Cetacean population density in the eastern Pacific Ocean: Analyzing patterns with predictive spatial models

This dissertation introduces methods to quantitatively analyze data from shipboard line-transect surveys of cetaceans in the eastern Pacific Ocean in order to identify patterns in, and make predictions of, cetacean population density. Chapter One provides an introduction to the research questions, the study area, and the methods used to address the research questions. Chapter Two discusses a stratified line-transect analysis of delphinid (family Delphinidae) and Cuvier's beaked whale (Ziphius cavirostris) density based upon data from shipboard surveys conducted during the summer and fall between 1986--1996. Spatial patterns were found in delphinid and Cuvier's beaked whale densities, although the analytical methods limited the spatial resolution of the results to relatively large scales. Chapter Three introduces a method to predict cetacean density on smaller scales from line-transect survey sighting data by relating delphinid encounter rates (number of groups per unit distance) and group sizes to environmental variables in the eastern tropical Pacific. Areas with the highest predicted delphinid densities were the Gulf of California, the equatorial cold tongue, and coastal waters, including the west coast of the Baja Peninsula and the Costa Rica Dome. Chapter Four is a review of quantitative beaked whale habitat studies conducted worldwide; a common theme pervades all studies: to better understand beaked whale habitat, it is necessary to better understand their prey. In Chapter Five, the methods introduced in Chapter Three are used to predict Cuvier's and Mesoplodon beaked whale density in the eastern tropical Pacific Ocean. Results from Chapter Five provide evidence that the standard definition of beaked whale habitat proposed in the past may be too narrow, and that beaked whales may be found from the continental slope to the abyssal plain, in waters ranging from well-mixed to highly stratified. In Chapter Six, variance is estimated for the Cuvier's beaked whale density predictions presented in the previous chapter and the relative importance of the predictor variables that were used to predict encounter rate and group size is examined.