Last Updated April 26th
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 7,640 terns (24 April) and 2,970 terns (23 April), respectively. The drier weather in the estuary this past week allowed the standing water to drain from the colony by 19 April. The flooding on the colony in previous weeks caused early nesting terns to occupy the higher ground around the edges of the colony; new nesting attempts, particularly during this past week, are mostly in the core area of the colony. There has been a dramatic increase in the number of terns digging scrapes and incubating eggs this week as compared to the previous week. 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. Rice Island and Other Upper Estuary Islands On 17 April, a total of 396 terns were seen on upland areas of Pillar Rocks Sands. Resource managers were informed of the situation and a USACE contractor (Ken Larson) deployed stakes fixed with brightly colored flagging and eagle silhouettes to dissuade terns from nesting at the site on 21–22 April. On 21 April, three terns were seen on an upland area of Miller Sands Spit. No terns were observed in upland sites on Pillar Rocks Sands or Miller Sands Spit on 24 April. During this past week, no terns were observed roosting or attempting to nest in upland areas of 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 terns have not been observed at other sites along the Mid-Columbia River.
For the week of 18–24 April, 28% of the identifiable fish delivered to the East Sand Island colony (N = 376) were salmonids. The majority of the prey items being delivered to the East Sand Island colony were smelt (31%). Year to date, 27% of the identifiable fish delivered to the East Sand Island tern colony (N = 738) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 13% salmonids in the diet by this time last year. For the week, 78% of the identifiable fish delivered to the Crescent Island tern colony (N = 137) were salmonids. Year to date, 85% of the identifiable fish delivered to the Crescent Island tern colony (N = 339) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 76% salmonids in the diet by this time last year. Net Pen Study On 19 April and 20 April, net pens were deployed in Burbank Slough and in the forebay of Ice Harbor Dam, respectively. Roughly 850 and 650 PIT-tagged trout were stocked in the Burbank Slough and Ice Harbor 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 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. Caspian terns showed interest in the fish held within the Burbank Slough net pen, with one trout removed from the pen by a tern on 24 April. No terns were observed foraging in the Ice Harbor net pen, although a tern was observed circling the pen on 23 April. We will continue to monitor bird activity at the net pens in the coming weeks.
At East Sand Island, bald eagle disturbances at the tern colony have declined this week as compared to previous weeks. A total of 5 colony disturbances were observed (all causes) at a rate of 0.3 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. There were no disturbances observed at the Crescent Island tern colony tern colony this past week. This past week, gull kleptoparasitism rates at East Sand Island were 2% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 7% 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.
This past week, Talina Gossen and Aaron Sahnow volunteered to help with research activities in the Columbia River estuary (thank you, Talina and Aaron!). |
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Up-river Sites (Crescent Island)
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.