Originally Posted by The Grocer
Get used to seeing them; until we all decide overfishing is a Bad Thing, what you saw will become more and more common.
Not sure that fishing is the cause of the situation. Jelly swarms are rather fickle; resulting from a myriad of coincidental environmental factors, often hundreds of miles away. Wind and currents transport them pretty far afield, often in huge numbers.
The moon jellies in the video are pretty benign to humans (clogging of coolant pipes notwithstanding); they sting to feed, but are incapable of penetrating human skin.
Here (US west coast) we've been graced by a heavy influx of Brown Sea Nettles for the past couple of years: they come and go, but when they're in, they're in pretty heavy. These guys do sting (though the effects depend on your sensitivity to the toxin: I feel like I've been insufficiently novocained - locally numb and itchy; my cousin swells up for a week or more unless she takes large doses of antihistamines.)
Attached are a couple of pics of a heavy nettle day taken a couple of years ago. Shot with the wrong lens (not particularly wide angle), looking up from about 70 feet.