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.
