A petrel is a fairly common bird to sailors. To landsmen, particularly those who live in the far Northern Hemisphere and inland, it may not be familiar at all. They are cousins to the albatross but are generally smaller. All are some pattern of black or white and they eat - you guessed it - fish.
The thing about petrels is that they tend to mass around a ship in blue water when a storm is coming up. Even before the humans aboard can identify what is going on, the petrels are staying close to the thing they perceive as a wooden island, ready to settle down on deck during the worst of it. Wayward seagulls and even huge albatross have been known to do this too, but not with the frequency that petrels do.
So, over the years, the smaller, predominantly black birds that are most common around ships and shipping came to be called storm petrels. Because sailors, like so many others who do hazardous and dirty jobs, are superstitious to the core, the title "storm petrel" came to mean someone who brings bad luck or trouble. Just as the presence of the bird around the ship meant a storm was coming, the presence of the person meant bad luck for ship and crew alike. Time to plan a marooning, perhaps.
Finally, and most humorously to me, the bird is sometimes referred to as Mother Cary's chicken. This particularly among English speaking sailors who had seen service in the Mediterranean, leading linguists to believe that the term is a corruption of the Latin mater cara. The words mean "dear mother" in deference to the Virgin and may have been uttered by Catholics in the face of a storm. Or a storm petrel.
One final extrapolation finds sailors using a similar slang term for snow: Mother Cary's chickens.
Mind your storm petrels, mates. And don't count your eggs before they're in the pudding. See you tomorrow for Seafaring Sunday