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Aquatic Biology

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AB 4:297-309 (2009)  -  DOI:

Estimating seabird densities from vessel transects: distance sampling and implications for strip transects

Robert A. Ronconi1,2,*, Alan E. Burger1

1Department of Biology, University of Victoria, PO Box 3020 Stn CSC, Victoria, BC, V8W 3N5, Canada
2Present address: Department of Biology, Dalhousie University, 1355 Oxford St., Halifax, Nova Scotia, B3H 4J1, Canada

ABSTRACT: Vessel-based transects have been a mainstay of seabird research for decades, yet there has been surprisingly little effort to evaluate the detectability of seabirds within strip transects. Distance sampling methods offer an opportunity to quantify detectability and assess the proportions of birds on the water that may be missed during strip transects. Three summers of line-transect surveys were analyzed using the program Distance to model detection functions of 6 seabird taxa (marbled murrelet Brachyramphus marmoratus, rhinoceros auklet Cerorhinca monocerata, common murre Uria aalge, pigeon guillemot Cepphus columba, pelagic cormorant Phalacrocorax pelagicus, gulls Larus spp.). We tested the effects of covariates (year, observers, sea state, and cluster size) on detection function estimation. Year and observer were the most important covariates for some but not all species. For a 300 m wide transect (150 m on either side), 20 to 80% of birds were detected, depending on species, year, and observer. Detectability was highest for cormorants (60 to 90%) and fairly similar among other species (typically 40 to 60%). Sea state had a significant effect on encounter rate for murrelets suggesting that g(0), detection probability near the transect line, may be <1 when wave height increases. These data emphasize that a high proportion of birds on the water remain undetected during strip-transect surveys. It is important that surveys develop independent estimates of detection probability to account for detectability among species, years, observers, vessel types/speed, and viewing conditions. The application of distance sampling to seabird surveys can provide more accurate abundance and population estimates, which can improve conservation and management efforts.

KEY WORDS: Abundance estimates · Detectability · Distance sampling · Line transects · Observer bias · Seabirds · Surveys

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Cite this article as: Ronconi RA, Burger AE (2009) Estimating seabird densities from vessel transects: distance sampling and implications for strip transects. Aquat Biol 4:297-309.

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