Note: The following data are preliminary and may change upon further review.

Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 14,119 terns (7 May) and 9,861 terns (4 May), respectively. The low count this week was preceded by a major disturbance to the colony caused by a bald eagle landing on the colony to consume a dead tern. The colony is slowly increasing in size, as indicated by an increase in density of nesting terns within plots compared to last week.

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. We are planning with BPA to conduct the annual photo census of the Caspian tern colony on East Sand Island during the week of 17 May.

Rice Island and Other Upper Estuary Islands

No aggregations of terns have been observed on upland areas of dredged material disposal islands 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.


Washington Coast (Willapa Bay, Grays Harbor)

During aerial surveys conducted along the southern Washington coast this past week (8 May), no terns were observed on any of the islands in Willapa Bay or Grays Harbor, either in upland or inter-tidal areas. During similar surveys in previous years, we have usually seen terns in these areas at this time of year, although the numbers of terns using these areas seems to have declined in recent years. We will continue to fly periodic surveys in these areas looking for nesting terns.

Up-river Sites (Crescent Island, Three Mile Canyon Island, Miller Rocks)

The high and low on-colony counts for the week on Crescent Island were 950 terns (5 May) and 758 terns (3 May), respectively. The first tern egg was observed at the Crescent Island colony on April 12th, 8 days earlier than the first egg date on the Crescent Island tern colony in 2003, and earlier than the first egg date in any previous year of this study.

No terns have been observed at other former or potential Caspian tern colony sites along the mid-Columbia River so far this season.

Diet Composition

For the week of 3 May – 9 May, 37% of the identifiable fish delivered to the East Sand Island tern colony (N = 359) were salmonids, the same percentage as the previous week. Most of the non-salmonids observed being delivered to the East Sand Island colony were clupeids (e.g., herring; 31%). Year to date, 23% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,501) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 33% 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 = 208) were salmonids. Most of the non-salmonids observed being delivered to the Crescent Island colony were centrarchids (e.g., bass, sunfish; 9% of total diet). Year to date, 80% of the identifiable fish delivered to the Crescent Island tern colony (N = 696) 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.

Net Pen Study

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.

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. 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 (24 – 25 April). On 26 April, Caspian terns began foraging on fish within the net pen. The net pen data for this past week were not available when this report was prepared. These data will be reported in the next weekly update.

Predator Activity

At East Sand Island, bald eagles have been observed at the colony throughout the week, frequently causing terns to take flight. 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 was one disturbance observed at the Crescent Island tern colony this past week, or 0.1 disturbances/hr of observation.

During plot watches, 3 tern eggs were preyed upon by gulls at East Island tern colony this past week, or 0.2 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 5% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 15% 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.

Media & Other Visitors

There were no visitors to the East Sand Island tern colony site this past week.

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