Last Updated May 29th
Caspian Tern Colony Counts East Sand Island The high and low on-colony counts for the past week on East Sand Island were 16,875 terns and 8,540 terns, respectively. The annual photo census of the Caspian tern colony on East Sand Island was conducted on 23 May. Once the aerial photos are counted (Fall 2007) we will be able to accurately estimate colony size and determine whether there has been a significant change in colony size since 2006.
Two boat-based surveys were conducted this past week (21 and 25 May) looking for nesting terns at dredged material disposal sites in the upper estuary. During these surveys, no terns were observed roosting or attempting to nest in upland areas of Pillar Rock Sands, Miller Sands Spit, or Rice Island. The total number of terns roosting at sites in the upper estuary has declined considerably from what was observed earlier in the season. 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.
The high and low on-colony counts for the week on Crescent Island were 560 terns and 501 terns, respectively. Counts of incubating or brooding adult terns on the Crescent Island colony indicate that about 355 pairs of Caspian terns attempted to nest at the colony this year. This represents about a 21% decline in the size of the colony compared to 2005, when about 450 pairs nested on Crescent Island. The first Caspian tern chick hatched at the Crescent Island colony on 16 May, 8 days later than the first chick hatched on the Crescent Island colony last year. The tern colony site at Rock Island was visited on 14 May, when 101 Caspian terns were observed on the island, which is located in John Day Pool. Of these, 24 terns were sitting in nest scrapes. Although no eggs were spotted during this survey, egg-laying, if it has not happened already, appears to be imminent. Caspian terns were first observed nesting on Rock Island in 2005, when 6 pairs attempted to nest there and at least one young was fledged from the colony. The Rock Island Caspian tern colony increased dramatically to 110 nesting pairs in 2006, but subsequently failed due to mink predation on all tern eggs and chicks produced at the colony. Nesting Caspian terns have not been observed at other potential colony sites along the Mid-Columbia River so far this year.
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, few 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. This year, we initiated a pilot study whereby run-of-the-river steelhead smolts encountered at the juvenile fish facilities 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, 978 and 140 steelhead smolts were PIT-tagged at Lower Monumental Dam and Ice Harbor Dam, respectively. The majority of fish were in either excellent (51%) or good (37%) physical condition, with some fish classified as being in fair (9%) or poor (3%) condition. To date, 5,010 steelhead have been PIT-tagged at either Lower Monumental or Ice Harbor dams as part of this study.
This past week at East Sand Island, a total of 17 colony disturbances were observed (all causes) at a rate of 1.0 disturbances/hr of observation. With the exception of the pelican disturbances mentioned above, 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 this past week. This past week, gull kleptoparasitism rates at East Sand Island were 3% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were also 3% 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 21 May, Steve Duncan and Eric Stone from BPA’s Photogrammetry Department visited the East Sand Island bird colonies to set up ground controls for the aerial photo census that was conducted on 23 May (see above). |
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The first Caspian tern chick of the season on East Sand Island was seen on 27 May. This is 4 days later than the first chick was observed on the East Sand Island colony last year, which previous to this year, was the latest first-chick date we have observed at the East Sand Island tern colony. The late first-chick date this year was apparently due to early nests failing because of gull predation. 
