Who’s eating all the fish?

What are the major causes of salmon mortality? Unsurprisingly, this is one of the major questions in salmonid research. Of the thousands of eggs that an individual adult female salmon lays, on average only 1-2 adults return to successfully spawn. Salmon have a complex life history, spanning both freshwater and marine realms; there are many opportunities for them to die along the way, but it’s not always easy for researchers to parse out what’s killing them, and at which life stage.

This week, I helped out one of my lab-mates who’s studying one aspect of this complex question. She’s trying to figure out whether bird predation is a major cause of juvenile steelhead mortality in several small creeks just north of Santa Cruz, California. However, quantifying predation can be extremely difficult—the challenge is not just to show whether one animal is eating another, but also to quantify the predation rate (i.e. what percentage of the out-migrating juvenile salmon population is being eaten?). To do this, she and several researchers at the National Marine Fisheries Service lab in Santa Cruz came up with an ingenious method. A local biologist discovered a PIT tag (a small tag used to individually ID fish) on the nearby Año Nuevo Island, sparking the question: are birds eating young salmon in the creek and estuary and then depositing the tags (i.e. crapping them out) on the island?

The old foghorn keeper's house from the late 19th century

Año Nuevo Island is a beautiful state reserve just off the coast north of Santa Cruz, and provides important breeding and resting habitat for Northern Elephant Seals, California and Stellar’s Sea Lions, Rhinoceros Auklets, Brandt’s Cormorants, and Western gulls. At this time of year, its beaches and rocky terraces are teeming with wildlife—elephant seal pups, abandoned by their mothers and resting until they’re ready to start their own ocean journey, lie in adorable, fat, glassy-eyed piles. Huge droves of California sea lions blanket the beaches as well, barking noisily. And Western gulls add to the relentless cacophony; it’s a place that is at once peaceful and frantic, depending on your mood and ability to filter out the constant noise.

Sea lions and elephant seals blanketing the beach of the island

Wallowing baby elephant seals

We made a research trip out to the island yesterday to search for the PIT tags, heading across the ~1km stretch of ocean in a tiny dingy, banging the sides to scare off over-curious marine mammals. Our mission was to use a PIT tag detector to scan as much of the island as possible (marine mammals permitting)—the detector picks up the individual tag ID if it’s near a tag. Luckily, we didn’t need to actually find or retrieve the tags, since they are about the size of a grain of rice. Data on how many PIT tags are found and the detection likelihood, combined with data on total out-migrating salmon population size, will allow us to estimate avian predation rates on juvenile salmon in nearby creeks. In addition, the tags will tell us which particular individual fish were eaten, allowing us to quantify the characteristics of these fish to see if there is size-based mortality (i.e. were these fish disproportionately small or large compared to the average size of fish in the out-migrating population?)

Western gulls staking out their territory

Scanning for PIT tags

Estimating predation rates, as well as pinpointing potential predators, is an important step towards good management practices. So what are the potential predators that could be depositing these tags on the island? Possible culprits include avian predators—Western gulls and Brandt’s cormorants both use Año Nuevo Island for breeding—but also California sea lions, who also eat salmon. Determining which of these possible predators is actually depositing the tags on the island requires more (past and ongoing) research, including several studies analyzing the diet and movement patterns of Western gulls.