Linking Rivers to the Sea(birds): Initial Surveys of River Otter Predation on Leach’s Storm-Petrels

By Eleanor Gnam, Seasonal Field Technician

The southern Oregon coast, between Port Orford to the north and Brookings to the south, hosts the largest colonies of Leach’s Storm-Petrels (Hydrobates leucorhous) in the lower-48. Goat Island, half a mile offshore from Harris Beach State Park, is estimated to host more than 100,000 of these small, dusky-colored seabirds. But looking at the island from the shore, you might never know that they’re there.

Leach’s Storm-Petrels

One of Goat Island’s many Leach’s Storm-Petrels.

Leach’s Storm-Petrels, which top out at just under 50 grams at the heaviest, return to their colonies only at night, and nest in underground burrows hidden beneath mats of long grass. Beachgoers who are in the know might be made aware of the colony from the distinctive, musky odor that petrels are famous for—which is strong enough to waft ashore—but otherwise, the colony is practically invisible from more than a few inches above the ground.

LHSPs feed on zooplankton and other planktonic creatures far out to sea, and only return to their colonies under the cover of darkness. During the breeding season, members of breeding pairs will take turns incubating their single egg or chick in the burrow, sometimes remaining underground for four or five days, while the other member of the pair forages. This cryptic, nocturnal behavior likely provides protection against diurnal avian predators. Southern Oregon’s LHSP colonies are close enough to shore, however, that nocturnal mammalian predators can pose a threat.

Project Goals

In collaboration with the U.S Fish and Wildlife Service, Luke Stuntz (MSc student, Seabird Oceanography Lab) and I (Eleanor Gnam, seasonal field tech) are investigating the impact of mammalian predators (mainly North American River Otters Lontra canadensis) on southern Oregon’s Leach’s Storm-Petrels.

We’re seeking to understand how, where, when, and to what extent these predators use petrels as a food source—knowledge that will help inform potential predator management in the future. River otters tend to operate either in loose social groups of unrelated males or in family units of a mother and her cubs. We’re hoping that our research will help us understand the social organizations of the river otters that are using these islands, as well.

Field Work (May-June)

Luke began fieldwork for this project in May, with trips to our focal islands to survey for predator sign and set up motion-activated game cameras.  Two of the four islands showed definite signs of predator activity (trampling, scat, and prey remains). Cameras on Goat Island quickly revealed activity from multiple social groups of river otters, including a pair of adults and a female with cubs. Because river otters commute between these islands and the mainland, surveys along the coastline are also important for monitoring their activity.

Luke’s initial surveys in May and early June revealed quite a bit of predator activity along the beaches and creeks near these colonies. River otters tend to deposit scat in shared, regularly-used locations called latrines, which aid territory marking and scent-based communication between individuals. They also need to return to freshwater sources frequently, especially after swimming in the ocean, both to drink and to groom their fur. The scat found in the latrines near our focal islands definitely contained digested storm-petrel remains—obvious from the distinct odor.

On June 15th, we moved into OSU’s Port Orford Field Station to commence full-time fieldwork on the project. Our first step was to revisit the coastline sites near our colony islands and to check out some new sites with the potential to be good river otter habitat. We were surprised by how little fresh river otter activity we found at some of our sites that were very active in May and early June. We’re also seeing them on our game cameras less often than before.

We found evidence of recent activity in several new locations, though, and we continue to see a lot of activity on the beach closest to Goat Island. This raises questions about how frequent and how seasonal these island-going behaviors might be. We’re also starting to wonder about the impact of Route 101 and its associated culverts on river otters’ use of coastal streams. Continuing predator sign surveys throughout the summer, both along the coast and on our colony islands, will help fill in our picture of this predator-prey dynamic and will help us understand how much of a problem it might be for these seabird colonies. We’re planning to expand our predator sign surveys up some of the larger creeks and rivers in the area, as well, using an inflatable sea kayak.