Balancing bias and precision in capture-recapture estimates of abundance

Authors

  • Nancy A. Friday,

    1. Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, U.S.A. E-mail: nancy.friday@noaa.gov
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      Currently at NOAA, NMFS, AFSC National Marine Mammal Laboratory, 7600 Sand Point Way NE, Seattle, Washington 98115-6349, U.S.A.

  • Tim D. Smith,

    1. Northeast Fisheries Science Center, 166 Water Street, Woods Hole, Massachusetts 02543, U.S.A.
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      Currently at 71 Sandpoint Shores, East Falmouth, Massachusetts 02536, U.S.A.

  • Peter T. Stevick,

    1. College of the Atlantic, 105 Eden Street, Bar Harbor, Maine 04609, U.S.A.
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      Currently at Hebridean Whale and Dolphin Trust, 28 Main Street, Tobermory, Isle of Mull, PA75 6NU, Scotland.

  • Judy Allen,

    1. College of the Atlantic, 105 Eden Street, Bar Harbor, Maine 04609, U.S.A.
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      Currently at Hebridean Whale and Dolphin Trust, 28 Main Street, Tobermory, Isle of Mull, PA75 6NU, Scotland.

  • Tom Fernald

    1. College of the Atlantic, 105 Eden Street, Bar Harbor, Maine 04609, U.S.A.
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      Currently at 11 Pine Street, Bar Harbor, Maine 04609, U.S.A.


Abstract

Capture-recapture estimates of abundance using photographic identification data are sensitive to the quality of photographs used and distinctiveness of individuals in the population. Here analyses are presented for examining the effects of photographic quality and individual animal distinctiveness scores and for objectively selecting a subset of data to use for capture-recapture analyses using humpback whale (Megaptera novaeangliae) data from a 2-year study in the North Atlantic. Photographs were evaluated for their level of quality and whales for their level of individual distinctiveness. Photographic quality scores had a 0.21 probability of changing by a single-quality level, and there were no changes by two or more levels. Individual distinctiveness scores were not independent of photographic quality scores. Estimates of abundance decreased as poor-quality photographs were removed. An appropriate balance between precision and bias in abundance estimates was achieved by removing the lowest-quality photographs and those of incompletely photographed flukes given our assumptions about the true population abundance. A simulation of the selection process implied that, if the estimates are negatively biased by heterogeneity, the increase in bias produced by decreasing the sample size is not more than 2%. Capture frequencies were independent of individual distinctiveness scores.

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