Last Updated April 27th
| Note:
The following data are
preliminary and may change upon further review.
On 13 April, Caspian terns were observed loafing on Pillar Rock Sands (a dredged material disposal island in the upper estuary) in upland areas (13 terns). On 16 April, as many as 297 terns were seen on the upland area of Pillar Rocks Sands during low tide. This is significant because if the terns were just loafing near a foraging site they would likely use the beach during low tide. Other indications of their intention to nest on Pillar Rock Sands were courtship displays, exchange of courtship meals, copulations, and the digging of nest scrapes. Resource managers were informed of the situation and the USACE conducted continuous monitoring and hazing of terns in upland areas on Pillar Rock Sands from 17-21 April. Passive measures to dissuade terns from nesting on Pillar Rock Sands were also deployed (200 stakes fixed with brightly colored flagging and 6 eagle silhouettes). No terns have been observed in upland sites on Pillar Rocks Sands since the continuous monitoring and hazing by a USACE contractor (Ken Larson) was discontinued on 21 April. A group of about 25 Caspian terns was observed on recent dredged material on Miller Sands Spit on 21 April. On subsequent trips to Miller Sands Spit on 23 April and 25 April, 17 and 0 terns were observed in upland areas, respectively. Resource managers have been informed of the situation and continuous monitoring and hazing of terns in upland sites on Miller Sands Spit have yet to be initiated. No aggregations of terns have been observed at upland areas on Rice Island this week, but because 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 Rice Island, Pillar Rock Sands, and Miller Sands Spit for early signs of nesting (e.g., nest 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, 73% of the identifiable fish delivered to the Crescent Island tern colony (N = 152) were salmonids. Most of the non-salmonids observed being delivered to the Crescent Island colony were centrarchids (e.g., bass, sunfish; 18% of total diet).
On 21 April, a 6-meter circular net pen was anchored in a backwater slough off the Columbia River near Crescent Island. The net pen frame was fitted with wire spike strips to prevent avian predators from roosting on the net pen frame and to deter predators other than terns from foraging on the fish held in the net pen. 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 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 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). 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. 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. One tern egg was preyed upon by California gulls at the Crescent Island tern colony this past week, or 0.1 eggs depredated/hr of observation. Egg predation rate data was not collected at East Sand Island this past week, but a number of tern eggs were removed from the colony by glaucous-winged/western gulls. 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 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.
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The high and low on-colony counts for the week on 
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.