lake again.