Last Updated May 24th
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 of 17 May – 23 May, 23% of the identifiable fish delivered to the East Sand Island tern colony (N = 363) were salmonids, a lower percentage as compared to the previous week (28%). Most of the non-salmonids observed being delivered to the East Sand Island colony were anchovy (31%) and clupeids (e.g., herring; 20%). Year to date, 23% of the identifiable fish delivered to the East Sand Island tern colony (N = 2,217) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 37% salmonids in the diet by this time last year . For the week, 70% of the identifiable fish delivered to the Crescent Island tern colony (N = 225) were salmonids, a lower percentage as compared to the previous week (78%). 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, 78% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,122) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 71% salmonids in the diet of Crescent Island terns by this time last year.
A 6-meter circular net pen was anchored in a backwater slough off the Columbia River near Crescent Island. 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. 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. More fish were removed from the net pen by Caspian terns this past week (n = 21) than were removed in the previous 4 weeks combined (n = 17), presumably due to the onset of the chick rearing period (i.e., 67% of the fish captured this week were not consumed by adults immediately following capture but instead carried in the tern’s bill as it flew away from the net pen in the direction of the colony). To date, 38 trout (13 large fish and 25 small fish) have been removed from the net pen by Caspian terns in 86 attempts (i.e., dives). As was the case last year, Forster’s terns have also been observed repeatedly feeding on fish within the net pen. A total of 133 plunge dives have been observed to date, 45 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 18 – 19 May, 30 breeding adult terns were captured on the East Sand Island colony early in the chick rearing 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 or three 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 30 terns that were force-fed fish, 26 terns (87%) successfully ingested the fish, and most (26 or 87%) returned to the colony to resume normal breeding behaviors within roughly 2 hours post-release. Two of the 30 terns used in the experiment (7%) already had PIT tags in their digestive track at the time of capture. A total of six tern eggs were lost as a result of this research activity, three due to predation by gulls and three due to researcher’s inadvertently damaging eggs when trying to quickly free adult terns from the traps. A total of five tern chicks wandered from their nests when traps were being attended to; it is likely that at least some of these chicks suffered mortality due to exposure or aggression by neighboring adult terns. No adult terns were injured during this experiment and all but one of the captured terns has been repeatedly resighted on the colony this past week. The capture of adult terns for force feeding experiments at Crescent and East Sand islands is now completed for the 2004 breeding season. 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, four colony disturbances were observed at a rate of 0.2 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, two tern eggs and one tern chick were preyed upon by gulls at East Island tern colony this past week, or 0.1 nest predations/hr of observation (this does not include the predation events discussed above). No nest predation was observed at the Crescent Island tern colony this past week. This past week, gull kleptoparasitism rates at East Sand Island were 1% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 10% 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 Californiagull. To date, there have been no signs of mammalian predators on any of the tern colonies monitored as part of this study.
Brad Ryan from NOAA Fisheries visited the East Sand Island tern colony site on 19 May. Colleen Spiering and Dorie Welch from Bonneville Power Administration and Joe MacMillan visited the East Sand Island tern colony site on 21 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.