Fisheries scientists are gaining unexpected insights from the serendipitous discovery on Año Nuevo Island of tiny tags that had been implanted in juvenile salmon and steelhead in coastal creeks.
The first tags were found by Patricia Morris, a research biologist at the University of California, Santa Cruz, and assistant manager of UCSC's Año Nuevo Island Reserve. The island, located a half-mile out to sea, hosts abundant populations of seals, sea lions, and seabirds. Researchers now suspect the tags came from fish that were eaten by birds.
When Morris was crawling through the muck of Año Nuevo Island in May of 2006 on gloved hands and padded knees, she wasn't trying to make a scientific discovery. She was mainly trying to get to her research terrace without disturbing the sea lions. Two unusual objects caught her eye as she crawled along--first, a flat, black plastic hexagon about half an inch across, and a week later, a glass object the size and shape of an uncooked grain of rice.
"You're kind of just sitting there, looking idly at the ground. In nature, things don't have perfect shapes--that's out of place, and it catches your eye," Morris said.
The glass grain of rice turned out to be a type of microchip called a passive integrated transponder (PIT), like the identification chips pet owners put in their dogs and cats. A PIT tag can be scanned, like a bag of frozen peas at the grocery store, to reveal identifying information about the animal.
Morris was able to scan the PIT tag and eventually figured out that it belonged to Sean Hayes and Morgan Bond, researchers in the Salmon Ecology Team at the NOAA Fisheries Lab near UCSC's Long Marine Laboratory. The black hexagonal tag was theirs, too--it was an implantable temperature logger designed to monitor the body temperature of tagged fish. (All three researchers are UCSC biology alumni--Morris with B.A. and M.A. degrees, Hayes with M.S. and Ph.D. degrees, and Bond with B.S. and M.A. degrees.)
The Salmon Ecology Team tags juvenile salmon and steelhead trout every year in Scotts Creek before they migrate out into the open ocean. The researchers hope to catch at least a small percentage of the fish over the next few years when they return to the creeks to spawn. Researchers scan captured fish for PIT tags and record each fish's tag number, size, and weight. If the fish is carrying a logger, the PIT tag lets the researchers know so they can take the fish to the lab and quickly remove the logger.
The logger takes a temperature reading every four hours. Because the fish is cold-blooded, its body temperature mirrors that of the water through which it swims. The scientists use data from the temperature loggers to try to figure out where the fish go and how they spend their time after they leave Scotts Creek.
After Morris got in touch with the Salmon Ecology Team, they came out and scanned the island for more PIT tags, most recently on January 24. So far they have recovered 60 PIT tags and 2 temperature loggers. Most of the PIT tags are from fish that were released in 2005 in Scotts Creek, but some came from as far away as Soquel Creek and as far back as 2003.
"This seems to be happening year after year," Bond said. "There are probably more older tags there, they're just buried."
But the first logger Morris found on Año Nuevo Island told a fascinating story. Hayes and Bond implanted the logger in a hatchery-raised Coho salmon on March 15, 2006. That salmon swam down Scotts Creek to the lagoon, where it lasted 13 days. On March 28, the temperature logger records it was eaten by a warm-blooded predator. The logger emerged on top of Año Nuevo Island on March 29.
Because it showed up on top of the island, researchers can rule out the theory that an elephant seal or a harbor seal ate the Coho. Hayes and Bond say the guilty predator is probably a bird, but they can't be sure.
"Unfortunately, the temperature loggers top out at about 25 degrees Celsius," Bond said. "The stomach temperature of the predator was much warmer than that. We never thought that we'd recover the tag of any of our fish that got eaten. We missed getting a good, precise measurement of the predator's body temperature."
To Hayes and Bond, the interesting thing about the temperature logger data is that it indicates that the Coho got eaten before it even made it out of the relative safety of the creek to the open ocean.
"The suspicion is that the predator is gulls," Hayes said. "It was a surprise only because we hadn't thought about it. In hindsight it makes sense."
Gulls spend a lot of time hanging out on the beach, where Scotts Creek broadens into a shallow stream only six inches deep, according to the researchers.
"If a big flock of gulls happens to be there at the same time that a big group of fish comes flooding out onto the beach, they're kind of running the gauntlet there," Bond said.
Also, the area on the island where Morris found the temperature logger is almost exclusively populated by nesting gulls. Hayes and Bond hope that further scans of the island will provide deeper insights into who's eating the fish. Bond is building a new scanner that will be more flexible and can scan farther than the improvised setup he initially used to scan the island.
"We've only scanned about half of the island. With the new scanner, I'll be able to scan under rocks and inside burrows," Bond said.
Hayes also hopes to raise funding for a study to determine if gulls are, in fact, eating the salmon and steelhead. He said that some researchers are skeptical that gulls are eating the fish, some of which were a foot long when they were consumed. Prior to this discovery, most researchers thought gulls ate only a very small percentage of young fish, while these results indicate that they may be a major predator.
"Seagulls have to eat something," Hayes said. "They haven't always lived off our dumps and McDonald's deposits."
Hayes and Bond will likely collect another year or two of data before they publish their results. They're grateful for this stroke of luck that allowed them to take a peek into the lives of young fish.
"Most of the time as a fish biologist, you tag a bunch of fish, they go out to sea, and most of them never come back," said Bond. "It was just pure luck that Pat Morris happened to see those tags on the island."
Note to reporters: You may contact Patricia Morris at (831) 429-5342 or firstname.lastname@example.org; Sean Hayes at (831) 420-3937 or email@example.com; and Morgan Bond at (831) 420-3955 or firstname.lastname@example.org.