Hey there! Say we're dealing with a race of merfolk who live somewhere in the North Sea and feed on plankton. Whereaboutish would they have to migrate if the gulf stream cut off? Could there be such a thing as a gradual disappearance or would there have to be some sort of catastrophic event that led to a sudden stop of warm water? I've tried searching for this but my google-fu is weak and oceanology utterly baffles me. Could you point me in the right direction? Thanks in advance!