Last Updated June 14th
Note: The following data are preliminary and may change upon further review.
Caspian Tern Colony Counts East Sand Island The annual photo census of the Caspian tern colony on East Sand Island was conducted on 17 May. Visual estimates of colony size from the ground suggest that the East Sand Island colony may have declined somewhat since the 2004 nesting season. Once the aerial photos are counted (Fall 2005) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2004.
During this past week, no terns were observed roosting or attempting to nest in upland areas of Pillar Rock Sands, Miller Sands Spit, or Rice Island. Given that no Caspian terns have been seen roosting on upland portions of any of these dredged material disposal islands in recent weeks, we do not expect that terns will attempt to nest at these sites in 2005. Our weekly boat-based surveys to monitor Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands looking for early signs of nesting (e.g., scrape digging) have been discontinued; we may check these dredged material disposal islands occasionally, but we do not anticipate the formation of any new colonies this late in the season.
Upper Columbia River Sites (Crescent Island, Three Mile Canyon, Miller Rocks) The annual photo census of the Caspian tern colony on Crescent Island was conducted on 17 May. Preliminary visual counts on the ground suggest that the Crescent Island tern colony may have declined by about 10% since the 2004 nesting season. Once the aerial photos are counted (Fall 2005) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2004. Nesting Caspian terns have not been observed at former colony sites along the Mid-Columbia River at Three Mile Canyon Island (John Day Pool), Miller Rocks (The Dalles Pool), and Cabin Island (Priest Rapids Pool). A small, new Caspian tern breeding colony has been discovered recently by our up-river field crew. The colony is on Rock Island in the Blalock Islands, John Day pool (between the towns of Boardman and Irrigon, Oregon). It only consists of six breeding pairs, and is situated close to a nesting colony of Forster’s terns (ca. 60 pairs). This small Caspian tern colony has both eggs and chicks.
For the week, 63% of the identifiable fish delivered to the Crescent Island tern colony (N = 217) were salmonids. Year to date, 75% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,865) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 74% salmonids in the diet at this time last year.
On 19 April and 20 April, net pens were deployed in the forebay of Ice Harbor Dam and in Burbank Slough, respectively. Roughly 650 and 850 PIT-tagged trout were stocked in the Ice Harbor and Burbank Slough net pens, respectively. PIT-tagged fish removed from the net pens by Caspian terns will be used to determine (1) PIT tag deposition rates at the Crescent Island tern colony and (2) vulnerability of fish in different size classes to tern predation.
At East Sand Island, one colony disturbance was observed at a rate of 0.1 disturbances/hr of observation. Generally, disturbances of the East Sand Island tern colony by bald eagles have been more frequent and more intense than last year, including observations of eagles preying on adult Caspian terns on or near the colony. During these eagle disturbances, a large proportion of the adult terns on the colony may flush, leaving tern eggs and chicks vulnerable to predation by gulls nesting near the tern colony. At Crescent Island, no colony disturbances were observed this past week. This past week, gull kleptoparasitism rates were 1% of all known fates of fish delivered by terns at the East Sand Island Caspian tern colony. At Crescent Island, gull kleptoparasitism rates were 1% of all known fates of fish delivered by terns this past week. At the East Sand Island tern colony, kleptoparasitism is by glaucous-winged/western gulls, while at Crescent Island kleptoparasitism is by the smaller California gull. To date, there have been no signs of mammalian predators on any of the tern colonies monitored as part of this study.
On 7–9 June, Dave Craig, Paul Swenson, and Jacquelyn Grace from Willamette University visited East Sand Island. |
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The high and low on-colony counts for the week on East Sand Island were 10,570 terns (7 June) and 5,720 terns (10 June), respectively. The vast majority of tern eggs that were laid on the colony this year have either hatched or been depredated by gulls, and there are many tern chicks on the colony that are half-grown. 
For the week of 6 June – 12 June, 16% of the identifiable fish delivered to the East Sand Island colony (N = 354) were salmonids. The majority of the non-salmonid prey items being delivered to the East Sand Island colony were anchovies (41%) and surfperch (18%). Year to date, 34% of the identifiable fish delivered to the East Sand Island tern colony (N = 3,226) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 23% salmonids in the diet at this time last year. In the last two weeks the percent salmonids in the diet of East Sand Island Caspian terns has declined to the levels seen last year at this time, suggesting that there has been an influx of anchovies and other marine forage fishes into the estuary. 
This past week, a total of 15 trout were removed from the Burbank Slough net pen by Caspian terns. In total, Caspian terns have removed 65 trout from the Burbank Slough net pen. We recorded no tern activity at the Ice Harbor net pen this past week and have yet to observe a tern remove a fish from that net pen. We will continue our daily monitoring of the Burbank Slough net pen until late June.