MEPS 453:227-240 (2012)  -  DOI: https://doi.org/10.3354/meps09636

Global analysis of cetacean line-transect surveys: detecting trends in cetacean density

R. Jewell1,2,*, L. Thomas3, C. M. Harris2,3, K. Kaschner4, R. Wiff5, P. S. Hammond2, N. J. Quick1,6

1SMRU Ltd., New Technology Centre, North Haugh, St. Andrews, Fife, KY16 9SR, Scotland
2Sea Mammal Research Unit, Scottish Oceans Institute, University of St. Andrews, St. Andrews, Fife, KY16 8LB, UK
3Centre for Research into Ecological and Environmental Modelling, Buchanan Gardens, University of St. Andrews, St. Andrews, Fife, KY16 9LZ, UK
4Evolutionary Biology and Ecology Lab, Institute of Zoology, Albert-Ludwigs-University, 79104 Freiburg, Germany
5Instituto de Fomento Pesquero (IFOP), Blanco 839, Valparaíso, Chile
6School of Biology, University of St. Andrews, St. Andrews, Fife, KY16 9TF, UK

ABSTRACT: Measuring the effect of anthropogenic change on cetacean populations is hampered by our lack of understanding about population status and a lack of power in the available data to detect trends in abundance. Often long-term data from repeated surveys are lacking, and alternative approaches to trend detection must be considered. We utilised an existing database of line-transect survey records to determine whether temporal trends could be detected when survey effort from around the world was combined. We extracted density estimates for 25 species and fitted generalised additive models (GAMs) to investigate whether taxonomic, spatial or methodological differences among systematic line-transect surveys affect estimates of density and whether we can identify temporal trends in the data once these factors are accounted for. The selected GAM consisted of 2 parts: an intercept term that was a complex interaction of taxonomic, spatial and methodological factors and a smooth temporal term with trends varying by family and ocean basin. We discuss the trends found and assess the suitability of published density estimates for detecting temporal trends using retrospective power analysis. In conclusion, increasing sample size through combining survey effort across a global scale does not necessarily result in sufficient power to detect trends because of the extent of variability across surveys, species and oceans. Instead, results from repeated dedicated surveys designed specifically for the species and geographical region of interest should be used to inform conservation and management.


KEY WORDS: Marine mammal density · Population trends · Generalised additive modelling · Power analysis · Monitoring


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Cite this article as: Jewell R, Thomas L, Harris CM, Kaschner K, Wiff R, Hammond PS, Quick NJ (2012) Global analysis of cetacean line-transect surveys: detecting trends in cetacean density. Mar Ecol Prog Ser 453:227-240. https://doi.org/10.3354/meps09636

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