The phrase "mass strandings" refers to an event where a large number of marine animals, such as whales or dolphins, get stuck or stranded on a beach or shallow water. Full definition
When investigating mass strandings of whales, likely the result of the use of navy sonar in open waters, (srfer.com)
Our militaries deploy sonar that induces the bends in dolphins, porpoises, and whales, so that they arrive in mass strandings on beaches with blood on their brains and coming from their ears; with air bubbles in their lungs; with all the signs that unfortunate divers display when they rise too soon through water. (scientificamerican.com)
We saw Norman Reedus - apparently portraying the game's protagonist - wake up on a beach under a grey sky, surrounded by dead sealife engaged in some sort of mass stranding event. (allgamers.com)