(water running) - [Narrator] Twice a month, Oregon State University researcher, Jennifer Fisher, heads out to sea to check on the health of the ocean.

She uses special nets to collect plankton and krill, tiny organisms that are a huge driver in the ocean food chain.

- [Jennifer] Can you see all the phytoplankton in there?

- [Narrator] Normally, what comes up in the net is a green brown sludge.

It tells her how much nutrition fish like salmon are getting from their food.

But recently, something else started coming up in her net, a strange, unfamiliar sea creature that she and other researchers are hoping to capture today, if not in their nets, then at least on camera.

- [Jennifer] Are you ready?

- [Olivia] Yep.

- [Jennifer] Let's do it.

(water splashing) - [Narrator] A couple months earlier, a similar effort revealed a stunning sight.

Thousands of pickle-shaped organisms called pyrosomes filled the ocean as far as the eye could see.

- [Jennifer] When we saw how many there were, and it was just like, "Oh my gosh, this is mind-boggling."

This is definitely something that we haven't seen before and we don't know why they're here.

That's the big question now.

(wheel whirring) - [Narrator] From California to Alaska, the ocean is so loaded with pyrosomes, they're clogging the nets of researchers and fishermen.

- At first, it was irritation because they were clogging our research nets, and then it was kind of like, "Wow, okay, what is going on here?

Why are there so many?

Why now?"

- [Narrator] There's pyrosomes mingling with schools of fish and becoming fish food.

Biologists are finding them inside the bellies of fish and whales.

But normally, they're found in tropical waters.

Biology student, Olivia Blondheim wants to learn more about how these creatures work, so she's gathering underwater video.

- Oh, yeah.

- [Narrator] Finally, she spots one.

- So this is my first pyrosome that I've caught on video, so I'm really excited.

(all laughing) It's my pyrosome.

(laughing) - "It's my pyrosome!"

(laughing) (water splashing) - [Narrator] But on this trip, the pyrosomes are few and far between.

Not until nightfall do they get one on board.

- Pyrosome!

(machine whirring) This is what we've been searching for.

They're kind of rubbery, they're actually not very slimy.

- [Narrator] Pyrosomes look a lot like lumpy pickles, though they're translucent and hollow in the middle.

In some places, they glow, hence their name, which comes from the Greek words fire body.

They can get up to 60 feet long, but here, two feet is about as big as they get.

They're known as colonial tunicates, which means each hollow tube is actually made up of hundreds of smaller animals.

Bound together, they move through the water to feed, but what do they eat?

- Okay, so we're going to be going to the lab to look at some specimens.

Okay.

(door squeaking) So these are some pyrosomes that we just collected in June, and these have been frozen in preparation for chemical analysis.

- [Narrator] By grinding up pyrosome tissue, researchers can analyze what they've been eating.

That could be key to figuring out why they suddenly bloomed in such huge numbers.

- [Ric] It's been exponential.

Every year has been substantially more than the previous year.

- [Narrator] The pyrosomes seem to show up along with a large blob of warm water off the west coast.

As the ocean returns to more normal conditions, experts say the pyrosomes will likely die off and sink to the sea floor, but no one knows whether they'll come back.

- And they could disappear for good, or they could be back every year.

We really don't know that, but we're hoping that at least we can learn something about what kind of habitat they like to reside in that we can figure out if they might be coming back.

(boat whirring) - We're treading in the unknown.

We have never had this happen here.

I'm thinking that when they leave, that then if we see a change in ocean conditions at that point, then we can say, "Aha!

We're piecing this puzzle together, 'cause we really know very little."

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