Other Piscivorous Waterbirds
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Compared to Caspian terns and double-crested cormorants, other piscivorous colonial waterbirds that nest along the mid-Columbia River (i.e., California gulls, ring-billed gulls, American white pelicans) are having less impact on the survival of juvenile salmonids from the Columbia and Snake rivers. One gull colony that may be having a significant impact on salmonid smolt survival, however, is the large California gull colony (ca. 5,300 nesting pairs) on Miller Rocks in The Dalles Pool, where ca. 5,300 smolt PIT tags were recovered in 2009. Previous research in 1997 and 1998 indicated that salmonid smolts, and fish in general, constituted a very small proportion of the diet of California and ring-billed gulls nesting at up-river colonies. At the American white pelican colony on Badger Island 2,050 smolt PIT tags were recovered in 2009; this represents less than one PIT-tagged smolts consumed per nesting adult at this growing colony. In comparison, double-crested cormorants nesting at Foundation Island and Caspian terns nesting on Crescent Island consumed 16.2 and 16.5 PIT-tagged smolts per nesting adult, respectively. The size of some up-river gull colonies (>10,000 breeding pairs on several islands) and the Badger Island white pelican colony (> 1,000 pairs), however, exceeds that of the up-river tern and cormorant colonies and should be taken into account when evaluating overall impacts of avian predation on salmonid smolt survival on the Columbia Plateau. Further research and monitoring is necessary to determine whether particular gull and pelican colonies might be having a significant effect on survival of juvenile salmonids in the lower and mid-Columbia River.
In contrast to the gull and pelican colonies on the Columbia Plateau, previous research on glaucous-winged/western gulls nesting in the Columbia River estuary indicated that these birds consumed significant numbers of juvenile salmonids. Gulls nesting on Rice Island (river km 34) ate mostly riverine fishes, including out-migrating salmonids, whereas gulls nesting on East Sand Island (river km 8) ate primarily marine fishes. In 1997 and 1998, juvenile salmonids comprised 10.9% and 4.2% of the diet (by mass) of glaucous-winged/western gulls nesting on Rice Island/Miller Sands Spit and East Sand Island, respectively. PIT tag recoveries at gull colonies in the Columbia River estuary also suggest that gulls nesting at East Sand Island are having a larger per-capita impact on smolt survival as compared to gulls nesting further upriver in the upper estuary (i.e., Miller Sands Spit and Rice Island).
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