Graduate Thesis Or Dissertation

 

Foraging ecology of caspian terns and double-crested cormorants in the Columbia River Estuary Public Deposited

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  • A detailed understanding of the foraging ecology of species preying upon threatened or endangered prey may contribute to identifying and evaluating management options to reduce predation, when such management is deemed appropriate. In the Columbia River estuary, Caspian terns (Sterna caspia) and double-crested cormorants (Phalacrocorax auritus) have been identified as significant predators on juvenile salmonids (Oncorhynchus spp.), many populations of which are listed under the U.S. Endangered Species Act. In 1998 and 1999, we studied the foraging ecology of Caspian terns and double-crested cormorants in the estuary using point count surveys. We also flew aerial strip transect surveys throughout the estuary for terns, and in 1999 we used radiotelemetry to track tems during the chick-rearing period. Terns and cormorants generally used habitat in relation to availability, with tidal flats and deep water channels both important foraging habitats, while tributaries, sloughs, and areas near ocean jetties were less important. Higher densities of cormorants were observed foraging in locations with pile dikes and/or pilings in 1999. More terns foraged in the freshwater portion of the estuary in 1998, than in 1999, when some tems nested on East Sand Island in the marine zone of the estuary. During the latter half of both seasons, use of upriver foraging sites became less prevalent for both terns and cormorants and use of sites in the marine and mixing zone more prevalent. Terns were observed foraging 50 km from the Rice Island colony (where all terns nested in 1998, and most did in 1999); however, 5% of foraging occurred 27 km from this colony in both years. In 1999, we compared the foraging ecology of radio-tagged Caspian terns raising young at the main estuary colony on Rice Island, in the freshwater zone of the estuary, to tems raising young at a newly restored colony site at East Sand Island in the marine zone. Early in the chick-rearing period, radio-tagged terns nesting at Rice Island (river km 34) foraged close to the colony in the freshwater zone of the estuary, while terns nesting on East Sand Island (river km 8) foraged in the marine or estuarine mixing zones close to that colony. Late in the chick-rearing period, Rice Island terns shifted their foraging to the marine and mixing zones lower in the estuary; East Sand Island terns continued to forage in these areas. Tern diets at each colony corresponded to foraging location (freshwater zone vs. marine/mixing zone) of radio-tagged individuals: Rice Island terns relied heavily on juvenile salmonids (71% of identified prey) early in chick-rearing but this declined late in chick-rearing (46%). East Sand Island terns relied less upon salmonids (42% and 16%, respectively), instead utilizing marine fishes such as anchovy (Engraulis mordax) and herring (Clupea pallasi). Throughout chickrearing, Rice Island terns foraged farther from the colony (median distance: 12.3 km during early chick-rearing and 16.9 km during late chick-rearing) than did East Sand Island terns (9.6 and 7.7 km, respectively). Colony attendance decreased for terns at both sites from similar high levels during early chickrearing (60-70% of daylight hours) to lower levels (40-50%) during late chickrearing, with attendance decreasing significantly more at Rice Island. We conclude that Caspian terns and double-crested cormorants are generalist foragers and make use of the forage fish resources most available near the breeding colony. Predation rates on salmonids should decline if terns are attracted to colony sites, such as East Sand Island, where alternative prey are readily available. Precluding cormorant roosting at pile dikes and pilings, if feasible, might reduce consumption of salmonids, but additional studies would be required for verification.
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