Columbia Plateau

As was the case in the Columbia River estuary, Caspian terns were identified as the most significant avian predator of juvenile salmonids on the Columbia Plateau in the late 1990's. In the year 2000, Caspian terns nested at five sites on the Columbia Plateau: Crescent Island (39 river km above McNary Dam on the Columbia River; ca. 550 breeding pairs), Three Mile Canyon Island (54 river km below McNary Dam on the Columbia River; ca. 275 breeding pairs), and three sites off the Columbia River (Potholes Reservoir, ca. 200 breeding pairs; Banks Lake, ca. 20 breeding pairs; Sprague Lake, ca. 25 breeding pairs). The proportion of juvenile salmonids in the diets of Caspian terns nesting at colonies on Crescent Island (61 – 70%) and Three Mile Canyon Island (81%) rivaled that of the former colony of Caspian terns on Rice Island (73 – 90%) in the Columbia River estuary. An estimated 465,000 and 679,000 juvenile salmonids were consumed by Caspian terns nesting on Crescent Island in 2000 and 2001, respectively. While this is an order of magnitude fewer juvenile salmonids than are consumed annually by Caspian terns nesting in the Columbia River estuary, predation rates on some Snake River and Upper Columbia River salmonid Evolutionarily Significant Units (ESUs) are surprisingly high. For example, predation rates by Caspian terns on in-river migrating, PIT-tagged smolts from the threatened Snake River Steelhead ESU were quite similar, if not higher, for terns nesting at the Crescent Island colony as compared to those nesting at the East Sand Island colony (13.9% and 10.8%, respectively; averaged over 2004-2005). This result is surprising because the Crescent Island tern colony is roughly 1/20th the size of the East Sand Island tern colony in the Columbia River estuary. During years of low river flow (e.g., 2004), Caspian tern predation on steelhead is especially high; an estimated 19% of in-river migrating PIT-tagged smolts from the threatened Snake River Steelhead ESU fell prey to Caspian terns nesting on Crescent Island in 2004. Tern predation rates on the Mid-Columbia River Steelhead ESU (threatened) and the Upper Columbia River Steelhead ESU (endangered) are also high for Caspian terns nesting on Crescent Island and for Caspian terns nesting at Potholes Reservoir.

Currently, there are fewer Caspian terns nesting on the Columbia Plateau than was recorded in the past; ca. 980 breeding pairs in 2009 versus ca. 1,200 breeding pairs in 1997. The Caspian tern colony on Three Mile Canyon Island is no longer extant (abandoned in 2001 due to mink predation), and a new, smaller Caspian tern colony has started at Rock Island (above John Day Dam). Currently, there are three active Caspian tern colonies on the Columbia Plateau off the Columbia River: Goose Island in Potholes Reservoir, ca. 490 breeding pairs; Twining Island in Banks Lake, ca. 60 breeding pairs; and Harper Island in Sprague Lake, 4 breeding pairs. The Caspian tern colony on Crescent Island (ca. 350 breeding pairs) is no longer the largest of its kind on the Columbia Plateau, eclipsed in size for the first time in 2009 by the Goose Island tern colony in Potholes Reservoir. Annual smolt consumption by the Crescent Island Caspian tern colony in recent years (2003-2009) has ranged from 330,000 to 470,000 smolts. Further study is needed to determine the magnitude and dynamics of predation on juvenile salmonids by Caspian terns nesting on the Columbia Plateau, especially in light of current and future management of Caspian terns in the Columbia River estuary that may result in some of these birds relocating to colonies on the Columbia Plateau.