Last Updated June 19th
Caspian Tern Colony Counts East Sand Island The high and low on-colony counts for the past week on East Sand Island were 17,430 terns and 9,975 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.
One boat-based survey was conducted this past week (13 June) looking for nesting terns at dredged material disposal sites in the upper estuary. During this survey, 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 discontinue monitoring Rice Island and the other upper estuary dredge disposal islands for early signs of nesting (e.g., nest scrape digging) because the chances of Caspian terns initiating nesting in the upper estuary this late in the season are essentially nil. During aerial surveys conducted along the southern Washington coast on 5 June, roughly 85 Caspian terns were counted on islands in Willapa Bay and no terns were seen in Grays Harbor. The Caspian terns seen in Willapa Bay were all observed roosting below the high tide line. The numbers of terns counted in Willapa Bay and Grays Harbor on 5 June were similar to counts of terns in these areas at this time last year. The aerial survey conducted on 5 June was the last for this season; the nesting season has progressed sufficiently so there is little chance of Caspian terns initiating nests this late in the season at sites where terns are not already nesting.
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. Based on numbers of tern chicks counted on the Crescent Island colony in the last week, it appears that tern productivity this year will be only fair. In late May we conducted aerial and boat surveys of several islands on the mid-Columbia River, on Potholes Reservoir, on Sprague Lake, and on Banks Lake in order to assess colony size, nesting chronology, and nesting success for Caspian terns. During these surveys nesting Caspian terns were observed on Rock Island (located on the mid-Columbia River in the John Day Pool), Goose Island (located in Potholes Reservoir), and Dry Falls Island (located in Banks Lake). No proof of nesting by Caspian terns was observed on Harper Island (located in Sprague Lake). A total of 42 active tern nests were counted on Rock Island and 23 active tern nests were counted on Dry Falls Island. Nesting chronology at these two sites was several weeks behind that observed at the Crescent Island tern colony. Tern nesting has been documented at both of these locations in the past. 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. A total of 21 tern chicks and some nests with eggs were counted on Dry Falls Island in 2006. 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. This past week, 357 and 27 steelhead smolts were PIT-tagged at Lower Monumental Dam and Ice Harbor Dam, respectively. The majority of fish were in either excellent (52%) or good (33%) physical condition, with some fish classified as being in fair (13%) or poor (2%) condition. To date, 6,803 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 5 colony disturbances were observed (all causes) at a rate of 0.3 disturbances/hr of observation. 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 <1% of all known fates of fish delivered by terns. At Crescent Island, gull kleptoparasitism rates were 6% 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 4 June, Ben Sandford (NOAA Fisheries), Joy Sandford, and Scott Sebring (NOAA Fisheries) visited the East Sand Island tern colony. On 15 June, Scott Dunmire (COE, Walla Walla District) visited the East Sand Island cormorant colony. |
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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 on tern eggs.
The high and low on-colony counts for the week on Crescent Island were 387 terns and 309 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 2006, when about 450 pairs nested on Crescent Island.