Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the past week on East Sand Island were 9,140 terns and 3,235 terns, respectively. The average number of terns on-colony this week (6,188 terns) was similar to counts conducted at the colony during this time last year (6,564 terns). For the past two weeks, the weather in the Columbia River estuary was more typical for this time of year (i.e., some high winds and light rain); whereas in early April the weather was mostly dry with calm winds.

The first Caspian tern egg of the season on East Sand Island was seen on 19 April. This is 7 days earlier than the first egg was observed on the East Sand Island colony last year, but later than the first-egg dates in 2005 (14 April) and 2004 (16 April).

Rice Island and Other Upper Estuary Islands

Two surveys were conducted this past week (26 and 29 April) looking for nesting terns at dredged material disposal sites in the upper estuary. During these surveys no terns were observed in upland areas on Rice Island, Pillar Rock Sands, or Miller Sands Spit.

We will continue to monitor Rice Island and the other upper estuary dredge disposal islands for early signs of nesting (e.g., nest scrape digging) and will promptly inform resource managers of any such activity

Up-river Sites (Crescent Island, Rock Island, and Other Up-river Islands)

The high and low on-colony counts for the week on Crescent Island were 515 terns (28 April) and 394 terns (24 April), respectively. This represents a two-fold increase in the average number of terns counted on colony this week as compared to the previous week. Still, the numbers of Caspian terns counted on the Crescent Island colony significantly lag the numbers counted on the colony at this time last year.

The first Caspian tern egg of the season on Crescent Island was seen on 19 April, the same first-egg date observed at East Sand Island this year. This is 3 days later than the first egg was observed on the Crescent Island colony last year.

Nesting Caspian terns have not been observed at other sites along the Mid-Columbia River.

Diet Composition

For the week of 23–29 April, 38% of the identifiable fish delivered to the East Sand Island colony (n = 100 prey items) were salmonids. The majority of the non-salmonid prey items being delivered to the East Sand Island colony were smelt (19%), clupeids (i.e., herring or sardines; 11%), and surfperch (11%). Year to date, 43% of the identifiable fish delivered to the East Sand Island tern colony (n = 758) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 41% salmonids in the diet by this time last year.

Relative Vulnerability - Steelhead Tagging Study

Avian predation rates often vary by salmonid species, run-type, stock, and rearing type (hatchery vs. wild). Furthermore, predation rates for specific groups of fish can vary greatly from one year to the next. Despite this well-documented variation, limited data are available to determine which biotic and abiotic factors account for differences in smolt vulnerability to avian predators. One method to evaluate the potential intrinsic and extrinsic factors affecting relative vulnerability of smolts to avian predation is to PIT-tag smolts and subsequently recover their tags on bird colonies following the breeding season.

In 2007, we initiated a pilot study whereby run-of-the-river steelhead smolts encountered at the juvenile fish facility at Lower Monumental and Ice Harbor dams on the lower Snake River are PIT-tagged, weighed, measured, photographed (for condition scoring), and released back into the river to resume their out-migration. Following the breeding season, PIT tags will be recovered from various bird colonies in order to test hypotheses concerning how differences in smolt morphology, condition, abundance, origin, river conditions, and dam operations are associated with differences in smolt vulnerability to avian predation.

PIT-tagging commenced on 5 April at Lower Monumental Dam and on 10 April at Ice Harbor Dam. This past week, 550 and 41 steelhead were PIT-tagged at Lower Monumental Dam and Ice Harbor Dam, respectively. The majority of fish were in either good (27%) or fair (47%) physical condition, with some fish classified as being in poor (20%) or bad (6%) condition. We observed a four-fold increase in the percentage of fish in the sample that were classified in poor or bad condition this week as compared to the previous week (more detailed results on fish condition will be provided in a subsequent report). To date, 841 steelhead have been PIT-tagged at either Lower Monumental or Ice Harbor dams as part of this study.

Predator Activity and Colony Disturbance

At the East Sand Island tern colony this past week, disturbances by peregrine falcons and bald eagles have decreased significantly as compared to previous weeks. Perhaps as a result, the tern colony appears to be more settled, with a larger proportion of terns that were counted on colony sitting in scrapes this past week as compared to the previous week.

There were no disturbances observed at the Crescent Island tern colony this past week. Bald eagles, which have caused disturbance at the Crescent Island tern colony in previous weeks, were not observed at the colony this past week.

Data regarding gull kleptoparasitism rates at East Sand Island were unavailable at the time of this report. At Crescent Island, gull kleptoparasitism rates were 1% of all known fish fates observed at that colony. 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

On 28 April, Tom Banse (National Public Radio) visited East Sand Island.

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