Caspian terns (Hydroprogne caspia) and double-crested cormorants
(Phalacrocorax auritus) nest in large colonies on East Sand Island in the Columbia
River estuary, the largest known colonies for the two species in the world. Both
species of piscivorous colonial waterbirds have been identified as predators with a
significant impact on the survival of juvenile salmonids (Oncorhynchus spp.) listed
under the U.S. Endangered Species Act. To better understand and address issues
related to seabird-fisheries interactions in the Columbia River estuary, I conducted
studies related to the ecology, conservation, and management of these two species of
piscivorous waterbirds.
I evaluated the demographics and inter-colony movements of Caspian terns
belonging to the Pacific Coast metapopulation, with special emphasis on two breeding
colonies, one on East Sand Island in the Columbia River estuary and the other on
Crescent Island in the mid-Columbia River, based on re-sightings of color-banded
individuals. Apparent annual adult survival at both colonies was high, and age at first
reproduction was greater than previously reported for the species. Colony site
philopatry of breeding adults at both colonies was high; however, some individuals
prospected for breeding colonies over much of the Pacific Coast region and moved to
other colonies over distances of up to 3,000 km. Some terns from the large colony in
the Columbia River estuary responded quickly to the availability of new colony sites
as distant as 550 km from the estuary, and established successful breeding colonies
within less than a year of the new sites becoming available. The Caspian tern colony
on East Sand Island appears to be an important source colony for a number of smaller,
less productive colonies distributed over an extensive area from the Salton Sea,
California to the Copper River Delta, Alaska, an area with limited and ephemeral
nesting opportunities.
Environmental contaminants have been a conservation concern for wildlife in
the Columbia River estuary, especially species that consume fish and are therefore
likely to bioaccumulate persistent organic pollutants. I measured and compared levels
of polychlorinated biphenyls (PCBs) in eggs and chicks of Caspian terns and doublecrested
cormorants, as well as their primary prey fish types, at colonies on East Sand
Island and farther up-river. Based on differences in tern and cormorant diet
composition at the various study colonies, higher PCB levels in eggs and chicks were
associated with diets dominated by resident freshwater and estuarine fishes. PCB
levels in prey fish were positively correlated with lipid content; however, PCB levels
in the livers of chicks were negatively correlated with chick fat scores, suggesting that
chick fat reserves are a sink for ingested PCBs. Lower PCB levels in terns and
cormorants from East Sand Island compared to colonies farther up-river reflected diets
with a higher proportion of marine forage fishes at East Sand Island; marine forage
fishes had lower average levels of PCBs than their resident freshwater and estuarine
counterparts.
In order to explore non-destructive techniques for managing nesting colonies
of double-crested cormorants, I evaluated habitat enhancement and social attraction,
two techniques that have proven effective for relocating Caspian tern colonies to sites
where impacts on fish stocks of conservation concern would be minimal. Cormorants
were attracted to nest and successfully raised young at test plots on East Sand Island
and on islands in the estuary with a previous history of cormorant nesting or
unsuccessful nesting attempts. On an island with no history of cormorant nesting or
prospecting, however, no cormorants were attracted to nest. My results suggest that
attraction of nesting cormorants using these techniques is dependent on the previous
history of cormorant nesting or nesting attempts, the frequency and intensity of
disturbance by potential predators, and the presence of breeding cormorants nearby.
While habitat enhancement and social attraction have potential as methods for
redistributing nesting cormorants away from areas where fish stocks of concern are
highly susceptible to predation, successful establishment of new colonies using these
techniques will likely require a focus on sites with a history of cormorant nesting.
Findings from this dissertation raise some concerns over the management of
Caspian tern and double-crested cormorant colonies on East Sand Island in order to redistribute
parts of these colonies to alternative sites and mitigate the impact of those
piscivorous colonial waterbirds on ESA-listed salmonids. East Sand Island has
supported source colonies of piscivorous colonial waterbirds for many smaller
colonies throughout the region and is close to an abundant and relatively
uncontaminated food supply. Also, alternative colony sites that can substitute for East
Sand Island are not readily apparent, especially for double-crested cormorants.
Therefore, management of Caspian tern and double-crested cormorant colonies on
East Sand Island to benefit Columbia Basin salmonids needs to proceed cautiously and
reversibly because of the implications for the region-wide populations of these
piscivorous colonial waterbirds. / Graduation date: 2012
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28030 |
| Date | 13 January 2012 |
| Creators | Suzuki, Yasuko |
| Contributors | Roby, Daniel D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Relation | Lower Columbia Explorer, Wetlands Explorer, Wildlife Explorer |
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