Last Updated May 18th
Note: The following data are preliminary and may change upon further review. East Sand Island
This past week, no aggregations of terns have been observed on upland areas of dredged material disposal islands in the upper estuary ( Rice Island, Miller Sands Spit, and Pillar Rock Sands). Given that the entire former colony area on Rice Island is now vegetated, we do not expect that terns will attempt to nest at the former colony site in 2004. We will continue to monitor upper estuary dredge disposal islands looking for early signs of nesting (e.g., scrape digging) and will promptly inform resource managers of any such activity.
No terns have been observed at other former or potential Caspian tern colony sites along the mid-Columbia River so far this season.
For the week, 78% of the identifiable fish delivered to the Crescent Island tern colony (N = 201) were salmonids, a lower percentage as compared to the previous week (84%). Most of the non-salmonids observed being delivered to the Crescent Island colony were centrarchids (e.g., bass, sunfish; 14% of total diet). Year to date, 80% of the identifiable fish delivered to the Crescent Island tern colony (N = 897) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 73% salmonids in the diet of Crescent Island terns by this time last year.
On 21 April, a 6-meter circular net pen was anchored in a backwater slough off the Columbia River near Crescent Island. On 22 – 23 April, approximately equal numbers of juvenile rainbow trout in two different size classes (mean fork length 12 cm and 20 cm, respectively) were PIT-tagged and placed in the net pen. All fish were certified, disease-free triploids (sterile as adults) obtained from the Trout Lodge Hatchery, WA. The net pen and the surrounding slough will be monitored 8 – 15 hrs/day, 7 days/week until a minimum of 60 fish are seen being removed from the net pen by Caspian terns. A net will be placed over the net pen after each observation period to prevent terns and other predators from feeding on fish held in the pen when observers are not present. Following the breeding season and after all the terns and gulls have left their respective colonies, the Crescent Island tern and gull colonies will be scanned for PIT tags by NOAA Fisheries. The number of PIT tags that are removed from the net pen by terns and subsequently detected on the Crescent Island tern and gull colonies will be used to address the research objectives listed above. Caspian terns showed interest in fish held within the net pen (i.e., circling and hovering over the pen) during the first two days following the stocking of fish within the pen (24 – 25 April). On 26 April, Caspian terns began foraging on fish within the net pen. As many as 7 Caspian terns have been observed foraging in the vicinity of the net pen at one time and many more commute past the site each day to other foraging areas. To date, 17 trout (7 large fish and 10 small fish) have been removed from the net pen by Caspian terns in 44 attempts (i.e., dives). The majority of the trout removed from the pen (n = 16) were consumed by the adult tern immediately following capture; 1 trout was seen being carried in the adult tern’s bill as it flew away from the net pen in the direction of the colony. As was the case last year, Forster’s terns have also been observed repeatedly feeding on fish within the net pen. A total of 122 plunge dives have been observed to date, 36 of which were successful. All of the fish removed from the pen by Forster’s terns were small. No other predators have been observed at the net pen. We will continue to monitor bird activity at the net pen in the coming weeks.
On 10 May, 28 breeding adult terns were captured on the Crescent Island colony late in the incubation period using noose-mat traps placed over active nests. Each captured adult was weighed, measured, individually marked, and force-fed one fish (9 – 12 cm in length) containing two PIT tags. All fish were certified, disease-free triploid trout obtained from the Trout Lodge Hatchery, WA. Terns were held for a short period (approx. 5 minutes) to ensure that the force-fed fish was ingested (i.e., not regurgitated after force feeding) and then 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 28 terns that were force-fed fish, 26 terns (93%) successfully ingested the fish, and the majority of these (25 or 96%) returned to the colony to resume normal breeding behaviors 1 – 6 hours post-release. Two of the 28 terns used in the experiment (7%) already had PIT tags in their digestive track at the time of capture. A total of 17 tern eggs were lost as a result of this research activity, the majority (10 or 59%) due to predation by California gulls. The remaining egg loss was caused by terns struggling to free themselves from the noose mats (n = 4) and researchers inadvertently damaging eggs (n = 3) when trying to quickly free adult terns from the traps. No adult terns were injured during this experiment and all of the captured terns have been repeatedly resighted on the colony this past week. We are planning to conduct a similar force-feeding experiment at the East Sand Island tern colony during the week of 17 May. 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.
At East Sand Island, three colony disturbances were observed at a rate of 0.1 disturbances/hr of observation. Generally, these disturbances are short, causing terns to flush for only 10 – 30 seconds before returning to the colony. There were no disturbances (other than those described above) observed at the Crescent Island tern colony this past week. During plot watches, 2 tern eggs were preyed upon by gulls at East Island tern colony this past week, or 0.1 eggs depredated/hr of observation. No egg predations (other than those described above) were observed at the Crescent Island tern colony this past week. 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 14% 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.
Geoff Dorsey, Scott Dunmire, Karla Ellis, Fred Higginbotham, John Kranda, and Karen Zelch from the U.S. Army Corps of Engineers visited the East Sand Island tern colony site on 12 May. |
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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.