Abstract: A water pressure-activated end line severing device is configured to be attached to a lobster, crab or snow crab trawl end line and to automatically sever the end line if a large marine animal has snagged, bitten, seized, caught, become entangled with, or otherwise somehow attached itself to that end line. When any large marine animal, such as a whale, snags or becomes entangled in the end line, the normal, instinctive response of the large marine animal is to dive rapidly to a greater depth, which pulls the end line and the device attached to the end line to a greater depth in the water column. As the device descends to a greater depth in the water column, the water pressure exerted against the device rapidly increases.