ROXANNE KHAMSI: This is Science Friday. I’m Roxanne Khamsi. If you’re about to eat lunch, fair warning. You might want to put down that sandwich for this next one. How do you map out a picture of how marine parasite populations have changed over time? Unfortunately, there’s no great archive of historical samples. Or is there?

Researchers recently gained access to a trove of canned salmon reaching back over 40 years. And they’re using that information to try to build a baseline for what’s normal and what’s not in the seas. Joining me now to talk about this is Natalie Mastick, who’s a PhD candidate at the University of Washington’s School of Aquatic and Fishery Sciences in Seattle. And she recently presented her work on this at the Ecological Society of America meeting. Welcome, Natalie, to Science Friday.

NATALIE MASTICK: Thank you for having me.

ROXANNE KHAMSI: So Natalie, can you talk to me about these salmon samples? Tell me a little bit more about where they came from.

NATALIE MASTICK: So the lab that I work in is Dr. Chelsea Wood’s lab at the School of Aquatic and Fishery Sciences. And we have kind of made a reputation for ourselves as being historical ecologists. So we use any type of historical data we can get our hands on to reconstruct baselines that might have been lost, especially for parasites, which are understudied.

And there’s an organization local to Seattle called the Seafood Products Association that heard about us in our lab and said, we have a basement full of canned salmon from the ’70s and on. Are you interested? And after sitting on that invitation for a while, I heard about it and jumped on it because I’ve been trying to figure out how parasites have changed in salmon for a long time. But there really aren’t that many museum specimens available for salmon.

ROXANNE KHAMSI: And so all of sudden, there was this trove of salmon. But what does 40 years of salmon in cans look like?

NATALIE MASTICK: I was expecting to see really old vintage labels and ruptured cans and things like that. I didn’t see that at all. I saw plastic totes full of unlabeled aluminum cans. It looks a lot like canned tuna.

It’s cooked fish inside of a can. It comes in different sizes. Some of them are those traditional kind of squatty cans. Some of them are about the size of a can of beans. And then there are mega cans that are huge, like Costco size.

[LAUGHTER]

ROXANNE KHAMSI: So you got out, I would assume, a can opener. But once you have the cans open, how do you analyze that salmon flesh, or whatever you call it?

NATALIE MASTICK: That was the question because we’re really used to analyzing frozen, or fresh, or museum specimens of fish. But we hadn’t looked at cooked fish before for worms. So it took some trial and error between me and a post-doc in our lab, Rachel Welicky, and eventually found out that the most effective way of finding these worms is by using two pairs of forceps– so basically, two pairs of tweezers– and picking through all of the meat.

ROXANNE KHAMSI: So you guys were picking through these specimens with your special tweezers. What do you actually see when you’re looking, I assume, with a microscope?

NATALIE MASTICK: These worms, you can see with your naked eye. So once we pick through, we would find a worm that was about a centimeter long, usually coiled up. And once you get this search image in your head, you can pick them out pretty easily.

And they form these little pockets in the muscle. So you’ll find a little pocket with this little worm coiled up. And you can pull it out and see everything that you need to with your naked eye.

ROXANNE KHAMSI: What’s the kind of name these worms have?

NATALIE MASTICK: These are anisakid nematodes. So they are parasitic nematodes of the family Anisakidae. And they are generalist parasites that infect a whole wide range of fish in the North Pacific and also throughout the world.

And they use marine mammals as their definitive hosts. So these fish get eaten by a seal or a whale, and then those parasites infect that marine mammal and reproduce. And then those eggs are released into the ocean through the marine mammal’s feces.

ROXANNE KHAMSI: So it’s the cycle of life. But it seems like we wouldn’t have a risk of getting infected with them if we happened to eat some canned salmon or canned fish. I’m guessing that that’s not going to hurt us.

NATALIE MASTICK: So these parasites are cooked, and the cooking process completely kills them. So if you eat canned fish that have these types of parasites, you’re completely fine. Raw fish, on the other hand, if it hasn’t been frozen, those worms are still alive and kicking. And they can hurt us. They cause food poisoning-like symptoms for about a day or two.

ROXANNE KHAMSI: Well, I’m glad there’s a vegan sushi place near my house.

NATALIE MASTICK: Yeah.

[LAUGHTER]

ROXANNE KHAMSI: So you did all this searching, but can you say a little bit more about what you found?

NATALIE MASTICK: Yes. So after dissecting a lot of cans, we found a significant increase in the number of these parasitic nematodes in pink and chum salmon over this 40-year period. And we also a dissected cans of sockeye and coho salmon but did not find the same trend. So in some species of salmon– like, pink and chum– we’re seeing this increase in parasite burden.

ROXANNE KHAMSI: And is it a lot? Is it double or triple?

NATALIE MASTICK: It’s hard to say. We are still running all of the numbers. But it varies. There are some cans that are worm-free. Of the cans that do have worms, the average is about three worms per can, which you definitely wouldn’t notice if you were just the average consumer. We did have one more recent can that had 115 worms in it. So they can reach pretty big quantities, but that’s not typical.

ROXANNE KHAMSI: Yeah. Well, three is frankly too many.

NATALIE MASTICK: Yeah. [LAUGHS]

ROXANNE KHAMSI: Can you tell me if there’s any idea why there’s this increase? Is it a bad thing?

NATALIE MASTICK: So there are a couple of different possible reasons for this. One that I think is pretty obvious to me is that, generally, in this area, marine mammals have recovered. So in the early 1970s, marine mammals in the US were protected by the Marine Mammal Protection Act. And with that increase in marine mammals, you’re increasing the number of definitive hosts for these parasites. So there are more hosts in the environment that can eat these infected fish and release eggs into the environment.

It also could be differences in the salmon diet or time spent in marine systems. There might have been a shift in their diets that could have led to an increase in eating prey that are parasitized. But I think it’s probably the marine mammal hypothesis.

ROXANNE KHAMSI: So it’s not a appetizing story, but there’s a real silver lining here because it means that mammal species in the waters are rebounding.

NATALIE MASTICK: Yes, that’s exactly right. It seems to me like this is kind of a sign of ecosystem recovery. Because these parasites need so many different host species in the environment, if they’re doing well, that means that their host species are also doing well. So I think this is actually a good thing.

ROXANNE KHAMSI: Well, I’m glad we could get to a happy ending here. Natalie Mastick– she’s a PhD candidate at the University of Washington’s School of Aquatic and Fishery Sciences in Seattle. Thanks so much, Natalie, for taking the time to talk with me today.

NATALIE MASTICK: Thank you so much for having me.

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