Validating the use of generalized additive models and at‐sea surveys to estimate size and temporal trends of seabird populations

Validating the use of generalized additive models and at‐sea surveys to estimate size and... Summary 1 The population size of seabirds is often difficult to estimate from surveys at breeding colonies because of factors such as burrow nesting and nocturnal breeding. The reliability of alternative surveys conducted at sea has not previously been validated. Sample‐based estimates from at‐sea surveys could be biased and have poor precision due to non‐random survey design and the uneven distribution of seabirds at sea. 2 We conducted at‐sea surveys of western gulls Larus occidentalis and common guillemots Uria aalge of the Farallon Islands, California, USA, and waved albatrosses Phoebastria irrorata of the Galápagos. The observed counts were modelled using generalized additive models (GAMs), with a correction factor for bird movement relative to the ship included in an offset. The models were used to produce estimates of the size of the seabird populations at sea, which were compared with independent colony‐based estimates, adjusted to account for the number of non‐breeders. 3 Gull and guillemot populations were estimated separately for each of 10 survey years. Temporal trends were estimated by smoothing through the annual values. The albatross data from 7 survey years were pooled to obtain a single estimate of average population size. 4 The coefficients of variation (CVs) of the annual estimates were approximately 10%, 15–20% and 15–45% prior to smoothing for the albatross, gull and guillemot, respectively. The CVs of the smoothed estimates were about 10% for the gull and 15% for the guillemot. These represent substantial improvements in precision over previous sample‐based estimates from at‐sea surveys. 5 The colony‐based estimates usually lay within the 95% confidence intervals (CIs) of both the annual and smoothed sea‐based estimates, showing that the sea‐based methods worked well. 6 Synthesis and applications. We conclude that GAMs of at‐sea seabird survey data, collected under suitable protocols and corrected for bird movement, can accurately estimate population size. Given sufficient demographic information, these methods can provide a valuable tool for the management of populations that are difficult to census at the breeding colony. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Applied Ecology Wiley

Validating the use of generalized additive models and at‐sea surveys to estimate size and temporal trends of seabird populations

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Publisher
Wiley
Copyright
Copyright © 2003 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0021-8901
eISSN
1365-2664
DOI
10.1046/j.1365-2664.2003.00802.x
Publisher site
See Article on Publisher Site

Abstract

Summary 1 The population size of seabirds is often difficult to estimate from surveys at breeding colonies because of factors such as burrow nesting and nocturnal breeding. The reliability of alternative surveys conducted at sea has not previously been validated. Sample‐based estimates from at‐sea surveys could be biased and have poor precision due to non‐random survey design and the uneven distribution of seabirds at sea. 2 We conducted at‐sea surveys of western gulls Larus occidentalis and common guillemots Uria aalge of the Farallon Islands, California, USA, and waved albatrosses Phoebastria irrorata of the Galápagos. The observed counts were modelled using generalized additive models (GAMs), with a correction factor for bird movement relative to the ship included in an offset. The models were used to produce estimates of the size of the seabird populations at sea, which were compared with independent colony‐based estimates, adjusted to account for the number of non‐breeders. 3 Gull and guillemot populations were estimated separately for each of 10 survey years. Temporal trends were estimated by smoothing through the annual values. The albatross data from 7 survey years were pooled to obtain a single estimate of average population size. 4 The coefficients of variation (CVs) of the annual estimates were approximately 10%, 15–20% and 15–45% prior to smoothing for the albatross, gull and guillemot, respectively. The CVs of the smoothed estimates were about 10% for the gull and 15% for the guillemot. These represent substantial improvements in precision over previous sample‐based estimates from at‐sea surveys. 5 The colony‐based estimates usually lay within the 95% confidence intervals (CIs) of both the annual and smoothed sea‐based estimates, showing that the sea‐based methods worked well. 6 Synthesis and applications. We conclude that GAMs of at‐sea seabird survey data, collected under suitable protocols and corrected for bird movement, can accurately estimate population size. Given sufficient demographic information, these methods can provide a valuable tool for the management of populations that are difficult to census at the breeding colony.

Journal

Journal of Applied EcologyWiley

Published: Apr 1, 2003

References

  • Seabirds and marine oil incidents: is it possible to predict the spatial distribution of pelagic seabirds?
    Fauchald, Fauchald; Erikstad, Erikstad; Systad, Systad
  • Analysis of population trends for farmland birds using generalized additive models
    Fewster, Fewster; Buckland, Buckland; Siriwardena, Siriwardena; Baillie, Baillie; Wilson, Wilson
  • Measuring the range to animals at sea from boats using photographic and video images
    Gordon, Gordon
  • Trends in the abundance of farmland birds: a quantitative comparison of smoothed Common Birds Census indices
    Siriwardena, Siriwardena; Baillie, Baillie; Buckland, Buckland; Fewster, Fewster; Marchant, Marchant; Wilson, Wilson
  • Modelling the impact of fishery by‐catches on albatross populations
    Tuck, Tuck; Polacheck, Polacheck; Croxall, Croxall; Weimerskirch, Weimerskirch
  • Modelling and smoothing parameter estimation with multiple quadratic penalties
    Wood, Wood

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