Last Updated June 1st
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. This concludes our weekly monitoring of 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); occasionally, we may check these dredge material disposal islands, but we do not anticipate the formation of any new colonies after early June.
This past week, we conducted aerial surveys looking for nesting and loafing Caspian terns on the (1) Columbia River from The Dalles Dam to Chelan, Washington; (2) Yakima River from the mouth to Selah, Washington; (3) Snake River from the mouth to Lewiston, Idaho; and (4) Potholes Reservoir (near Moses Lake, Washington). No new nesting colonies were discovered during these surveys; the existing tern colonies in Potholes Reservoir on Solstice and Goose islands, are active with approximately 100 and 200 terns counted at each site, respectively. Only two Caspian tern loafing aggregations were spotted during these surveys, one on the Snake River near Lower Monumental Dam (10 terns) and the other on the Columbia River near Wanapum Dam (27 terns).
For the week, 64% of the identifiable fish delivered to the Crescent Island tern colony (N = 200) were salmonids, a lower percentage as compared to the previous week (70%). Most of the non-salmonids observed being delivered to the Crescent Island colony were centrarchids (e.g., bass, sunfish; 17% of total diet) and cyprinids (e.g., northern pikeminnow, peamouth; 17% of total diet). Year to date, 76% of the identifiable fish delivered to the Crescent Island tern colony (N = 1,322) 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 (Burbank 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. This past week, a total of 37 trout were removed from the net pen by Caspian terns. To date, 79 trout (30 large fish and 49 small fish) have been removed from the net pen by Caspian terns in 167 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, although not recently. A total of 133 plunge dives have been observed to date, 43 of which resulted in a fish being captured. All of the fish removed from the pen by Forster’s terns were small. No other predators have been observed at the net pen. The net pen experiment will conclude on 4 June, at which time the pen will be removed from Burbank Slough.
At East Sand Island, two 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 observed at the Crescent Island tern colony this past week. During plot watches, no tern eggs or chicks were preyed upon by gulls at the East Island tern colony this past week. One tern chick was preyed upon by a California gull at the Crescent Island tern colony this past week, or 0.4 nest predations/hr of observation. 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 8% 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.
Scott Dunmire (U.S. Army Corps of Engineers, Walla Walla District) and Brad Ryan (NOAA Fisheries) visited the net pen on 25 May. |
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For the week of 24 May – 30 May, 22% of the identifiable fish delivered to the East Sand Island tern colony (N = 359) were salmonids,