What the fish in Alaska's oceans are eating -- and what that can tell us

Ever wonder what eats the salmon that don't make it back to Bristol Bay? Or what fish are cannibals?

A new database released online in late March by the National Marine Fisheries Service offers a glimpse into fish diets, based on decades of study of their stomach contents. It ranges from commonly known species like halibut, Pacific cod and pollock, to lesser-known fish, like sculpins, snailfish and even alligatorfish. Herring and salmon also make an appearance, although they aren't the focus.

Kerim Aydin, a supervisory fisheries biologist at the Alaska Fisheries Science Center in Seattle, said the database includes collections that have been in the works since the 1980s, and is largely focused on commercial groundfish species.

"We've collected hundreds of thousands of fish for the region, both the Bering Sea generally and Bristol Bay," Aydin said. "The new database has that information in a very raw form, fish by fish, but we also have summaries each year of which fish was eating which other fish and what other things they were eating."

That information can help researchers better understand the connectivity of the food web, and learn more about how a given species is doing.

Information about halibuts' stomach contents has helped provide information about what might be happening to the stock.

"Both cod and halibut are very similar," Aydin said. "When they're smaller, maybe about 10 or 15 inches long, they're primarily on the bottom and they're eating stuff on the bottom. Crabs, starfish."


As they grow, they start eating more pollock. But when there were fewer pollock out in the ocean, the halibut seemed to have less to eat.

"In the last couple years … we've seen more pollock again being consumed by halibut," Aydin said. "It's a bit too early to say if in the next few years this is going to lead to improved conditions, but the signs are positive."

So what does eat salmon? Due to how and when the data is collected, that's actually one of the lesser-known questions.

"It's interesting that salmon don't show up a lot in our data," Aydin said. "There's a couple reasons for that. One of the reasons is a lot of our samples are in the later summer when salmon smolts aren't so much out there in the system. But the other thing is, there really tends to be a separation in the ecosystem between salmon that are up on the surface, and they're getting eaten by all kinds of things, birds and mammals, but the fish that we see them in are down at the bottom."

Aydin said there is another interesting detail of salmon life in the database: they, like many fish, eat juvenile pollock.

"When the salmon smolts are out there, they're eating a lot of the tiny pollock," he said.

Pollock are one of the species with a relative abundance of data, collected during summer trawl surveys and also by fisheries observers in the winters.

"There's a lot of cannibalism that goes on in that species," Aydin said. "It happens more in the Bering Sea than in the Gulf of Alaska because of the way the juveniles and adults overlap. And it's also something that varies in different years. In fact, cannibalism isn't necessarily the best way for a species to survive."

But that varies from year to year. In cold years, plankton is fatter and has more nutritional value. Pollock eat more of that, and less of the juveniles.

"But when it gets really hot out there, the zooplankton don't do as well, and there end up being a lot of hungry little fish, a lot of hungry little pollock, and those hungry little pollock end up getting eaten by the big pollock in those warm years," he said.

Beyond just enabling access to data and providing interesting information to the public, Aydin said the agency is hoping that making the information available will help the agency tease more ideas out of it, too, with other scientists looking at it.

"A lot of these species seemed to be linked in the way they move through the ecosystem," he said. "You can look back 30 years and say these cycles are happening, and it gives us a window on when conditions will be good, when conditions will be poor."

This story first appeared in The Bristol Bay Times/Dutch Harbor Fisherman and is republished here with permission.