You could call it revenge on the birds.
While many marine birds are well known for their skills at diving into the sea to pluck out fishy meals, there is now solid evidence that some Pacific cod have turned the tables on the avian species.
The practice came to light a few years ago when seafood workers in Dutch Harbor noticed that some of the cod they were processing came with extra features -- partially digested birds in the fish stomachs.
Scientists from the Alaska SeaLife Center and University of Alaska have now examined remains of 74 birds collected from cod stomachs in 2011 and have some findings described in a study published online in the journal Marine Ornithology.
The bird remains come from cod caught in the Aleutian Islands region, off Cape Sarichef in Unimak Pass, using trawl and pot gear. The fish were processed at the UniSea plant in Dutch Harbor; the plant froze the bird remains and sent them to the scientists for analysis.
There have been other known cases of big fish eating small seabirds elsewhere in the world, the new study says, and past surveys by the National Oceanic and Atmospheric Administration have turned up, in very rare instances, bits of birds inside cod. In one case, a NOAA researcher found a murre foot in a cod stomach.
But the evidence from Dutch Harbor appears to be the first documentation of Pacific cod making a practice of eating birds, said study co-author Tuula Hollmen, science director at the SeaLife Center and an associate professor at the University of Alaska Fairbanks' School of Fisheries and Ocean Sciences.
The discovery sparks a lot of questions about what causes birds to be cod food, along with questions about how often this happens and what the biological significance may be, she said. "That's just an area that we didn't know much about," she said.
The recovered bird specimens were in varying conditions, said the study's lead author.
"It appears that they were probably swallowed whole, but they were in different stages of digestion," said Sadie Ulman, a SeaLife Center researcher. Some of the recovered birds were nearly intact, she said. "Others, it was just bones and feathers," she said.
In some cases, it was quite obvious that they had been consumed by cod, Ulman said. "There were a bunch of birds that were still within the stomach lining," she said.
The scientists were able to identify the species of 59 of the birds. For the rest of the bird remains, they were able to identify the genus, or group of species.
By far, the bird species most likely to wind up in a Pacific cod stomach -- if the Dutch Harbor sample is any indication – is the crested auklet. Of the 74 dead and partially digested birds collected from cod stomachs at the UniSea plant, 55 were crested auklets, small black birds that populate the Bering Sea and Sea of Okhotsk and forage for food in flocks. Crested auklets dive from the water's surface to find krill and other small fishy food, swimming in a way that resembles underwater flight, according to the U.S. Fish and Wildlife Service.
The others that could be identified by species were common murres, thick-billed murres and one Cassin's auklet, according to the study.
Whether the cod preyed on the birds, swallowed them accidentally or scavenged carcasses is still unknown, Ulman and Hollmen said.
Indications are, from tests the scientists performed on the birds' lung tissue, that some of them were alive when eaten by the cod but that more were already dead. The scientists found that lungs of three of the recovered crested auklets had air in them -- indicating those birds had been alive -- but that lungs of 30 did not, suggesting that they had drowned before winding up in the cods' stomachs.
Other evidence of fish eating birds comes from New England, where stomachs of big monkfish have been found with remains of dovekies, small bird in the puffin family, as detailed in a 2013 study.
A study in Sweden found evidence that pike introduced to freshwater lakes may be preying on ducks, though the predation might also be removing food sources that the ducks need. Tiger sharks are known to include birds in their diet, including songbirds in the Gulf of Mexico and young albatrosses in the tropical Pacific that are learning to fly.