Why do some swallow tattoos have a dagger through them?

I heard that if I sailor lost a good mate at sea he would put a dagger through his swallow tattoo to symbolize his lost friend. My grandfather told me this and showed me a picture of his sailer friend. He had 2 swallows on his chest and one had a dagger through it. My grandpa then told me about thow he had lost his brother (a sailor too)at sea.