Last Updated May 04th
Note: The following data are preliminary and may change upon further review.
The first Caspian tern egg of the season on East Sand Island was seen on 16 April. This is 7 days earlier than the first egg was laid on the East Sand Island colony last year, and the earliest lay date so far recorded for Caspian terns in the Columbia River estuary. Currently, the majority of nesting terns on the colony have at least one egg.
No aggregations of terns have been observed at upland areas in the upper estuary ( Rice Island, Miller Sands Spit, Pillar Rock Sands) this past week. 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 on Rice Island 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.
For the week of 26 April – 2 May, 37% of the identifiable fish delivered to the East Sand Island tern colony (N = 354) were salmonids. Most of the non-salmonids observed being delivered to the East Sand Island colony were clupeids (e.g., herring; 34%). Year to date, 19% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,142) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 24% salmonids in the diet by this time last year. For the week, 84% of the identifiable fish delivered to the Crescent Island tern colony (N = 248) were salmonids. Most of the non-salmonids observed being delivered to the Crescent Island colony were centrarchids (e.g., bass, sunfish; 11% of total diet). Year to date, 79% of the identifiable fish delivered to the Crescent Island tern colony (N = 488) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 68% 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. Buoys and floats will be placed within 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. A total of 14 plunge dives were observed this past week, 9 of which were successful (i.e., resulting in a fish being captured). 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. As was the case last year, Forster’s terns have also been observed repeatedly feeding on fish within the net pen. A total of 30 plunge dives were observed this past week, 12 of which were successful. 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.
At East Sand Island, bald eagles have been observed at the colony throughout the week, frequently causing terns to take flight. Eight colony disturbances were observed at a rate of 0.3 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, one tern egg was preyed upon by a gull at East Island tern colony this past week, or 0.1 eggs depredated/hr of observation. No egg 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 16% 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 1 May, Dr. David Craig, Assistant Professor at Willamette University, and seven of his students, Amy Vasereno, Chris Eide, Minh Ngnyen, Mariah Mayfield, Anton Chiono, Andrea Countner, and Samantha Lantz, visited the East Sand Island tern colony. |
On 24 April, a net pen was deployed near Crescent Island and stocked with roughly 1,000 PIT-tagged juvenile rainbow trout to investigate (1) PIT tag deposition rates at the Crescent Island tern colony, (2) vulnerability of fish in different size classes to tern predation, and (3) gull kleptoparasitism rates on fish captured by terns at the net pen.