Last Updated May 17th
Note: The following data are preliminary and may change upon further review.
Caspian Tern Colony Counts East Sand Island The high and low on-colony counts for the week on East Sand Island were 13,960 terns (10 May) and 9,740 terns (11 May), respectively. The drier weather in the estuary the past several 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 on the upper beach to the southeast of the core colony area. Recent nesting attempts, particularly during the past two weeks, are mostly in the core area of the colony.
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. The former colony sites at Three Mile Canyon Island and Miller Rocks were visited on May 11 and no indications of terns nesting at either site were observed.
For the week, 63% of the identifiable fish delivered to the Crescent Island tern colony (N = 201) were salmonids. Year to date, 81% of the identifiable fish delivered to the Crescent Island tern colony (N = 941) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 80% 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 four 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.
On 9-10 May, 59 breeding adult terns were captured on the Crescent Island colony late in the egg incubation period using noose-mat traps placed over active nests. Each captured adult was weighed, measured, banded, and force-fed one fish (5-9 cm in length) containing one or two PIT tags. All fish were certified, disease-free triploid trout obtained from the Trout Lodge Hatchery, WA. Following sampling and force-feeding terns were released back onto the colony. Individually marked terns were observed following release to determine if and when normal on-colony behavior was resumed. Of the 59 terns that were force-fed fish, 100% successfully ingested the fish, and most (56 or 95%) returned to the colony to resume normal breeding behaviors within 6 hours following release. Five of the 59 terns used in the experiment (8%) already had PIT tags in their digestive track at the time of capture (i.e., from a naturally ingested PIT-tagged fish). A total of 26 tern eggs were lost as a result of this research activity, a majority due to adult terns crushing eggs while captured by the noose mats. Predation by gulls (n=4) and researchers inadvertently damaging eggs when trying to quickly free adult terns from the traps (n=3) were other causes of egg loss. No adult terns were injured during this experiment. The capture of adult terns for force-feeding experiments at Crescent Island is now completed for the 2005 breeding season. Force-feeding is scheduled to take place at East Sand Island next week (May 18 and 19 th). Following the breeding season and after all the terns have left their respective colonies, East Sand and Crescent islands will be scanned for PIT tags by NOAA Fisheries and the proportion of force-fed tags that are detected on-colony will be determined.Predator Activity At East Sand Island, a total of 8 colony disturbances were observed at a rate of 0.4 disturbances/hr of observation. At Crescent Island, one colony disturbance was observed at a rate of 0.1 disturbances/hr of observation. These disturbance data for the Crescent Island tern colony do not include disturbances associated with efforts to catch adult terns for the force feeding experiment. 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 4% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 3% 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 10 May, Ann Stephenson from the Yakima Klickitat Fisheries Project visited Crescent Island. On 14 May, Eric Harmon visited East Sand Island. |
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Up-river Sites (Crescent Island, Three Mile Canyon, Miller Rocks)
For the week of 9 May – 15 May, 55% 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 (18%) and clupeids (e.g., herring; 9%) . Year to date, 37% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,805) 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. 
The recovery of PIT tags on piscivorous waterbird colonies has been used as a direct measure of predation rates on ESA-listed salmonid populations from the Columbia and Snake rivers. Predation rates based on PIT tag recoveries are, however, underestimates; some proportion of tags consumed by terns is deposited off-colony, weather and other environmental factors remove PIT tags that are deposited on-colony, and some PIT tags are damaged and rendered unreadable before or after egestion on the colony. Understanding the magnitude of the bias in estimates of tern predation on smolts based on PIT tag recoveries on-colony is critical for deriving ESU-specific tern predation rates. One method we will use to estimate deposition rates will be to force-feed PIT-tagged fish to adult terns captured on the East Sand and Crescent island tern colonies and then determine the proportion of force-fed tags that are subsequently deposited at each colony.