The Los Angeles Times reports that the weird, gooey beasts, called salp, usually stay far off the coasts, but for some unknown reason they've migrated to the Diablo Canyon plant in San Luis Obispo. The plants operator, PG&E has been forced to cut production because of the event:

Workers on Monday discovered an influx of the creatures, called salp, clogging screens that are used to keep marine life out of the seawater used as a coolant, Cuddy said. Often thronging many square miles of ocean in huge, gelatinous masses, salp are tubular, transparent organisms that can be roughly the size of a human thumb. No one knows how many are at the Avila Beach plant or how long they will remain.

The creatures aren't each that big—there are just too many of them. According to officials, the salp will likely disperse after a few days. Apparently it's not that weird for jellyfish to swarm power plants. It's just that no one seems to have a good explanation.

They pose no threat you say? Surrrrrrrrrrre they don't.

Nuclear Event in Japan on Friday, 24 June, 2011 at 04:51 (04:51 AM) UTC. Description A Japanese nuclear reactor that slowed down when jellyfish partly blocked a seawater intake resumed normal operations on Friday, operator Chugoku Electric Power Co said. Output from the 820 megawatt reactor fell to 770 MW on Thursday after the jellyfish partly covered the entrance of a seawater supply pipe, the Hiroshima-based utility said. A fall in the number of jellyfish enabled the No.2 reactor at its sole Shimane plant return to operating at full capacity on Friday, it said.

Originally posted by Darkblade71
Perhaps they "know" something we don't about the nuclear power plant.

Maybe this plant is the next one that will fail and the only way the animal kingdom can fight back is to prevent it from happening in the first place or at least attempt to.

I personally like that thought, although it is just a thought.