Barges as temporary breeding sites for Caspian terns: assessing potential sites for colony restoration

Management proposals to reduce Caspian tern (Sterna caspia) predation on juvenile salmonids (Oncorhynchus spp.) in the Columbia River estuary include relocating some terns from the large colony in the estuary to several smaller colonies outside the Columbia River basin. The welfare of other listed or beleaguered salmonid stocks has been a primary concern in areas considered for restoration of Caspian tern colonies, demonstrating a need for empirical evidence on the effects of tern predation on fisheries prior to restoration of permanent colonies. The main objectives of this study were to determine 1) whether Caspian terns would readily use a barge as a temporary nesting site and 2) whether tern diet composition and productivity data could be collected at the barge. A small, sand-covered barge equipped with tern decoys and sound systems was anchored in Commencement Bay, Washington in 2001. Approximately 388 tern nests were initiated on the barge in a 17-day time period. We monitored diet composition at the barge site by direct observation of fish (n=1,097) in the bills of nesting adults. Tern diets during May were 65% juvenile salmonids; marine forage fishes comprised the remainder of the diet. Predation on tern eggs by glaucous-winged gulls (Larus glaucescens) was frequently observed during the first 10 days following the onset of egg laying; however, predation declined once incubating terns were densely packed on the barge. This study demonstrated that terns might rapidly colonize a barge and that diet composition and productivity data can be collected at the barge site. Barges may be used to assess prospective colony restoration sites; however, these efforts must be carefully conceived and coordinated with resource managers to avoid new resource management conflicts.