Spatial models of delphinid (family Delphinidae) encounter rate and group size in the eastern tropical Pacific Ocean

Abstract

We use temporally dynamic environmental variables and fixed geographic variables to construct generalized additive models to predict delphinid (family Delphinidae) encounter rates (number of groups per unit survey effort) and group sizes in the eastern tropical Pacific Ocean. The delphinid sighting data and environmental data were collected simultaneously during the Southwest Fisheries Science Center's cetacean line-transect surveys conducted during the summer and fall of 1986–1990 and 1993. Predictions from the encounter rate and group size models were combined with previously published estimates of line-transect sighting parameters to describe patterns in the density (number of individuals per unit area) of delphinids throughout the study area. Areas with the highest predicted 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. Offshore waters in the northern and southern subtropical gyres had the lowest predicted densities. For both encounter rate and group size models, there was no geographic pattern evident in the residuals as measured by the ratio of pooled predicted to pooled observed values within geographic strata.