ECHIZEN, Fukui Pref. — A blood-orange blob the size of a small refrigerator emerged from the dark waters, its venomous tentacles trapped in a fishing net. Within minutes, hundreds more were being hauled up, a pulsating mass crowding out the catch of mackerel and sea bass.

The fishermen leaned into the nets, grunting and grumbling as they tossed the translucent jellyfish back into the bay, giants weighing up to 200 kg, marine invaders that are putting the men's livelihoods at risk.

The venom of the Nomura's jellyfish, the world's largest species at up to 2 meters in diameter, can ruin a whole day's catch by tainting or killing fish stung when ensnared with them in the maze of nets here in Wakasa Bay.