Density and Abundance of Cetaceans in the Eastern Tropical Pacific Based on a Spatial/Temporal Analysis of 1986–2006 Line‐Transect Surveys

ABSTRACT The abundance of cetaceans in the eastern tropical Pacific Ocean (ETP) is estimated using spatial/temporal density modeling and conventional line‐transect approaches. Line‐transect data are analyzed from 10 ship‐based visual sighting surveys from 1986 to 2006 that include 281,000 km of survey effort in a 19.4 million km 2 study area. Three generalized additive density models are evaluated for each of 26 taxonomic categories (e.g., species, U.S. Marine Mammal Protection Act stocks, or genera), and the best‐fit model is used to estimate abundance. The three models include varying levels of complexity in spatial (latitude, longitude, and distance from the coast) and temporal (survey years) predictors. Group‐size, Beaufort state, and availability bias corrections are applied, which results in abundance estimates that are typically 2–6 times greater than those from previous analyses of the same data. After correcting for these biases, conventional design‐based estimates are similar to model‐based estimates. Our higher abundance estimates have significant implications for interpretating the status of depleted stocks of dolphins that have been impacted by the ETP tuna purse‐seine fishery. Additionally, the spatial/temporal models provide new insights into how these species are distributed in the ETP and how those distributions have changed through time.