Last Updated May 3rd
Note: The following data are preliminary and may change upon further review.
Caspian Tern Colony Counts
The high and low on-colony counts for the week on East Sand Island were 10,900 terns (1 May) and 4,720 terns (25 April), respectively. The drier weather in the estuary these past two weeks has allowed the standing water to drain from the colony. The flooding on the colony in previous weeks caused early nesting terns to occupy the higher ground around the edges of the colony and has likely contributed to the expansion of a satellite colony (510 terns) on the upper beach to the southeast of the core colony area. Recent nesting attempts, particularly during the past week, are mostly in the core area of the colony. The first Caspian tern egg of the season on East Sand Island was seen on 14 April. This is 2 days earlier than the first egg was observed on the East Sand Island colony last year, and the earliest lay date so far recorded for Caspian terns in the Columbia River estuary. Despite this early egg date, nest initiation on East Sand Island has been somewhat slowed by the inclement weather during most of April.
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 the entire former colony site on Rice Island is now vegetated, we do not expect that terns will attempt to nest there in 2005. We will continue to monitor Rice Island and the other upper estuary dredged material disposal islands for early signs of nesting (e.g., nest scrape digging) and will promptly inform resource managers of any such activity.
Nesting Caspian terns have not been observed at other potential colony sites along the Mid-Columbia River.
For the week, 87% of the identifiable fish delivered to the Crescent Island tern colony (N = 201) were salmonids. Year to date, 85% of the identifiable fish delivered to the Crescent Island tern colony (N = 540) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 79% salmonids in the diet by 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. The net pen frames were fitted with wire spike strips to prevent avian predators from roosting on the net pen frames and to deter predators other than terns from foraging on the fish held in the net pens. Juvenile rainbow trout in two different size classes (mean fork length 11 cm and 19 cm, respectively) were PIT-tagged and placed in the net pens, with approximately equal numbers of both size classes within each net pen. All fish were certified, disease-free triploids (sterile as adults) obtained from the Trout Lodge Hatchery, WA. The net pens will be monitored 8 hrs/day, 7 days/week until a minimum of 120 fish have been removed from each net pen by Caspian terns. Mesh netting will be placed over the net pens after each observation period to prevent terns and other predators from feeding on fish held in the pens when observers are not present. Following the breeding season and after all the terns have left the colony, the Crescent Island tern colony will be scanned for PIT tags by NOAA Fisheries. The number of PIT tags that are removed from the net pens by terns and subsequently detected on the Crescent Island tern colony will be used to address the research objectives listed above. Since fish were stocked in the net pens, Caspian terns have removed two fish from the Burbank Slough net pen; terns have yet to forage on fish held within the Ice Harbor net pen, although terns have been observed foraging in the vicinity of the pen. We will continue to monitor bird activity at the net pens in the coming weeks.
At East Sand Island, a total of 5 colony disturbances were observed at a rate of 0.4 disturbances/hr of observation. One colony disturbance was observed at Crescent Island at a rate of 0.1 disturbances/hr of observation. Generally, disturbances at the tern colony are short, causing terns to flush for only 10-30 seconds before returning to the colony. This past week, gull kleptoparasitism rates at East Sand Island were 3% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 6% 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.
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E
Up-river Sites (Crescent Island, Three Mile Canyon, Miller Rocks)
For the week of 25 April – 1 May, 34% of the identifiable fish delivered to the East Sand Island colony (N = 358) were salmonids. The majority of the non-salmonid prey items being delivered to the East Sand Island colony were clupeids (e.g., herring; 25%) and anchovies (21%). Year to date, 29% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,096) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 19% salmonids in the diet by this time last year.